figure tag
- title: figure
- tags: figure
- This can be used to mark up a photo. The figure element can also contain a figure caption.
<figure>
<img src="zzz.jpg" alt="zzz" style="width:100%">
<figcaption>ZZZ</figcaption>
</figure>
video tag
- title: video
- tags: video
-
This allows you to embed a media player for video playback. For example, you can upload your video on AWS S3 and use the video tag to embed it on your website.
-
Using YouTube for that might come off as unprofessional. And Vimeo doesn't allow you to embed your videos without paying. You can specify certain attributes, such as width, height, autoplay, loop, controls, etc.
<video autoplay="" loop="" controls="" width="640" height="480">
<source type="video/mp4" src="zzz.mp4">
</video>
- You can also embed a live stream using getUserMedia() or WebRTC
picture tag
- title: picture
- tags: picture
-
This tag helps you display images in a responsive manner, by showing an alternative image version for smaller viewports.
-
It needs to contain one or more source tags and one img tag. The img tag will be used only if the viewport doesn't match any of the source tags or if the browser does not support it.
<picture>
<source media="(min-width: 968px)" srcset="large_img.jpg">
<source media="(min-width: 360px)" srcset="small_img.jpg">
<img src="default_img.jpg" alt="avatar">
</picture>
progress tag
- title: progress
- tags: progress
- The progress tag represents the progress of a task. The progress tag should not be confused with the meter tag (which represents a gauge).
<progress value="63" max="100">
</progress>
meter tag
- title: meter
- tags: meter
- You can use the meter element to measure data within a given range (a gauge).
- This can be achieved with min and max values or with a percentage.
<meter value="2" min="0" max="10">2 out of 10</meter>
<meter value="0.6">60%</meter>
map tag
- title: map
- tags: map
-
The map tag is used to define a client-side image-map. An image-map is an image with clickable areas. All you have to do is mention the X and Y coordinates in the elements from the map.
-
This means that you create a map of our Solar System and define areas for each planet and take the visitors to a separate page for each planet they click on.
<img src="solar_system.png" width="500" height="300" alt="Planets" usemap="#planetmap">
<map name="planetmap">
<area shape="rect" coords="0,0,52,92" href="earth.htm" alt="Earth">
<area shape="circle" coords="60,48,5" href="mars.htm" alt="Mars">
<area shape="circle" coords="135,48,12" href="saturn.htm" alt="Saturn">
</map>
contenteditable attribute
- title: contenteditable
- tags: contenteditable
- This attribute specifies whether the content of an element is editable or not.
<p contenteditable="true">This is an editable paragraph.</p>
input suggestions
- title: input suggestions
- tags: input suggestions
<input type="text" list="planets">
<datalist id="planets">
<option value="Mercury"></option>
<option value="Venus"></option>
<option value="Earth"></option>
<option value="Mars"></option>
<option value="Jupiter"></option>
<option value="Saturn"></option>
<option value="Uranus"></option>
<option value="Neptune"></option>
</datalist>
noscript tag
- title: noscript
- tags: noscript
-
The content inside the noscript element is rendered by the browser only when JavaScript is disabled.
-
It provides a fallback mechanism for the components that will stop working without JavaScript.
<noscript><h2>JavaScript is disabled in your browser.</h2></noscript>
html native search
- title: HTML Native Search
- tags: native, search
<div class="wrapper">
<h1>
Native HTML Search
</h1>
<input list="items">
<datalist id="items">
<option value="Marko Denic">
<option value="FreeCodeCamp">
<option value="FreeCodeTools">
<option value="Web Development">
<option value="Web Developer">
</datalist>
</div>
fieldset element
- title: Fieldset Element
- tags: fieldset, element
- You can use fieldset element to group several controls as well as labels (label) within a web form.
<form>
<fieldset>
<legend>Choose your favorite language</legend>
<input type="radio" id="javascript" name="language">
<label for="javascript">JavaScript</label><br/>
<input type="radio" id="python" name="language">
<label for="python">Python</label><br/>
<input type="radio" id="java" name="language">
<label for="java">Java</label>
</fieldset>
</form>
window opener
- title: window opener
- tags: window, opener
-
Pages opened with target=”_blank” allow the new page to access the original's window.opener.
-
This can have security and performance implications.
-
Include rel=”noopener” or rel=”noreferrer” to prevent this.
<a href="https://freecodetools.org" target="_blank" rel="noopener">
Free Code Tools
</a>
base element
- title: base element
- tags: base, element
- If you want to open all links in the document in a new tab, you can use base element:
<head>
<base target="_blank">
</head>
<!-- This link will open in a new tab. -->
<div class="wrapper">
This link will be opened in a new tab:
<a href="https://freecodetools.org/">
Free Code Tools
</a>
</div>
details element
- title: Details Element
- tags: details, element
- You can use the
details
element to create native HTML accordion.
<div class="wrapper">
<details>
<summary>
Click me to see more details
</summary>
<p>
More details.
</p>
</details>
</div>
liveReload
- title: livereload
- tags: livereload
- Heard of VsCode LiveServer Extension? We are going to create something similar to that with a single line of HTML, all you have to do is change the content value to your desired value Example: content = "3000".
<meta http-equiv="refresh" content="value(seconds)">
border-with-top-triangle
- title: Border with top triangle
- tags: visual,beginner
Creates a content container with a triangle at the top.
- Use the
:before
and:after
pseudo-elements to create two triangles. - The colors of the two triangles should be the same as the container's
border-color
and the container'sbackground-color
respectively. - The
border-width
of the one triangle (:before
) should be1px
wider than the other one (:after
), in order to act as the border. - The smaller triangle (
:after
) should be1px
to the right of the larger triangle (:before
) to allow for its left border to be shown.
<div class="container">Border with top triangle</div>
.container {
position: relative;
background: ##ffffff;
padding: 15px;
border: 1px solid ##dddddd;
margin-top: 20px;
}
.container:before,
.container:after {
content: '';
position: absolute;
bottom: 100%;
left: 19px;
border: 11px solid transparent;
border-bottom-color: ##dddddd;
}
.container:after {
left: 20px;
border: 10px solid transparent;
border-bottom-color: ##ffffff;
}
bouncing-loader
- title: Bouncing loader
- tags: animation,intermediate
Creates a bouncing loader animation.
- Use
@keyframes
to define an animation with two states, where the element changesopacity
and is translated up on the 2D plane usingtransform: translate3d()
. Use a single axis translation ontransform: translate3d()
to achieve better animation performance. - Create a parent container,
.bouncing-loader
, for the bouncing circles and usedisplay: flex
andjustify-content: center
to position them in the center. - Give the three bouncing circle
<div>
elements awidth
andheight
of16px
and useborder-radius: 50%
to make them circular. - Apply the
bouncing-loader
animation to each of the three bouncing circles, using a differentanimation-delay
for each one andanimation-direction: alternate
to create the appropriate effect.
<div class="bouncing-loader">
<div></div>
<div></div>
<div></div>
</div>
@keyframes bouncing-loader {
to {
opacity: 0.1;
transform: translate3d(0, -16px, 0);
}
}
.bouncing-loader {
display: flex;
justify-content: center;
}
.bouncing-loader > div {
width: 16px;
height: 16px;
margin: 3rem 0.2rem;
background: ##8385aa;
border-radius: 50%;
animation: bouncing-loader 0.6s infinite alternate;
}
.bouncing-loader > div:nth-child(2) {
animation-delay: 0.2s;
}
.bouncing-loader > div:nth-child(3) {
animation-delay: 0.4s;
}
box-sizing-reset
- title: Box-sizing reset
- tags: layout,beginner
Resets the box-model so that width
and height
are not affected by border
or padding
.
- Use
box-sizing: border-box
to include the width and height ofpadding
andborder
when calculating the element'swidth
andheight
. - Use
box-sizing: inherit
to pass down thebox-sizing
property from parent to child elements.
<div class="box">border-box</div>
<div class="box content-box">content-box</div>
div {
box-sizing: border-box;
}
*,
*:before,
*:after {
box-sizing: inherit;
}
.box {
display: inline-block;
width: 120px;
height: 120px;
padding: 8px;
margin: 8px;
background: ##F24333;
color: white;
border: 1px solid ##BA1B1D;
border-radius: 4px;
}
.content-box {
box-sizing: content-box;
}
button-border-animation
- title: Button border animation
- tags: animation,intermediate
Creates a border animation on hover.
- Use the
:before
and:after
pseudo-elements to create two boxes24px
wide opposite each other above and below the box. - Use the
:hover
pseudo-class to extend thewidth
of those elements to100%
on hover and animate the change usingtransition
.
<button class="animated-border-button">Submit</button>
.animated-border-button {
background-color: ##141414;
border: none;
color: ##ffffff;
outline: none;
padding: 12px 40px 10px;
position: relative;
}
.animated-border-button:before,
.animated-border-button:after {
border: 0 solid transparent;
transition: all 0.3s;
content: '';
height: 0;
position: absolute;
width: 24px;
}
.animated-border-button:before {
border-top: 2px solid ##141414;
right: 0;
top: -4px;
}
.animated-border-button:after {
border-bottom: 2px solid ##141414;
bottom: -4px;
left: 0;
}
.animated-border-button:hover:before,
.animated-border-button:hover:after {
width: 100%;
}
checkerboard-pattern
- title: Checkerboard background pattern
- tags: visual,intermediate
Creates a checkerboard background pattern.
- Use
background-color
to set a white background. - Use
background-image
with twolinear-gradient()
values, each one with a different angle to create the checkerboard pattern. - Use
background-size
to specify the pattern's size. - Note: The fixed
height
andwidth
of the element is for demonstration purposes only.
<div class="checkerboard"></div>
.checkerboard {
width: 240px;
height: 240px;
background-color: ##fff;
background-image: linear-gradient(
45deg,
##000 25%,
transparent 25%,
transparent 75%,
##000 75%,
##000
),
linear-gradient(
-45deg,
##000 25%,
transparent 25%,
transparent 75%,
##000 75%,
##000
);
background-size: 60px 60px;
background-repeat: repeat;
}
circle
- title: Circle
- tags: visual,beginner
Creates a circular shape with pure CSS.
- Use
border-radius: 50%
to curve the borders of the element to create a circle. - Since a circle has the same radius at any given point, the
width
andheight
must be the same. Differing values will create an ellipse.
<div class="circle"></div>
.circle {
border-radius: 50%;
width: 32px;
height: 32px;
background: ##9C27B0;
}
clearfix
- title: Clearfix
- tags: layout,beginner
Ensures that an element self-clears its children.
- Use the
:after
pseudo-element and applycontent: ''
to allow it to affect layout. - Use
clear: both
to make the element clear past both left and right floats. - For this technique to work properly, make sure there are no non-floating children in the container and that there are no tall floats before the clearfixed container but in the same formatting context (e.g. floated columns).
- Note: This is only useful if you are using
float
to build layouts. Consider using a more modern approach, such as the flexbox or grid layout.
<div class="clearfix">
<div class="floated">float a</div>
<div class="floated">float b</div>
<div class="floated">float c</div>
</div>
.clearfix:after {
content: '';
display: block;
clear: both;
}
.floated {
float: left;
padding: 4px;
}
constant-width-to-height-ratio
- title: Constant width to height ratio
- tags: layout,beginner
Ensures that an element with variable width
will retain a proportionate height
value.
- Apply
padding-top
on the:before
pseudo-element, making theheight
of the element equal to a percentage of itswidth
. - The proportion of
height
towidth
can be altered as necessary. For example apadding-top
of100%
will create a responsive square (1:1 ratio).
<div class="constant-width-to-height-ratio"></div>
.constant-width-to-height-ratio {
background: ##9C27B0;
width: 50%;
}
.constant-width-to-height-ratio:before {
content: '';
padding-top: 100%;
float: left;
}
.constant-width-to-height-ratio:after {
content: '';
display: block;
clear: both;
}
counter
- title: Counter
- tags: visual,advanced
Creates a custom list counter that accounts for nested list elements.
- Use
counter-reset
to initialize a variable counter (default0
), the name of which is the value of the attribute (i.e.counter
). - Use
counter-increment
on the variable counter for each countable element (i.e. each<li>
). - Use
counters()
to display the value of each variable counter as part of thecontent
of the:before
pseudo-element for each countable element (i.e. each<li>
). The second value passed to it ('.'
) acts as the delimiter for nested counters.
<ul>
<li>List item</li>
<li>List item</li>
<li>
List item
<ul>
<li>List item</li>
<li>List item</li>
<li>List item</li>
</ul>
</li>
</ul>
ul {
counter-reset: counter;
list-style: none;
}
li:before {
counter-increment: counter;
content: counters(counter, '.') ' ';
}
custom-scrollbar
- title: Custom scrollbar
- tags: visual,advanced
Customizes the scrollbar style for the an elements with scrollable overflow.
- Use
::-webkit-scrollbar
to style the scrollbar element. - Use
::-webkit-scrollbar-track
to style the scrollbar track (the background of the scrollbar). - Use
::-webkit-scrollbar-thumb
to style the scrollbar thumb (the draggable element). - Note: Scrollbar styling doesn't appear to be on any standards track. This technique only works on WebKit-based browsers.
<div class="custom-scrollbar">
<p>
Lorem ipsum dolor sit amet consectetur adipisicing elit.<br />
Iure id exercitationem nulla qui repellat laborum vitae, <br />
molestias tempora velit natus. Quas, assumenda nisi. <br />
Quisquam enim qui iure, consequatur velit sit?
</p>
</div>
.custom-scrollbar {
height: 70px;
overflow-y: scroll;
}
.custom-scrollbar::-webkit-scrollbar {
width: 8px;
}
.custom-scrollbar::-webkit-scrollbar-track {
background: ##1E3F20;
border-radius: 12px;
}
.custom-scrollbar::-webkit-scrollbar-thumb {
background: ##4A7856;
border-radius: 12px;
}
custom-text-selection
- title: Custom text selection
- tags: visual,beginner
Changes the styling of text selection.
- Use the
::selection
pseudo-selector to style text within it when selected.
<p class="custom-text-selection">Select some of this text.</p>
::selection {
background: aquamarine;
color: black;
}
.custom-text-selection::selection {
background: deeppink;
color: white;
}
disable-selection
- title: Disable selection
- tags: interactivity,beginner
Makes the content unselectable.
- Use
user-select: none
to make the content of the element not selectable. - Note: This is not a secure method to prevent users from copying content.
<p>You can select me.</p>
<p class="unselectable">You can't select me!</p>
.unselectable {
user-select: none;
}
display-table-centering
- title: Display table centering
- tags: layout,intermediate
Vertically and horizontally centers a child element within its parent element, using display: table
.
- Use
display: table
to make the.center
element behave like a<table>
element. - Set
height
andwidth
to100%
to make the element fill the available space within its parent element. - Use
display: table-cell
on the child element to make it behave like a<td>
elements. - Use
text-align: center
andvertical-align: middle
on the child element to center it horizontally and vertically. - The outer parent (
.container
) must have a fixedwidth
andheight
.
<div class="container">
<div class="center"><span>Centered content</span></div>
</div>
.container {
border: 1px solid ##9C27B0;
height: 250px;
width: 250px;
}
.center {
display: table;
height: 100%;
width: 100%;
}
.center > span {
display: table-cell;
text-align: center;
vertical-align: middle;
}
donut-spinner
- title: Donut spinner
- tags: animation,intermediate
Creates a donut spinner that can be used to indicate the loading of content.
- Use a semi-transparent
border
for the whole element, except one side that will serve as the loading indicator for the donut. - Define and use an appropriate animation, using
transform: rotate()
to rotate the element.
<div class="donut"></div>
@keyframes donut-spin {
0% {
transform: rotate(0deg);
}
100% {
transform: rotate(360deg);
}
}
.donut {
display: inline-block;
border: 4px solid rgba(0, 0, 0, 0.1);
border-left-color: ##7983ff;
border-radius: 50%;
width: 30px;
height: 30px;
animation: donut-spin 1.2s linear infinite;
}
drop-cap
- title: Drop cap
- tags: visual,beginner
Makes the first letter of the first paragraph bigger than the rest of the text.
- Use the
:first-child
selector to select only the first paragraph. - Use the
::first-letter
pseudo-element to style the first element of the paragraph.
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam commodo ligula quis tincidunt cursus. Integer consectetur tempor ex eget hendrerit. Cras facilisis sodales odio nec maximus. Pellentesque lacinia convallis libero, rhoncus tincidunt ante dictum at. Nullam facilisis lectus tellus, sit amet congue erat sodales commodo.</p>
<p>Donec magna erat, imperdiet non odio sed, sodales tempus magna. Integer vitae orci lectus. Nullam consectetur orci at pellentesque efficitur.</p>
p:first-child::first-letter {
color: ##5f79ff;
float: left;
margin: 0 8px 0 4px;
font-size: 3rem;
font-weight: bold;
line-height: 1;
}
dynamic-shadow
- title: Dynamic shadow
- tags: visual,intermediate
Creates a shadow similar to box-shadow
but based on the colors of the element itself.
- Use the
:after
pseudo-element withposition: absolute
andwidth
andheight
equal to100%
to fill the available space in the parent element. - Use
background: inherit
to inherit thebackground
of the parent element. - Use
top
to slightly offset the pseudo-element,filter: blur()
to create a shadow andopacity
to make it semi-transparent. - Use
z-index: 1
on the parent andz-index: -1
on the pseudo-element to position it behind its parent.
<div class="dynamic-shadow"></div>
.dynamic-shadow {
position: relative;
width: 10rem;
height: 10rem;
background: linear-gradient(75deg, ##6d78ff, ##00ffb8);
z-index: 1;
}
.dynamic-shadow:after {
content: '';
width: 100%;
height: 100%;
position: absolute;
background: inherit;
top: 0.5rem;
filter: blur(0.4rem);
opacity: 0.7;
z-index: -1;
}
etched-text
- title: Etched text
- tags: visual,intermediate
Creates an effect where text appears to be "etched" or engraved into the background.
- Use
text-shadow
to create a white shadow offset0px
horizontally and2px
vertically from the origin position. - The background must be darker than the shadow for the effect to work.
- The text color should be slightly faded to make it look like it's engraved/carved out of the background.
<p class="etched-text">I appear etched into the background.</p>
.etched-text {
text-shadow: 0 2px white;
font-size: 1.5rem;
font-weight: bold;
color: ##b8bec5;
}
evenly-distributed-children
- title: Evenly distributed children
- tags: layout,intermediate
Evenly distributes child elements within a parent element.
- Use
display: flex
to use the flexbox layout. - Use
justify-content: space-between
to evenly distributes child elements horizontally. The first item is positioned at the left edge, while the last item is positioned at the right edge. - Alternatively, you can use
justify-content: space-around
to distribute the children with space around them, rather than between them.
<div class="evenly-distributed-children">
<p>Item1</p>
<p>Item2</p>
<p>Item3</p>
</div>
.evenly-distributed-children {
display: flex;
justify-content: space-between;
}
fit-image-in-container
- title: Fit image in container
- tags: layout,visual,intermediate
Fits an positions an image appropriately inside its container while preserving its aspect ratio.
- Use
object-fit: contain
to fit the entire image within the container while preserving its aspect ratio. - Use
object-fit: cover
to fill the container with the image while preserving its aspect ratio. - Use
object-position: center
to position the image at the center of the container.
<img class="image image-contain" src="https://picsum.photos/600/200" />
<img class="image image-cover" src="https://picsum.photos/600/200" />
.image {
background: ##34495e;
border: 1px solid ##34495e;
width: 200px;
height: 200px;
}
.image-contain {
object-fit: contain;
object-position: center;
}
.image-cover {
object-fit: cover;
object-position: right top;
}
flexbox-centering
- title: Flexbox centering
- tags: layout,beginner
Horizontally and vertically centers a child element within a parent element using flexbox.
- Use
display: flex
to create a flexbox layout. - Use
justify-content: center
to center the child horizontally. - Use
align-items: center
to center the child vertically.
<div class="flexbox-centering">
<div>Centered content.</div>
</div>
.flexbox-centering {
display: flex;
justify-content: center;
align-items: center;
height: 100px;
}
floating-list-titles
- title: List with floating section headings
- tags: visual,advanced
Creates a list with floating headings for each section.
- Use
overflow-y: auto
to allow the list container to overflow vertically. - Use
display: grid
on the inner container (<dl>
) to create a layout with two columns. - Set headings (
<dt>
) togrid-column: 1
and content (<dd>
) togrid-column: 2
- Finally, apply
position: sticky
andtop: 0.5rem
to headings to create a floating effect.
<div class="container">
<div class="floating-stack">
<dl>
<dt>A</dt>
<dd>Algeria</dd>
<dd>Angola</dd>
<dt>B</dt>
<dd>Benin</dd>
<dd>Botswana</dd>
<dd>Burkina Faso</dd>
<dd>Burundi</dd>
<dt>C</dt>
<dd>Cabo Verde</dd>
<dd>Cameroon</dd>
<dd>Central African Republic</dd>
<dd>Chad</dd>
<dd>Comoros</dd>
<dd>Congo, Democratic Republic of the</dd>
<dd>Congo, Republic of the</dd>
<dd>Cote d'Ivoire</dd>
<dt>D</dt>
<dd>Djibouti</dd>
<dt>E</dt>
<dd>Egypt</dd>
<dd>Equatorial Guinea</dd>
<dd>Eritrea</dd>
<dd>Eswatini (formerly Swaziland)</dd>
<dd>Ethiopia</dd>
</dl>
</div>
</div>
.container {
display: grid;
place-items: center;
min-height: 400px;
}
.floating-stack {
background: ##455A64;
color: ##fff;
height: 80vh;
height: 320px;
border-radius: 1rem;
overflow-y: auto;
}
.floating-stack > dl {
margin: 0 0 1rem;
display: grid;
grid-template-columns: 2.5rem 1fr;
align-items: center;
}
.floating-stack dt {
position: sticky;
top: 0.5rem;
left: 0.5rem;
font-weight: bold;
background: ##263238;
color: ##cfd8dc;
height: 2rem;
width: 2rem;
border-radius: 50%;
padding: 0.25rem 1rem;
grid-column: 1;
display: inline-flex;
align-items: center;
justify-content: center;
box-sizing: border-box;
}
.floating-stack dd {
grid-column: 2;
margin: 0;
padding: 0.75rem;
}
.floating-stack > dl:first-of-type > dd:first-of-type {
margin-top: 0.25rem;
}
focus-within
- title: Focus Within
- tags: visual,interactivity,intermediate
Changes the appearance of a form if any of its children are focused.
- Use the pseudo-class
:focus-within
to apply styles to a parent element if any child element gets focused.
<form>
<label for="username">Username:</label>
<input id="username" type="text" />
<br />
<label for="password">Password:</label>
<input id="password" type="text" />
</form>
form {
border: 2px solid ##52B882;
padding: 8px;
border-radius: 2px;
}
form:focus-within {
background: ##7CF0BD;
}
label {
display: inline-block;
width: 72px;
}
input {
margin: 4px 12px;
}
full-width
- title: Full-width image
- tags: layout,intermediate
Creates a full-width image.
- Use
left: 50%
andright: 50%
to position the image in the middle of the parent element. - Use
margin-left: -50vw
andmargin-right: -50vw
to offset the image on both sides relative to the size of the viewport. - Use
width: 100vw
andmax-width: 100vw
to size the image relative to the viewport.
<main>
<h4>Lorem ipsum dolor sit amet</h4>
<p>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris molestie
lobortis sapien, sit amet iaculis est interdum tincidunt. Nunc egestas nibh
ut metus elementum consequat. Integer elit orci, rhoncus efficitur lectus
eu, faucibus interdum felis.
</p>
<p>
<img class="full-width" src="https://picsum.photos/id/257/2560/1440.jpg" />
</p>
<p>
Orci varius natoque penatibus et magnis dis parturient montes, nascetur
ridiculus mus. Nullam pretium lectus sed ex efficitur, ac varius sapien
gravida. Sed facilisis elit quis sem sollicitudin, ut aliquam neque
eleifend. Maecenas sagittis neque sapien, ac tempus nulla tempus nec.
Curabitur tellus est, convallis id dolor ut, porta hendrerit quam.
</p>
</main>
main {
margin: 0 auto;
max-width: 640px;
}
img {
height: auto;
max-width: 100%;
}
.full-width {
position: relative;
left: 50%;
right: 50%;
margin-left: -50vw;
margin-right: -50vw;
max-width: 100vw;
width: 100vw;
}
fullscreen
- title: Fullscreen
- tags: visual,advanced
Applies styles to an element when in fullscreen mode.
The :fullscreen
CSS pseudo-element represents an element that's displayed when the browser is in fullscreen mode.
- Use the
:fullscreen
CSS pseudo-element selector to select and style an element that is displayed in fullscreen mode. - Use a
<button>
andElement.requestFullscreen()
to create a button that makes the element fullscreen for the purposes of previewing the style.
<div class="container">
<p><em>Click the button below to enter the element into fullscreen mode. </em></p>
<div class="element" id="element"><p>I change color in fullscreen mode!</p></div>
<br />
<button onclick="var el = document.getElementById('element'); el.requestFullscreen();">
Go Full Screen!
</button>
</div>
.container {
margin: 40px auto;
max-width: 700px;
}
.element {
padding: 20px;
height: 300px;
width: 100%;
background-color: skyblue;
box-sizing: border-box;
}
.element p {
text-align: center;
color: white;
font-size: 3em;
}
.element:-ms-fullscreen p {
visibility: visible;
}
.element:fullscreen {
background-color: ##e4708a;
width: 100vw;
height: 100vh;
}
gradient-text
- title: Gradient text
- tags: visual,intermediate
Gives text a gradient color.
- Use
background
with alinear-gradient
value to give the text element a gradient background. - Use
webkit-text-fill-color: transparent
to fill the text with a transparent color. - Use
webkit-background-clip: text
to clip the background with the text, filling the text with the gradient background as the color.
<p class="gradient-text">Gradient text</p>
.gradient-text {
background: linear-gradient(##70D6FF, ##00072D);
-webkit-text-fill-color: transparent;
-webkit-background-clip: text;
font-size: 32px;
}
grid-centering
- title: Grid centering
- tags: layout,beginner
Horizontally and vertically centers a child element within a parent element using grid
.
- Use
display: grid
to create a grid layout. - Use
justify-content: center
to center the child horizontally. - Use
align-items: center
to center the child vertically.
<div class="grid-centering">
<div class="child">Centered content.</div>
</div>
.grid-centering {
display: grid;
justify-content: center;
align-items: center;
height: 100px;
}
hamburger-button
- title: Hamburger Button
- tags: interactivity,intermediate
Displays a hamburger menu which transitions to a cross button on hover.
- Use a
.hamburger-menu
containerdiv
which contains the top, bottom, and middle bars. - Set the container to
display: flex
withflex-flow: column wrap
. - Add distance between the bars using
justify-content: space-between
. - Use
transform: rotate()
to rotate the top and bottom bars by 45 degrees andopacity: 0
to fade the middle bar on hover. - Use
transform-origin: left
so that the bars rotate around the left point.
<div class="hamburger-menu">
<div class="bar top"></div>
<div class="bar middle"></div>
<div class="bar bottom"></div>
</div>
.hamburger-menu {
display: flex;
flex-flow: column wrap;
justify-content: space-between;
height: 2.5rem;
width: 2.5rem;
cursor: pointer;
}
.hamburger-menu .bar {
height: 5px;
background: black;
border-radius: 5px;
margin: 3px 0px;
transform-origin: left;
transition: all 0.5s;
}
.hamburger-menu:hover .top {
transform: rotate(45deg);
}
.hamburger-menu:hover .middle {
opacity: 0;
}
.hamburger-menu:hover .bottom {
transform: rotate(-45deg);
}
height-transition
- title: Height transition
- tags: animation,intermediate
Transitions an element's height from 0
to auto
when its height is unknown.
- Use
transition
to specify that changes tomax-height
should be transitioned over. - Use
overflow: hidden
to prevent the contents of the hidden element from overflowing its container. - Use
max-height
to specify an initial height of0
. - Use the
:hover
pseudo-class to change themax-height
to the value of the--max-height
variable set by JavaScript. - Use
Element.scrollHeight
andCSSStyleDeclaration.setProperty()
to set the value of--max-height
to the current height of the element. - Note: Causes reflow on each animation frame, which will be laggy if there are a large number of elements beneath the element that is transitioning in height.
<div class="trigger">
Hover me to see a height transition.
<div class="el">Additional content</div>
</div>
.el {
transition: max-height 0.3s;
overflow: hidden;
max-height: 0;
}
.trigger:hover > .el {
max-height: var(--max-height);
}
let el = document.querySelector('.el');
let height = el.scrollHeight;
el.style.setProperty('--max-height', height + 'px');
horizontal-scroll-snap
- title: Horizontal scroll snap
- tags: interactivity,intermediate
Creates a horizontally scrollable container that will snap on elements when scrolling.
- Use
display: grid
andgrid-auto-flow: column
to create a horizontal layout. - Use
scroll-snap-type: x mandatory
andoverscroll-behavior-x: contain
to create a snap effect on horizontal scroll. - You can use
scroll-snap-align
with eitherstart
,stop
orcenter
to change the snap alignment.
<div class="horizontal-snap">
<a href="##"><img src="https://picsum.photos/id/1067/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/122/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/188/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/249/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/257/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/259/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/283/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/288/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/299/640/640"></a>
</div>
.horizontal-snap {
margin: 0 auto;
display: grid;
grid-auto-flow: column;
gap: 1rem;
height: calc(180px + 1rem);
padding: 1rem;
width: 480px;
overflow-y: auto;
overscroll-behavior-x: contain;
scroll-snap-type: x mandatory;
}
.horizontal-snap > a {
scroll-snap-align: center;
}
.horizontal-snap img {
width: 180px;
max-width: none;
object-fit: contain;
border-radius: 1rem;
}
hover-additional-content
- title: Show additional content on hover
- tags: visual,intermediate
Creates a card that displays additional content on hover.
- Use
overflow: hidden
on the card to hide elements that overflow vertically. - Use the
:hover
and:focus-within
pseudo-class selectors to change the card's styling as necessary when it's hovered or it or its contents are focused. - Set
transition: 0.3s ease all
to create a transition effect on hover/focus.
<div class="card">
<img src="https://picsum.photos/id/404/367/267"/>
<h3>Lorem ipsum</h3>
<div class="focus-content">
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.<br/> <a href="##">Link to source</a>
</p>
</div>
</div>
.card {
width: 300px;
height: 280px;
padding: 0;
box-shadow: 0 2px 4px 0 rgba(0,0,0,0.1);
border-radius: 8px;
box-sizing: border-box;
overflow: hidden;
}
.card * {
transition: 0.3s ease all;
}
.card img {
margin: 0;
width: 300px;
height: 224px;
object-fit: cover;
display: block;
}
.card h3 {
margin: 0;
padding: 12px 12px 48px;
line-height: 32px;
font-weight: 500;
font-size: 20px;
}
.card .focus-content {
display: block;
padding: 8px 12px;
}
.card p {
margin: 0;
line-height: 1.5;
}
.card:hover img, .card:focus-within img {
margin-top: -80px;
}
.card:hover h3, .card:focus-within h3 {
padding: 8px 12px 0;
}
hover-shadow-box-animation
- title: Hover shadow box animation
- tags: animation,intermediate unlisted: true
Creates a shadow box around the text when it is hovered.
- Set
transform: perspective(1px)
to give element a 3D space by affecting the distance between the Z plane and the user andtranslate(0)
to reposition thep
element along z-axis in 3D space. - Use
box-shadow
to make the box transparent. - Use
transition-property
to enable transitions for bothbox-shadow
andtransform
. - Use the
:hover
,:active
and:focus
pseudo-class selectors to apply a newbox-shadow
andtransform: scale(1.2)
to change the scale of the text.
<p class="hover-shadow-box-animation">Box it!</p>
.hover-shadow-box-animation {
display: inline-block;
vertical-align: middle;
transform: perspective(1px) translateZ(0);
box-shadow: 0 0 1px transparent;
margin: 10px;
transition-duration: 0.3s;
transition-property: box-shadow, transform;
}
.hover-shadow-box-animation:hover,
.hover-shadow-box-animation:focus,
.hover-shadow-box-animation:active {
box-shadow: 1px 10px 10px -10px rgba(0, 0, 24, 0.5);
transform: scale(1.2);
}
hover-underline-animation
- title: Hover underline animation
- tags: animation,advanced
Creates an animated underline effect when the text is hovered over.
- Use
display: inline-block
to prevent the underline from spanning the entire parent width rather than just the text content. - Use the
:after
pseudo-element with awidth
of100%
andposition: absolute
, placing it below the content. - Use
transform: scaleX(0)
to initially hide the pseudo-element. - Use the
:hover
pseudo-class selector to applytransform: scaleX(1)
and display the pseudo-element on hover. - Animate
transform
usingtransform-origin: left
and an appropriatetransition
.
<p class="hover-underline-animation">Hover this text to see the effect!</p>
.hover-underline-animation {
display: inline-block;
position: relative;
color: ##0087ca;
}
.hover-underline-animation:after {
content: '';
position: absolute;
width: 100%;
transform: scaleX(0);
height: 2px;
bottom: 0;
left: 0;
background-color: ##0087ca;
transform-origin: bottom right;
transition: transform 0.25s ease-out;
}
.hover-underline-animation:hover:after {
transform: scaleX(1);
transform-origin: bottom left;
}
image-hover-menu
- title: Menu on image hover
- tags: layout,animation,intermediate
Displays a menu overlay when the image is hovered.
- Use a
<figure>
to wrap the<img>
element and a<div>
element that will contain the menu links. - Use the
opacity
andright
attributes to animate the image on hover, creating a sliding effect. - Set the
left
attribute of the<div>
to the negative of the element'swidth
and reset it to0
when hovering over the parent element to slide in the menu. - Use
display: flex
,flex-direction: column
andjustify-content: center
on the<div>
to vertically center the menu items.
<figure class="hover-menu">
<img src="https://picsum.photos/id/1060/800/480.jpg"/>
<div>
<a href="##">Home</a>
<a href="##">Pricing</a>
<a href="##">About</a>
</div>
</figure>
.hover-menu {
position: relative;
overflow: hidden;
margin: 8px;
min-width: 340px;
max-width: 480px;
max-height: 290px;
width: 100%;
background: ##000;
text-align: center;
box-sizing: border-box;
}
.hover-menu * {
box-sizing: border-box;
}
.hover-menu img {
position: relative;
max-width: 100%;
top: 0;
right: 0;
opacity: 1;
transition: 0.3s ease-in-out;
}
.hover-menu div {
position: absolute;
top: 0;
left: -120px;
width: 120px;
height: 100%;
padding: 8px 4px;
background: ##000;
transition: 0.3s ease-in-out;
display: flex;
flex-direction: column;
justify-content: center;
}
.hover-menu div a {
display: block;
line-height: 2;
color: ##fff;
text-decoration: none;
opacity: 0.8;
padding: 5px 15px;
position: relative;
transition: 0.3s ease-in-out;
}
.hover-menu div a:hover {
text-decoration: underline;
}
.hover-menu:hover img {
opacity: 0.5;
right: -120px;
}
.hover-menu:hover div {
left: 0;
opacity: 1;
}
image-hover-rotate
- title: Image rotate on hover
- tags: animation,visual,intermediate
Creates a rotate effect for the image on hover.
- Use
scale
androtate
when hovering over the parent element (a<figure>
) to animate the image, using thetransition
property. - Use
overflow: hidden
on the parent container to hide the excess from the image transformation.
<figure class="hover-rotate">
<img src="https://picsum.photos/id/669/600/800.jpg"/>
</figure>
.hover-rotate {
overflow: hidden;
margin: 8px;
min-width: 240px;
max-width: 320px;
width: 100%;
}
.hover-rotate img {
transition: all 0.3s;
box-sizing: border-box;
max-width: 100%;
}
.hover-rotate:hover img {
transform: scale(1.3) rotate(5deg);
}
image-mosaic
- title: Responsive image mosaic
- tags: layout,intermediate
Creates a responsive image mosaic.
- Use
display: grid
to create an appropriate responsive grid layout. - Use
grid-row: span 2 / auto
andgrid-column: span 2 / auto
to create items that span two rows or two columns respectively. - Wrap the previous styles into a media query to avoid applying on small screen sizes.
<div class="image-mosaic">
<div
class="card card-tall card-wide"
style="background-image: url('https://picsum.photos/id/564/1200/800')"
></div>
<div
class="card card-tall"
style="background-image: url('https://picsum.photos/id/566/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/575/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/626/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/667/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/678/800/530')"
></div>
<div
class="card card-wide"
style="background-image: url('https://picsum.photos/id/695/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/683/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/693/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/715/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/610/800/530')"
></div>
<div
class="card"
style="background-image: url('https://picsum.photos/id/599/800/530')"
></div>
</div>
.image-mosaic {
display: grid;
gap: 1rem;
grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
grid-auto-rows: 240px;
}
.card {
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
background: ##353535;
font-size: 3rem;
color: ##fff;
box-shadow: rgba(3, 8, 20, 0.1) 0px 0.15rem 0.5rem, rgba(2, 8, 20, 0.1) 0px 0.075rem 0.175rem;
height: 100%;
width: 100%;
border-radius: 4px;
transition: all 500ms;
overflow: hidden;
background-size: cover;
background-position: center;
background-repeat: no-repeat;
padding: 0;
margin: 0;
}
@media screen and (min-width: 600px) {
.card-tall {
grid-row: span 2 / auto;
}
.card-wide {
grid-column: span 2 / auto;
}
}
image-overlay-hover
- title: Image overlay on hover
- tags: visual,animation,advanced
Displays an image overlay effect on hover.
- Use the
:before
and:after
pseudo-elements for the top and bottom bars of the overlay respectively, setting theiropacity
,transform
andtransition
to produce the desired effect. - Use the
<figcaption>
for the text of the overlay, settingdisplay: flex
,flex-direction: column
andjustify-content: center
to center the text into the image. - Use the
:hover
pseudo-selector to update theopacity
andtransform
of all the elements and produce the desired effect.
<figure class="hover-img">
<img src="https://picsum.photos/id/200/440/320.jpg"/>
<figcaption>
<h3>Lorem <br/>Ipsum</h3>
</figcaption>
</figure>
.hover-img {
background-color: ##000;
color: ##fff;
display: inline-block;
margin: 8px;
max-width: 320px;
min-width: 240px;
overflow: hidden;
position: relative;
text-align: center;
width: 100%;
}
.hover-img * {
box-sizing: border-box;
transition: all 0.45s ease;
}
.hover-img:before,
.hover-img:after {
background-color: rgba(0, 0, 0, 0.5);
border-top: 32px solid rgba(0, 0, 0, 0.5);
border-bottom: 32px solid rgba(0, 0, 0, 0.5);
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
content: '';
transition: all 0.3s ease;
z-index: 1;
opacity: 0;
transform: scaleY(2);
}
.hover-img img {
vertical-align: top;
max-width: 100%;
backface-visibility: hidden;
}
.hover-img figcaption {
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
align-items: center;
z-index: 1;
display: flex;
flex-direction: column;
justify-content: center;
line-height: 1.1em;
opacity: 0;
z-index: 2;
transition-delay: 0.1s;
font-size: 24px;
font-family: sans-serif;
font-weight: 400;
letter-spacing: 1px;
text-transform: uppercase;
}
.hover-img:hover:before,
.hover-img:hover:after {
transform: scale(1);
opacity: 1;
}
.hover-img:hover > img {
opacity: 0.7;
}
.hover-img:hover figcaption {
opacity: 1;
}
input-with-prefix
- title: Input with prefix
- tags: interactivity,visual,intermediate
Creates an input with a visual, non-editable prefix.
- Use
display: flex
to create a container element. - Remove the border and outline from the
<input>
field and apply them to the parent element instead to make it look like an input box. - Use the
:focus-within
pseudo-class selector to style the parent element accordingly, when the user interacts with the<input>
field.
<div class="input-box">
<span class="prefix">+30</span>
<input type="tel" placeholder="210 123 4567"/>
</div>
.input-box {
display: flex;
align-items: center;
max-width: 300px;
background: ##fff;
border: 1px solid ##a0a0a0;
border-radius: 4px;
padding-left: 0.5rem;
overflow: hidden;
font-family: sans-serif;
}
.input-box .prefix {
font-weight: 300;
font-size: 14px;
color: ##999;
}
.input-box input {
flex-grow: 1;
font-size: 14px;
background: ##fff;
border: none;
outline: none;
padding: 0.5rem;
}
.input-box:focus-within {
border-color: ##777;
}
masonry-layout
- title: Masonry Layout
- tags: layout,advanced
Creates a masonry-style layout that is especially useful when working with images.
- Create a masonry-style layout that consists of "bricks" that fall into each other with either a fixed
width
(vertical layout) or a fixedheight
(horizontal layout), forming a perfect fit. Especially useful when working with images. - Define
.masonry-container
, the container for the masonry layout and.masonry-columns
an inner container in which.masonry-brick
elements will be placed. - Apply
display: block
to.masonry-brick
elements to allow the layout to flow properly. - Use the
:first-child
pseudo-element selector to apply a differentmargin
for the first element to account for its positioning. - Use CSS variables to allow for greater flexibility within the layout in combination with media queries to ensure that the layout is responsive in different viewport sizes.
<div class="masonry-container">
<div class="masonry-columns">
<img
class="masonry-brick"
src="https://picsum.photos/id/1016/384/256"
alt="An image"
/>
<img
class="masonry-brick"
src="https://picsum.photos/id/1025/495/330"
alt="Another image"
/>
<img
class="masonry-brick"
src="https://picsum.photos/id/1024/192/128"
alt="Another image"
/>
<img
class="masonry-brick"
src="https://picsum.photos/id/1028/518/345"
alt="One more image"
/>
<img
class="masonry-brick"
src="https://picsum.photos/id/1035/585/390"
alt="And another one"
/>
<img
class="masonry-brick"
src="https://picsum.photos/id/1074/384/216"
alt="Last one"
/>
</div>
</div>
/* Container */
.masonry-container {
--column-count-small: 1;
--column-count-medium: 2;
--column-count-large: 3;
--column-gap: 0.125rem;
padding: var(--column-gap);
}
/* Columns */
.masonry-columns {
column-gap: var(--column-gap);
column-count: var(--column-count-small);
column-width: calc(1 / var(--column-count-small) * 100%);
}
@media only screen and (min-width: 640px) {
.masonry-columns {
column-count: var(--column-count-medium);
column-width: calc(1 / var(--column-count-medium) * 100%);
}
}
@media only screen and (min-width: 800px) {
.masonry-columns {
column-count: var(--column-count-large);
column-width: calc(1 / var(--column-count-large) * 100%);
}
}
/* Bricks */
.masonry-brick {
width: 100%;
height: auto;
margin: var(--column-gap) 0;
display: block;
}
.masonry-brick:first-child {
margin: 0 0 var(--column-gap);
}
mouse-cursor-gradient-tracking
- title: Mouse cursor gradient tracking
- tags: visual,interactivity,advanced
A hover effect where the gradient follows the mouse cursor.
- Declare two CSS variables,
--x
and--y
, used to track the position of the mouse on the button. - Declare a CSS variable,
--size
, used to modify the gradient's dimensions. - Use
background: radial-gradient(circle closest-side, pink, transparent);
to create the gradient at the correct position. - Use
Document.querySelector()
andEventTarget.addEventListener()
to register a handler for the'mousemove'
event. - Use
Element.getBoundingClientRect()
andCSSStyleDeclaration.setProperty()
to update the values of the--x
and--y
CSS variables.
<button class="mouse-cursor-gradient-tracking">
<span>Hover me</span>
</button>
.mouse-cursor-gradient-tracking {
position: relative;
background: ##7983ff;
padding: 0.5rem 1rem;
font-size: 1.2rem;
border: none;
color: white;
cursor: pointer;
outline: none;
overflow: hidden;
}
.mouse-cursor-gradient-tracking span {
position: relative;
}
.mouse-cursor-gradient-tracking:before {
--size: 0;
content: '';
position: absolute;
left: var(--x);
top: var(--y);
width: var(--size);
height: var(--size);
background: radial-gradient(circle closest-side, pink, transparent);
transform: translate(-50%, -50%);
transition: width 0.2s ease, height 0.2s ease;
}
.mouse-cursor-gradient-tracking:hover:before {
--size: 200px;
}
let btn = document.querySelector('.mouse-cursor-gradient-tracking');
btn.addEventListener('mousemove', e => {
let rect = e.target.getBoundingClientRect();
let x = e.clientX - rect.left;
let y = e.clientY - rect.top;
btn.style.setProperty('--x', x + 'px');
btn.style.setProperty('--y', y + 'px');
});
navigation-list-item-hover-and-focus-effect
- title: Navigation list item hover and focus effect
- tags: visual,beginner
Creates a custom hover and focus effect for navigation items, using CSS transformations.
- Use the
:before
pseudo-element at the list item anchor to create a hover effect, hide it usingtransform: scale(0)
. - Use the
:hover
and:focus
pseudo-class selectors to transition totransform: scale(1)
and show the pseudo-element with its colored background. - Prevent the pseudo-element from covering the anchor element by using
z-index
.
<nav class="hover-nav">
<ul>
<li><a href="##">Home</a></li>
<li><a href="##">About</a></li>
<li><a href="##">Contact</a></li>
</ul>
</nav>
.hover-nav ul {
list-style: none;
margin: 0;
padding: 0;
overflow: hidden;
}
.hover-nav li {
float: left;
}
.hover-nav li a {
position: relative;
display: block;
color: ##222;
text-align: center;
padding: 8px 12px;
text-decoration: none;
z-index: 0;
}
li a:before {
position: absolute;
content: "";
width: 100%;
height: 100%;
bottom: 0;
left: 0;
background-color: ##f6c126;
z-index: -1;
transform: scale(0);
transition: transform 0.5s ease-in-out;
}
li a:hover:before,
li a:focus:before {
transform: scale(1);
}
offscreen
- title: Offscreen
- tags: layout,visual,intermediate
Completely hides an element visually and positionally in the DOM while still allowing it to be accessible.
- Remove all borders and padding and hide the element's overflow.
- Use
clip
to indicate that no part of the element should be shown. - Make the
height
andwidth
of the element1px
and negate them usingmargin: -1px
. - Use
position: absolute
so that the element does not take up space in the DOM. - Note: This provides an accessible and layout-friendly alternative to
display: none
(not readable by screen readers) andvisibility: hidden
(takes up physical space in the DOM).
<a class="button" href="https://google.com">
Learn More <span class="offscreen"> about pants</span>
</a>
.offscreen {
border: 0;
clip: rect(0 0 0 0);
height: 1px;
margin: -1px;
overflow: hidden;
padding: 0;
position: absolute;
width: 1px;
}
overflow-scroll-gradient
- title: Overflow scroll gradient
- tags: visual,intermediate
Adds a fading gradient to an overflowing element to better indicate there is more content to be scrolled.
- Use the
:after
pseudo-element to create alinear-gradient
that fades fromtransparent
towhite
(top to bottom). - Use
position: absolute
,width
andheight
to appropriately place and size the pseudo-element in its parent. - Use
pointer-events: none
to exclude the pseudo-element from mouse events, allowing text behind it to still be selectable/interactive.
<div class="overflow-scroll-gradient">
<div class="overflow-scroll-gradient-scroller">
Lorem ipsum dolor sit amet consectetur adipisicing elit. <br />
Iure id exercitationem nulla qui repellat laborum vitae, <br />
molestias tempora velit natus. Quas, assumenda nisi. <br />
Quisquam enim qui iure, consequatur velit sit? <br />
Lorem ipsum dolor sit amet consectetur adipisicing elit.<br />
Iure id exercitationem nulla qui repellat laborum vitae, <br />
molestias tempora velit natus. Quas, assumenda nisi. <br />
Quisquam enim qui iure, consequatur velit sit?
</div>
</div>
.overflow-scroll-gradient {
position: relative;
}
.overflow-scroll-gradient:after {
content: '';
position: absolute;
bottom: 0;
width: 250px;
height: 25px;
background: linear-gradient(transparent, white);
pointer-events: none;
}
.overflow-scroll-gradient-scroller {
overflow-y: scroll;
background: white;
width: 240px;
height: 200px;
padding: 15px;
line-height: 1.2;
}
polka-dot-pattern
- title: Polka dot background pattern
- tags: visual,intermediate
Creates a polka dot background pattern.
- Use
background-color
to set a black background. - Use
background-image
with tworadial-gradient()
values to create two dots. - Use
background-size
to specify the pattern's size andbackground-position
to appropriately place the two gradients. - Note: The fixed
height
andwidth
of the element is for demonstration purposes only.
<div class="polka-dot"></div>
.polka-dot {
width: 240px;
height: 240px;
background-color: ##000;
background-image: radial-gradient(##fff 10%, transparent 11%),
radial-gradient(##fff 10%, transparent 11%);
background-size: 60px 60px;
background-position: 0 0, 30px 30px;
background-repeat: repeat;
}
popout-menu
- title: Popout menu
- tags: interactivity,intermediate
Reveals an interactive popout menu on hover/focus.
- Use
left: 100%
to move the popout menu to the right of the parent. - Use
visibility: hidden
to hide the popout menu initially, allowing for transitions to be applied (unlikedisplay: none
). - Use the
:hover
,:focus
and:focus-within
pseudo-class selectors to applyvisibility: visible
to the popout menu, displaying it when the parent element is hovered/focused.
<div class="reference" tabindex="0">
<div class="popout-menu">Popout menu</div>
</div>
.reference {
position: relative;
background: tomato;
width: 100px;
height: 80px;
}
.popout-menu {
position: absolute;
visibility: hidden;
left: 100%;
background: ##9C27B0;
color: white;
padding: 16px;
}
.reference:hover > .popout-menu,
.reference:focus > .popout-menu,
.reference:focus-within > .popout-menu {
visibility: visible;
}
pretty-text-underline
- title: Pretty text underline
- tags: visual,intermediate
Provides a nicer alternative to text-decoration: underline
where descenders do not clip the underline.
- Use
text-shadow
to apply 4 values with offsets covering a 4x4 px area, ensuring the underline has a thick shadow that covers the line where descenders clip it. For the best results, use a color that matches thebackground
and adjust thepx
values for larger fonts. - Use
background-image
withlinear-gradient()
andcurrentColor
to create an appropriate gradient that will act as the actual underline. - Set
background-position
,background-repeat
andbackground-size
to place the gradient in the correct position. - Use the
::selection
pseudo-class selector to ensure the text shadow does not interfere with text selection. - Note: This is natively implemented as
text-decoration-skip-ink: auto
but it has less control over the underline.
<div class="container">
<p class="pretty-text-underline">Pretty text underline without clipping descenders.</p>
</div>
.container {
background: ##f5f6f9;
color: ##333;
padding: 8px 0;
}
.pretty-text-underline {
display: inline;
text-shadow: 1px 1px ##f5f6f9, -1px 1px ##f5f6f9, -1px -1px ##f5f6f9,
1px -1px ##f5f6f9;
background-image: linear-gradient(90deg, currentColor 100%, transparent 100%);
background-position: bottom;
background-repeat: no-repeat;
background-size: 100% 1px;
}
.pretty-text-underline::selection {
background-color: rgba(0, 150, 255, 0.3);
text-shadow: none;
}
pulse-loader
- title: Pulse loader
- tags: animation,beginner
Creates a pulse effect loader animation using the animation-delay
property.
- Use
@keyframes
to define an animation at two points in the cycle: at the start (0%
), the two<div>
elements have nowidth
orheight
and are positioned at the center and at the end (100%
), both<div>
elements have increasedwidth
andheight
, but theirposition
is reset to0
. - Use
opacity
to transition from1
to0
when animating to give the<div>
elements a disappearing effect as they expand. - Set a predefined
width
andheight
for the parent container,.ripple-loader
and useposition: relative
to position its children. - Use
animation-delay
on the second<div>
element, so that each element starts its animation at a different time.
<div class="ripple-loader">
<div></div>
<div></div>
</div>
.ripple-loader {
position: relative;
width: 64px;
height: 64px;
}
.ripple-loader div {
position: absolute;
border: 4px solid ##454ADE;
border-radius: 50%;
animation: ripple-loader 1s ease-out infinite;
}
.ripple-loader div:nth-child(2) {
animation-delay: -0.5s;
}
@keyframes ripple-loader {
0% {
top: 32px;
left: 32px;
width: 0;
height: 0;
opacity: 1;
}
100% {
top: 0;
left: 0;
width: 64px;
height: 64px;
opacity: 0;
}
}
reset-all-styles
- title: Reset all styles
- tags: visual,beginner
Resets all styles to default values using only one property.
- Use the
all
property to reset all styles (inherited or not) to their default values. - Note: This will not affect
direction
andunicode-bidi
properties.
<div class="reset-all-styles">
<h5>Title</h5>
<p>
Lorem ipsum dolor sit amet consectetur adipisicing elit. Iure id
exercitationem nulla qui repellat laborum vitae, molestias tempora velit
natus. Quas, assumenda nisi. Quisquam enim qui iure, consequatur velit sit?
</p>
</div>
.reset-all-styles {
all: initial;
}
responsive-layout-sidebar
- title: Responsive layout with sidebar
- tags: layout,intermediate
Creates a responsive layout with a content area and a sidebar.
- Use
display: grid
on the parent container, to create a grid layout. - Use
minmax()
for the second column (sidebar) to allow it to take up between150px
and20%
. - Use
1fr
for the first column (main content) to take up the rest of the remaining space.
<div class="container">
<main>
This element is 1fr large.
</main>
<aside>
Min: 150px / Max: 20%
</aside>
</div>
.container {
display: grid;
grid-template-columns: 1fr minmax(150px, 20%);
height: 100px;
}
main, aside {
padding: 12px;
text-align: center;
}
main {
background: ##d4f2c4;
}
aside {
background: ##81cfd9;
}
rotating-card
- title: Rotating Card
- tags: animation,advanced
Creates a two sided card which rotates on hover.
- Set the the
backface-visibility
of the cards to none. - Initially, set
rotateY
for the back side of the card to-180deg
and the front side to0deg
. - Upon hover, set
rotateY
for the front side to180deg
and backside to0deg
. - Set the appropriate
perspective
value to create the rotate effect.
<div class="card">
<div class="card-side front">
<div>Front Side</div>
</div>
<div class="card-side back">
<div>Back Side</div>
</div>
</div>
.card {
perspective: 150rem;
position: relative;
height: 40rem;
max-width: 400px;
margin: 2rem;
box-shadow: none;
background: none;
}
.card-side {
height: 35rem;
border-radius: 15px;
transition: all 0.8s ease;
backface-visibility: hidden;
position: absolute;
top: 0;
left: 0;
width: 80%;
padding:2rem;
color: white
}
.card-side.back {
transform: rotateY(-180deg);
background-color: ##4158D0;
background-image: linear-gradient(43deg, ##4158D0 0%,##C850C0 46%, ##FFCC70 100%);
}
.card-side.front {
background-color: ##0093E9;
background-image: linear-gradient(160deg, ##0093E9 0%, ##80D0C7 100%);
}
.card:hover .card-side.front {
transform: rotateY(180deg);
}
.card:hover .card-side.back {
transform: rotateY(0deg);
}
shape-separator
- title: Shape separator
- tags: visual,intermediate unlisted: true
Uses an SVG shape to create a separator between two different blocks.
- Use the
:after
pseudo-element to create the separator element. - Use
background-image
to add the SVG (a 24x12 triangle) shape via a data URI. The background image will repeat by default, covering the whole area of the pseudo-element. - Use the
background
of the parent element to set the desired color for the separator.
<div class="shape-separator"></div>
.shape-separator {
position: relative;
height: 48px;
background: ##9C27B0;
}
.shape-separator:after {
content: '';
background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 12'%3E%3Cpath d='m12 0l12 12h-24z' fill='transparent'/%3E%3C/svg%3E");
position: absolute;
width: 100%;
height: 12px;
bottom: 0;
}
sibling-fade
- title: Sibling fade
- tags: interactivity,intermediate
Fades out the siblings of a hovered item.
- Use a
transition
to animate changes toopacity
. - Use the
:hover
and:not
pseudo-class selectors to change theopacity
of all elements except for the one the mouse is over to0.5
.
<div class="sibling-fade">
<span>Item 1</span> <span>Item 2</span> <span>Item 3</span>
<span>Item 4</span> <span>Item 5</span> <span>Item 6</span>
</div>
span {
padding: 0 16px;
transition: opacity 0.3s;
}
.sibling-fade:hover span:not(:hover) {
opacity: 0.5;
}
staggered-animation
- title: Staggered animation
- tags: animation,advanced
Creates a staggered animation for the elements of a list.
- Set the
opacity
to0
andtransform
totranslateX(100%)
to make list elements transparent and move them all the way to the right. - Specify the appropriate
transition
properties for list elements, excepttransition-delay
which is specified relative to the--i
custom property. - Use inline styles to specify a value for
--i
for each list element, which will in turn be used fortransition-delay
to create the stagger effect. - Use the
:checked
pseudo-class selector for the checkbox to appropriately style list elements, settingopacity
to1
andtransform
totranslateX(0)
to make them appear and slide into view.
<div class="container">
<input type="checkbox" name="menu" id="menu" class="menu-toggler">
<label for="menu" class="menu-toggler-label">Menu</label>
<ul class="stagger-menu">
<li style="--i: 0">Home</li>
<li style="--i: 1">Pricing</li>
<li style="--i: 2">Account</li>
<li style="--i: 3">Support</li>
<li style="--i: 4">About</li>
</ul>
</div>
.container {
overflow-x: hidden;
width: 100%;
}
.menu-toggler {
display: none;
}
.menu-toggler-label {
cursor: pointer;
font-size: 20px;
font-weight: bold;
}
.stagger-menu {
list-style-type: none;
margin: 16px 0;
padding: 0;
}
.stagger-menu li {
margin-bottom: 8px;
font-size: 18px;
opacity: 0;
transform: translateX(100%);
transition-property: opacity, transform;
transition-duration: 0.3s;
transition-timing-function: cubic-bezier(0.750, -0.015, 0.565, 1.055);
}
.menu-toggler:checked ~ .stagger-menu li {
opacity: 1;
transform: translateX(0);
transition-delay: calc(0.055s * var(--i));
}
sticky-list-titles
- title: List with sticky section headings
- tags: visual,intermediate
Creates a list with sticky headings for each section.
- Use
overflow-y: auto
to allow the list container (<dl>
) to overflow vertically. - Set headings (
<dt>
)position
tosticky
and applytop: 0
to stick to the top of the container.
<div class="container">
<dl class="sticky-stack">
<dt>A</dt>
<dd>Algeria</dd>
<dd>Angola</dd>
<dt>B</dt>
<dd>Benin</dd>
<dd>Botswana</dd>
<dd>Burkina Faso</dd>
<dd>Burundi</dd>
<dt>C</dt>
<dd>Cabo Verde</dd>
<dd>Cameroon</dd>
<dd>Central African Republic</dd>
<dd>Chad</dd>
<dd>Comoros</dd>
<dd>Congo, Democratic Republic of the</dd>
<dd>Congo, Republic of the</dd>
<dd>Cote d'Ivoire</dd>
<dt>D</dt>
<dd>Djibouti</dd>
<dt>E</dt>
<dd>Egypt</dd>
<dd>Equatorial Guinea</dd>
<dd>Eritrea</dd>
<dd>Eswatini (formerly Swaziland)</dd>
<dd>Ethiopia</dd>
</dl>
</div>
.container {
display: grid;
place-items: center;
min-height: 400px;
}
.sticky-stack {
background: ##37474f;
color: ##fff;
margin: 0;
height: 320px;
border-radius: 1rem;
overflow-y: auto;
}
.sticky-stack dt {
position: sticky;
top: 0;
font-weight: bold;
background: ##263238;
color: ##cfd8dc;
padding: 0.25rem 1rem;
}
.sticky-stack dd {
margin: 0;
padding: 0.75rem 1rem;
}
.sticky-stack dd + dt {
margin-top: 1rem;
}
stripes-pattern
- title: Stripes background pattern
- tags: visual,beginner
Creates a stripes background pattern.
- Use
background-color
to set a white background. - Use
background-image
with aradial-gradient()
value to create a vertical stripe. - Use
background-size
to specify the pattern's size. - Note: The fixed
height
andwidth
of the element is for demonstration purposes only.
<div class="stripes"></div>
.stripes {
width: 240px;
height: 240px;
background-color: ##fff;
background-image: linear-gradient(90deg, transparent 50%, ##000 50%);
background-size: 60px 60px;
background-repeat: repeat;
}
system-font-stack
- title: System font stack
- tags: visual,beginner
Uses the native font of the operating system to get close to a native app feel.
- Define a list of fonts using
font-family
. - The browser looks for each successive font, preferring the first one if possible, and falls back to the next if it cannot find the font (on the system or defined in CSS).
-apple-system
is San Francisco, used on iOS and macOS (not Chrome however).BlinkMacSystemFont
is San Francisco, used on macOS Chrome.'Segoe UI'
is used on Windows 10.Roboto
is used on Android.Oxygen-Sans
is used on Linux with KDE.Ubuntu
is used on Ubuntu (all variants).Cantarell
is used on Linux with GNOME Shell.'Helvetica Neue'
andHelvetica
is used on macOS 10.10 and below.Arial
is a font widely supported by all operating systems.sans-serif
is the fallback sans serif font if none of the other fonts are supported.
<p class="system-font-stack">This text uses the system font.</p>
.system-font-stack {
font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto,
Oxygen-Sans, Ubuntu, Cantarell, 'Helvetica Neue', Helvetica, Arial,
sans-serif;
}
text-backdrop-overlay
- title: Image text overlay
- tags: visual,beginner
Displays a text on top of an image using an overlay.
- Use
backdrop-filter
to apply ablur(14px)
andbrightness(80%)
effect to make text readable regardless of background image and color.
<div>
<h3 class="text-overlay">Hello, World</h3>
<img src="https://picsum.photos/id/1050/1200/800">
</div>
div {
position: relative;
}
.text-overlay {
position: absolute;
top: 0;
left: 0;
padding: 1rem;
font-size: 2rem;
font-weight: 300;
color: white;
backdrop-filter: blur(14px) brightness(80%);
}
tile-layout-using-inline-block
- title: 3-tile layout
- tags: layout,beginner
Aligns items horizontally using display: inline-block
to create a 3-tile layout.
- Use
display: inline-block
to create a tiled layout, without usingfloat
,flex
orgrid
. - Use
width
in combination withcalc
to divide the width of the container evenly into 3 columns. - Set
font-size
for.tiles
to0
to avoid whitespace and to20px
for<h2>
elements to display the text. - Note: If you use relative units (e.g.
em
), usingfont-size: 0;
to fight whitespace between blocks might cause side effects.
<div class="tiles">
<div class="tile">
<img src="https://via.placeholder.com/200x150">
<h2>30 Seconds of CSS</h2>
</div>
<div class="tile">
<img src="https://via.placeholder.com/200x150">
<h2>30 Seconds of CSS</h2>
</div>
<div class="tile">
<img src="https://via.placeholder.com/200x150">
<h2>30 Seconds of CSS</h2>
</div>
</div>
.tiles {
width: 600px;
font-size: 0;
margin: 0 auto;
}
.tile {
width: calc(600px / 3);
display: inline-block;
}
.tile h2 {
font-size: 20px;
}
toggle-switch
- title: Toggle switch
- tags: visual,interactivity,beginner
Creates a toggle switch with CSS only.
- Use the
for
attribute to associate the<label>
with the checkbox<input>
element. - Use the
:after
pseudo-element of the<label>
to create a circular knob for the switch. - Use the
:checked
pseudo-class selector to change the position of the knob, usingtransform: translateX(20px)
and thebackground-color
of the switch. - Use
position: absolute
andleft: -9999px
to visually hide the<input>
element.
<input type="checkbox" id="toggle" class="offscreen" />
<label for="toggle" class="switch"></label>
.switch {
position: relative;
display: inline-block;
width: 40px;
height: 20px;
background-color: rgba(0, 0, 0, 0.25);
border-radius: 20px;
transition: all 0.3s;
}
.switch:after {
content: '';
position: absolute;
width: 18px;
height: 18px;
border-radius: 18px;
background-color: white;
top: 1px;
left: 1px;
transition: all 0.3s;
}
input[type='checkbox']:checked + .switch:after {
transform: translateX(20px);
}
input[type='checkbox']:checked + .switch {
background-color: ##7983ff;
}
.offscreen {
position: absolute;
left: -9999px;
}
transform-centering
- title: Transform centering
- tags: layout,beginner
Vertically and horizontally centers a child element within its parent element using CSS transforms.
- Set the
position
of the parent torelative
and that of the child toabsolute
to place it in relation to its parent. - Use
left: 50%
andtop: 50%
to offset the child 50% from the left and top edge of the containing block. - Use
transform: translate(-50%, -50%)
to negate its position, so that it is vertically and horizontally centered. - Note: The fixed
height
andwidth
of the parent element is for demonstration purposes only.
<div class="parent">
<div class="child">Centered content</div>
</div>
.parent {
border: 1px solid ##9C27B0;
height: 250px;
position: relative;
width: 250px;
}
.child {
left: 50%;
position: absolute;
top: 50%;
transform: translate(-50%, -50%);
text-align: center;
}
triangle
- title: Triangle
- tags: visual,beginner
Creates a triangular shape with pure CSS.
- Use three borders to create a triangle shape.
- All borders should have the same
border-width
(20px
). - The opposite side of where the triangle points towards (i.e. top if the triangle points downwards) should have the desired
border-color
, whereas the adjacent borders (i.e. left and right) should have aborder-color
oftransparent
. - Altering the
border-width
values will change the proportions of the triangle.
<div class="triangle"></div>
.triangle {
width: 0;
height: 0;
border-top: 20px solid ##9C27B0;
border-left: 20px solid transparent;
border-right: 20px solid transparent;
}
truncate-text-multiline
- title: Truncate multiline text
- tags: layout,intermediate
Truncates text that is longer than one line.
- Use
overflow: hidden
to prevent the text from overflowing its dimensions. - Set a fixed
width
to ensure the element has at least one constant dimension. - Set
height: 109.2px
as calculated from thefont-size
, using the formulafont-size * line-height * numberOfLines
(in this case26 * 1.4 * 3 = 109.2
). - Set
height: 36.4px
as calculated for the gradient container, based on the formulafont-size * line-height
(in this case26 * 1.4 = 36.4
). - Use
background
withlinear-gradient()
to create a gradient fromtransparent
to thebackground-color
.
<p class="truncate-text-multiline">
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy
eirmod tempor invidunt ut labore et.
</p>
.truncate-text-multiline {
position: relative;
overflow: hidden;
display: block;
height: 109.2px;
margin: 0 auto;
font-size: 26px;
line-height: 1.4;
width: 400px;
background: ##f5f6f9;
color: ##333;
}
.truncate-text-multiline:after {
content: '';
position: absolute;
bottom: 0;
right: 0;
width: 150px;
height: 36.4px;
background: linear-gradient(to right, rgba(0, 0, 0, 0), ##f5f6f9 50%);
}
truncate-text
- title: Truncate text
- tags: layout,beginner
Truncates text that is longer than one line, adding an ellipsis at the end (…
).
- Use
overflow: hidden
to prevent the text from overflowing its dimensions. - Use
white-space: nowrap
to prevent the text from exceeding one line in height. - Use
text-overflow: ellipsis
to make it so that if the text exceeds its dimensions, it will end with an ellipsis. - Specify a fixed
width
for the element to know when to display an ellipsis. - Only works for single line elements.
<p class="truncate-text">If I exceed one line's width, I will be truncated.</p>
.truncate-text {
overflow: hidden;
white-space: nowrap;
text-overflow: ellipsis;
width: 200px;
}
vertical-scroll-snap
- title: Vertical scroll snap
- tags: interactivity,intermediate
Creates a scrollable container that will snap on elements when scrolling.
- Use
display: gird
andgrid-auto-flow: row
to create a vertical layout. - Use
scroll-snap-type: y mandatory
andoverscroll-behavior-y: contain
to create a snap effect on vertical scroll. - You can use
scroll-snap-align
with eitherstart
,stop
orcenter
to change the snap alignment.
<div class="vertical-snap">
<a href="##"><img src="https://picsum.photos/id/1067/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/122/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/188/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/249/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/257/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/259/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/283/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/288/640/640"></a>
<a href="##"><img src="https://picsum.photos/id/299/640/640"></a>
</div>
.vertical-snap {
margin: 0 auto;
display: grid;
grid-auto-flow: row;
gap: 1rem;
width: calc(180px + 1rem);
padding: 1rem;
height: 480px;
overflow-y: auto;
overscroll-behavior-y: contain;
scroll-snap-type: y mandatory;
}
.vertical-snap > a {
scroll-snap-align: center;
}
.vertical-snap img {
width: 180px;
object-fit: contain;
border-radius: 1rem;
}
zebra-striped-list
- title: Zebra striped list
- tags: visual,beginner
Creates a striped list with alternating background colors.
- Use the
:nth-child(odd)
or:nth-child(even)
pseudo-class selectors to apply a differentbackground-color
to elements that match based on their position in a group of siblings. - Note: You can use it to apply different styles to other HTML elements like
<div>
,<tr>
,<p>
,<ol>
, etc.
<ul>
<li>Item 01</li>
<li>Item 02</li>
<li>Item 03</li>
<li>Item 04</li>
<li>Item 05</li>
</ul>
li:nth-child(odd) {
background-color: ##999;
}
zig-zag-pattern
- title: Zig zag background pattern
- tags: visual,advanced
Creates a zig zag background pattern.
- Use
background-color
to set a white background. - Use
background-image
with fourlinear-gradient()
values to create the parts of a zig zag pattern. - Use
background-size
to specify the pattern's size andbackground-position
to place the parts of the pattern in the correct locations. - Note: The fixed
height
andwidth
of the element is for demonstration purposes only.
<div class="zig-zag"></div>
.zig-zag {
width: 240px;
height: 240px;
background-color: ##fff;
background-image: linear-gradient(135deg, ##000 25%, transparent 25%),
linear-gradient(225deg, ##000 25%, transparent 25%),
linear-gradient(315deg, ##000 25%, transparent 25%),
linear-gradient(45deg, ##000 25%, transparent 25%);
background-position: -30px 0, -30px 0, 0 0, 0 0;
background-size: 60px 60px;
background-repeat: repeat;
}
CSVToArray
- title: CSVToArray
- tags: string,array,intermediate
Converts a comma-separated values (CSV) string to a 2D array.
- Use
Array.prototype.slice()
andArray.prototype.indexOf('\n')
to remove the first row (title row) ifomitFirstRow
istrue
. - Use
String.prototype.split('\n')
to create a string for each row, thenString.prototype.split(delimiter)
to separate the values in each row. - Omit the second argument,
delimiter
, to use a default delimiter of,
. - Omit the third argument,
omitFirstRow
, to include the first row (title row) of the CSV string.
const CSVToArray = (data, delimiter = ',', omitFirstRow = false) =>
data
.slice(omitFirstRow ? data.indexOf('\n') + 1 : 0)
.split('\n')
.map(v => v.split(delimiter));
CSVToArray('a,b\nc,d'); // [['a', 'b'], ['c', 'd']];
CSVToArray('a;b\nc;d', ';'); // [['a', 'b'], ['c', 'd']];
CSVToArray('col1,col2\na,b\nc,d', ',', true); // [['a', 'b'], ['c', 'd']];
CSVToJSON
- title: CSVToJSON
- tags: string,object,advanced
Converts a comma-separated values (CSV) string to a 2D array of objects. The first row of the string is used as the title row.
- Use
Array.prototype.slice()
andArray.prototype.indexOf('\n')
andString.prototype.split(delimiter)
to separate the first row (title row) into values. - Use
String.prototype.split('\n')
to create a string for each row, thenArray.prototype.map()
andString.prototype.split(delimiter)
to separate the values in each row. - Use
Array.prototype.reduce()
to create an object for each row's values, with the keys parsed from the title row. - Omit the second argument,
delimiter
, to use a default delimiter of,
.
const CSVToJSON = (data, delimiter = ',') => {
const titles = data.slice(0, data.indexOf('\n')).split(delimiter);
return data
.slice(data.indexOf('\n') + 1)
.split('\n')
.map(v => {
const values = v.split(delimiter);
return titles.reduce(
(obj, title, index) => ((obj[title] = values[index]), obj),
{}
);
});
};
CSVToJSON('col1,col2\na,b\nc,d');
// [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
CSVToJSON('col1;col2\na;b\nc;d', ';');
// [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
HSBToRGB
- title: HSBToRGB
- tags: math,intermediate
Converts a HSB color tuple to RGB format.
- Use the HSB to RGB conversion formula to convert to the appropriate format.
- The range of the input parameters is H: [0, 360], S: [0, 100], B: [0, 100].
- The range of all output values is [0, 255].
const HSBToRGB = (h, s, b) => {
s /= 100;
b /= 100;
const k = (n) => (n + h / 60) % 6;
const f = (n) => b * (1 - s * Math.max(0, Math.min(k(n), 4 - k(n), 1)));
return [255 * f(5), 255 * f(3), 255 * f(1)];
};
HSBToRGB(18, 81, 99); // [252.45, 109.31084999999996, 47.965499999999984]
HSLToRGB
- title: HSLToRGB
- tags: math,intermediate
Converts a HSL color tuple to RGB format.
- Use the HSL to RGB conversion formula to convert to the appropriate format.
- The range of the input parameters is H: [0, 360], S: [0, 100], L: [0, 100].
- The range of all output values is [0, 255].
const HSLToRGB = (h, s, l) => {
s /= 100;
l /= 100;
const k = n => (n + h / 30) % 12;
const a = s * Math.min(l, 1 - l);
const f = n =>
l - a * Math.max(-1, Math.min(k(n) - 3, Math.min(9 - k(n), 1)));
return [255 * f(0), 255 * f(8), 255 * f(4)];
};
HSLToRGB(13, 100, 11); // [56.1, 12.155, 0]
JSONToFile
- title: JSONToFile
- tags: node,intermediate
Writes a JSON object to a file.
- Use
fs.writeFileSync()
, template literals andJSON.stringify()
to write ajson
object to a.json
file.
const fs = require('fs');
const JSONToFile = (obj, filename) =>
fs.writeFileSync(`${filename}.json`, JSON.stringify(obj, null, 2));
JSONToFile({ test: 'is passed' }, 'testJsonFile');
// writes the object to 'testJsonFile.json'
JSONtoCSV
- title: JSONtoCSV
- tags: array,string,object,advanced
Converts an array of objects to a comma-separated values (CSV) string that contains only the columns
specified.
- Use
Array.prototype.join(delimiter)
to combine all the names incolumns
to create the first row. - Use
Array.prototype.map()
andArray.prototype.reduce()
to create a row for each object, substituting non-existent values with empty strings and only mapping values incolumns
. - Use
Array.prototype.join('\n')
to combine all rows into a string. - Omit the third argument,
delimiter
, to use a default delimiter of,
.
const JSONtoCSV = (arr, columns, delimiter = ',') =>
[
columns.join(delimiter),
...arr.map(obj =>
columns.reduce(
(acc, key) =>
`${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
''
)
),
].join('\n');
JSONtoCSV(
[{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
['a', 'b']
); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV(
[{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
['a', 'b'],
';'
); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'
RGBToHSB
- title: RGBToHSB
- tags: math,intermediate
Converts a RGB color tuple to HSB format.
- Use the RGB to HSB conversion formula to convert to the appropriate format.
- The range of all input parameters is [0, 255].
- The range of the resulting values is H: [0, 360], S: [0, 100], B: [0, 100].
const RGBToHSB = (r, g, b) => {
r /= 255;
g /= 255;
b /= 255;
const v = Math.max(r, g, b),
n = v - Math.min(r, g, b);
const h =
n && v === r ? (g - b) / n : v === g ? 2 + (b - r) / n : 4 + (r - g) / n;
return [60 * (h < 0 ? h + 6 : h), v && (n / v) * 100, v * 100];
};
RGBToHSB(252, 111, 48);
// [18.529411764705856, 80.95238095238095, 98.82352941176471]
RGBToHSL
- title: RGBToHSL
- tags: math,intermediate
Converts a RGB color tuple to HSL format.
- Use the RGB to HSL conversion formula to convert to the appropriate format.
- The range of all input parameters is [0, 255].
- The range of the resulting values is H: [0, 360], S: [0, 100], L: [0, 100].
const RGBToHSL = (r, g, b) => {
r /= 255;
g /= 255;
b /= 255;
const l = Math.max(r, g, b);
const s = l - Math.min(r, g, b);
const h = s
? l === r
? (g - b) / s
: l === g
? 2 + (b - r) / s
: 4 + (r - g) / s
: 0;
return [
60 * h < 0 ? 60 * h + 360 : 60 * h,
100 * (s ? (l <= 0.5 ? s / (2 * l - s) : s / (2 - (2 * l - s))) : 0),
(100 * (2 * l - s)) / 2,
];
};
RGBToHSL(45, 23, 11); // [21.17647, 60.71428, 10.98039]
RGBToHex
- title: RGBToHex
- tags: string,math,intermediate
Converts the values of RGB components to a hexadecimal color code.
- Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (
<<
) andNumber.prototype.toString(16)
. - Use
String.prototype.padStart(6, '0')
to get a 6-digit hexadecimal value.
const RGBToHex = (r, g, b) =>
((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');
RGBToHex(255, 165, 1); // 'ffa501'
URLJoin
- title: URLJoin
- tags: string,regexp,advanced
Joins all given URL segments together, then normalizes the resulting URL.
- Use
String.prototype.join('/')
to combine URL segments. - Use a series of
String.prototype.replace()
calls with various regexps to normalize the resulting URL (remove double slashes, add proper slashes for protocol, remove slashes before parameters, combine parameters with'&'
and normalize first parameter delimiter).
const URLJoin = (...args) =>
args
.join('/')
.replace(/[\/]+/g, '/')
.replace(/^(.+):\//, '$1://')
.replace(/^file:/, 'file:/')
.replace(/\/(\?|&|##[^!])/g, '$1')
.replace(/\?/g, '&')
.replace('&', '?');
URLJoin('http://www.google.com', 'a', '/b/cd', '?foo=123', '?bar=foo');
// 'http://www.google.com/a/b/cd?foo=123&bar=foo'
UUIDGeneratorBrowser
- title: UUIDGeneratorBrowser
- tags: browser,random,intermediate
Generates a UUID in a browser.
- Use
Crypto.getRandomValues()
to generate a UUID, compliant with RFC4122 version 4. - Use
Number.prototype.toString(16)
to convert it to a proper UUID.
const UUIDGeneratorBrowser = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(
c ^
(crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))
).toString(16)
);
UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'
UUIDGeneratorNode
- title: UUIDGeneratorNode
- tags: node,random,intermediate
Generates a UUID in Node.JS.
- Use
crypto.randomBytes()
to generate a UUID, compliant with RFC4122 version 4. - Use
Number.prototype.toString(16)
to convert it to a proper UUID.
const crypto = require('crypto');
const UUIDGeneratorNode = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
);
UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'
accumulate
- title: accumulate
- tags: math,array,intermediate
Creates an array of partial sums.
- Use
Array.prototype.reduce()
, initialized with an empty array accumulator to iterate overnums
. - Use
Array.prototype.slice(-1)
, the spread operator (...
) and the unary+
operator to add each value to the accumulator array containing the previous sums.
const accumulate = (...nums) =>
nums.reduce((acc, n) => [...acc, n + +acc.slice(-1)], []);
accumulate(1, 2, 3, 4); // [1, 3, 6, 10]
accumulate(...[1, 2, 3, 4]); // [1, 3, 6, 10]
addClass
- title: addClass
- tags: browser,beginner
Adds a class to an HTML element.
- Use
Element.classList
andDOMTokenList.add()
to add the specified class to the element.
const addClass = (el, className) => el.classList.add(className);
addClass(document.querySelector('p'), 'special');
// The paragraph will now have the 'special' class
addDaysToDate
- title: addDaysToDate
- tags: date,intermediate
Calculates the date of n
days from the given date, returning its string representation.
- Use
new Date()
to create a date object from the first argument. - Use
Date.prototype.getDate()
andDate.prototype.setDate()
to addn
days to the given date. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const addDaysToDate = (date, n) => {
const d = new Date(date);
d.setDate(d.getDate() + n);
return d.toISOString().split('T')[0];
};
addDaysToDate('2020-10-15', 10); // '2020-10-25'
addDaysToDate('2020-10-15', -10); // '2020-10-05'
addMinutesToDate
- title: addMinutesToDate
- tags: date,intermediate
Calculates the date of n
minutes from the given date, returning its string representation.
- Use
new Date()
to create a date object from the first argument. - Use
Date.prototype.getTime()
andDate.prototype.setTime()
to addn
minutes to the given date. - Use
Date.prototype.toISOString()
,String.prototype.split()
andString.prototype.replace()
to return a string inyyyy-mm-dd HH:MM:SS
format.
const addMinutesToDate = (date, n) => {
const d = new Date(date);
d.setTime(d.getTime() + n * 60000);
return d.toISOString().split('.')[0].replace('T',' ');
};
addMinutesToDate('2020-10-19 12:00:00', 10); // '2020-10-19 12:10:00'
addMinutesToDate('2020-10-19', -10); // '2020-10-18 23:50:00'
addMultipleEvents
- title: addMultipleListeners
- tags: browser,event,intermediate
Adds multiple event listeners with the same handler to an element.
- Use
Array.prototype.forEach()
andEventTarget.addEventListener()
to add multiple event listeners with an assigned callback function to an element.
const addMultipleListeners = (el, types, listener, options, useCapture) => {
types.forEach(type =>
el.addEventListener(type, listener, options, useCapture)
);
};
addMultipleListeners(
document.querySelector('.my-element'),
['click', 'mousedown'],
() => { console.log('hello!') }
);
addStyles
- title: addStyles
- tags: browser,beginner
Adds the provided styles to the given element.
- Use
Object.assign()
andElementCSSInlineStyle.style
to merge the providedstyles
object into the style of the given element.
const addStyles = (el, styles) => Object.assign(el.style, styles);
addStyles(document.getElementById('my-element'), {
background: 'red',
color: '##ffff00',
fontSize: '3rem'
});
addWeekDays
- title: addWeekDays
- tags: date,intermediate
Calculates the date after adding the given number of business days.
- Use
Array.from()
to construct an array withlength
equal to thecount
of business days to be added. - Use
Array.prototype.reduce()
to iterate over the array, starting fromstartDate
and incrementing, usingDate.prototype.getDate()
andDate.prototype.setDate()
. - If the current
date
is on a weekend, update it again by adding either one day or two days to make it a weekday. - NOTE: Does not take official holidays into account.
const addWeekDays = (startDate, count) =>
Array.from({ length: count }).reduce(date => {
date = new Date(date.setDate(date.getDate() + 1));
if (date.getDay() % 6 === 0)
date = new Date(date.setDate(date.getDate() + (date.getDay() / 6 + 1)));
return date;
}, startDate);
addWeekDays(new Date('Oct 09, 2020'), 5); // 'Oct 16, 2020'
addWeekDays(new Date('Oct 12, 2020'), 5); // 'Oct 19, 2020'
all
- title: all
- tags: array,beginner
Checks if the provided predicate function returns true
for all elements in a collection.
- Use
Array.prototype.every()
to test if all elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const all = (arr, fn = Boolean) => arr.every(fn);
all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // true
allEqual
- title: allEqual
- tags: array,beginner
Checks if all elements in an array are equal.
- Use
Array.prototype.every()
to check if all the elements of the array are the same as the first one. - Elements in the array are compared using the strict comparison operator, which does not account for
NaN
self-inequality.
const allEqual = arr => arr.every(val => val === arr[0]);
allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // true
allEqualBy
- title: allEqualBy
- tags: array,intermediate
Checks if all elements in an array are equal, based on the provided mapping function.
- Apply
fn
to the first element ofarr
. - Use
Array.prototype.every()
to check iffn
returns the same value for all elements in the array as it did for the first one. - Elements in the array are compared using the strict comparison operator, which does not account for
NaN
self-inequality.
const allEqualBy = (arr, fn) => {
const eql = fn(arr[0]);
return arr.every(val => fn(val) === eql);
};
allEqualBy([1.1, 1.2, 1.3], Math.round); // true
allEqualBy([1.1, 1.3, 1.6], Math.round); // false
allUnique
- title: allUnique
- tags: array,beginner
Checks if all elements in an array are unique.
- Create a new
Set
from the mapped values to keep only unique occurrences. - Use
Array.prototype.length
andSet.prototype.size
to compare the length of the unique values to the original array.
const allUnique = arr => arr.length === new Set(arr).size;
allUnique([1, 2, 3, 4]); // true
allUnique([1, 1, 2, 3]); // false
allUniqueBy
- title: allUniqueBy
- tags: array,intermediate
Checks if all elements in an array are unique, based on the provided mapping function.
- Use
Array.prototype.map()
to applyfn
to all elements inarr
. - Create a new
Set
from the mapped values to keep only unique occurrences. - Use
Array.prototype.length
andSet.prototype.size
to compare the length of the unique mapped values to the original array.
const allUniqueBy = (arr, fn) => arr.length === new Set(arr.map(fn)).size;
allUniqueBy([1.2, 2.4, 2.9], Math.round); // true
allUniqueBy([1.2, 2.3, 2.4], Math.round); // false
and
- title: and
- tags: math,logic,beginner unlisted: true
Checks if both arguments are true
.
- Use the logical and (
&&
) operator on the two given values.
const and = (a, b) => a && b;
and(true, true); // true
and(true, false); // false
and(false, false); // false
any
- title: any
- tags: array,beginner
Checks if the provided predicate function returns true
for at least one element in a collection.
- Use
Array.prototype.some()
to test if any elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const any = (arr, fn = Boolean) => arr.some(fn);
any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // true
aperture
- title: aperture
- tags: array,intermediate
Creates an array of n
-tuples of consecutive elements.
- Use
Array.prototype.slice()
andArray.prototype.map()
to create an array of appropriate length. - Populate the array with
n
-tuples of consecutive elements fromarr
. - If
n
is greater than the length ofarr
, return an empty array.
const aperture = (n, arr) =>
n > arr.length
? []
: arr.slice(n - 1).map((v, i) => arr.slice(i, i + n));
aperture(2, [1, 2, 3, 4]); // [[1, 2], [2, 3], [3, 4]]
aperture(3, [1, 2, 3, 4]); // [[1, 2, 3], [2, 3, 4]]
aperture(5, [1, 2, 3, 4]); // []
approximatelyEqual
- title: approximatelyEqual
- tags: math,beginner
Checks if two numbers are approximately equal to each other.
- Use
Math.abs()
to compare the absolute difference of the two values toepsilon
. - Omit the third argument,
epsilon
, to use a default value of0.001
.
const approximatelyEqual = (v1, v2, epsilon = 0.001) =>
Math.abs(v1 - v2) < epsilon;
approximatelyEqual(Math.PI / 2.0, 1.5708); // true
arithmeticProgression
- title: arithmeticProgression
- tags: math,algorithm,beginner
Creates an array of numbers in the arithmetic progression, starting with the given positive integer and up to the specified limit.
- Use
Array.from()
to create an array of the desired length,lim/n
, and a map function to fill it with the desired values in the given range.
const arithmeticProgression = (n, lim) =>
Array.from({ length: Math.ceil(lim / n) }, (_, i) => (i + 1) * n );
arithmeticProgression(5, 25); // [5, 10, 15, 20, 25]
arrayToCSV
- title: arrayToCSV
- tags: array,string,intermediate
Converts a 2D array to a comma-separated values (CSV) string.
- Use
Array.prototype.map()
andArray.prototype.join(delimiter)
to combine individual 1D arrays (rows) into strings. - Use
Array.prototype.join('\n')
to combine all rows into a CSV string, separating each row with a newline. - Omit the second argument,
delimiter
, to use a default delimiter of,
.
const arrayToCSV = (arr, delimiter = ',') =>
arr
.map(v =>
v.map(x => (isNaN(x) ? `"${x.replace(/"/g, '""')}"` : x)).join(delimiter)
)
.join('\n');
arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'
arrayToCSV([['a', '"b" great'], ['c', 3.1415]]);
// '"a","""b"" great"\n"c",3.1415'
arrayToHTMLList
- title: arrayToHTMLList
- tags: browser,array,intermediate
Converts the given array elements into <li>
tags and appends them to the list of the given id.
- Use
Array.prototype.map()
andDocument.querySelector()
to create a list of html tags.
const arrayToHTMLList = (arr, listID) =>
document.querySelector(`##${listID}`).innerHTML += arr
.map(item => `<li>${item}</li>`)
.join('');
arrayToHTMLList(['item 1', 'item 2'], 'myListID');
ary
- title: ary
- tags: function,advanced
Creates a function that accepts up to n
arguments, ignoring any additional arguments.
- Call the provided function,
fn
, with up ton
arguments, usingArray.prototype.slice(0, n)
and the spread operator (...
).
const ary = (fn, n) => (...args) => fn(...args.slice(0, n));
const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [6, 4, 8], [10]].map(x => firstTwoMax(...x)); // [6, 6, 10]
atob
- title: atob
- tags: node,string,beginner
Decodes a string of data which has been encoded using base-64 encoding.
- Create a
Buffer
for the given string with base-64 encoding and useBuffer.toString('binary')
to return the decoded string.
const atob = str => Buffer.from(str, 'base64').toString('binary');
atob('Zm9vYmFy'); // 'foobar'
attempt
- title: attempt
- tags: function,intermediate
Attempts to invoke a function with the provided arguments, returning either the result or the caught error object.
- Use a
try... catch
block to return either the result of the function or an appropriate error. - If the caught object is not an
Error
, use it to create a newError
.
const attempt = (fn, ...args) => {
try {
return fn(...args);
} catch (e) {
return e instanceof Error ? e : new Error(e);
}
};
var elements = attempt(function(selector) {
return document.querySelectorAll(selector);
}, '>_>');
if (elements instanceof Error) elements = []; // elements = []
average
- title: average
- tags: math,array,beginner
Calculates the average of two or more numbers.
- Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
. - Divide the resulting array by its length.
const average = (...nums) =>
nums.reduce((acc, val) => acc + val, 0) / nums.length;
average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2
averageBy
- title: averageBy
- tags: math,array,intermediate
Calculates the average of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
. - Divide the resulting array by its length.
const averageBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val) => acc + val, 0) / arr.length;
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5
bifurcate
- title: bifurcate
- tags: array,intermediate
Splits values into two groups, based on the result of the given filter
array.
- Use
Array.prototype.reduce()
andArray.prototype.push()
to add elements to groups, based onfilter
. - If
filter
has a truthy value for any element, add it to the first group, otherwise add it to the second group.
const bifurcate = (arr, filter) =>
arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [
[],
[],
]);
bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]);
// [ ['beep', 'boop', 'bar'], ['foo'] ]
bifurcateBy
- title: bifurcateBy
- tags: array,intermediate
Splits values into two groups, based on the result of the given filtering function.
- Use
Array.prototype.reduce()
andArray.prototype.push()
to add elements to groups, based on the value returned byfn
for each element. - If
fn
returns a truthy value for any element, add it to the first group, otherwise add it to the second group.
const bifurcateBy = (arr, fn) =>
arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [
[],
[],
]);
bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b');
// [ ['beep', 'boop', 'bar'], ['foo'] ]
binary
- title: binary
- tags: function,intermediate
Creates a function that accepts up to two arguments, ignoring any additional arguments.
- Call the provided function,
fn
, with the first two arguments given.
const binary = fn => (a, b) => fn(a, b);
['2', '1', '0'].map(binary(Math.max)); // [2, 1, 2]
binarySearch
- title: binarySearch
- tags: algorithm,array,beginner
Finds the index of a given element in a sorted array using the binary search algorithm.
- Declare the left and right search boundaries,
l
andr
, initialized to0
and thelength
of the array respectively. - Use a
while
loop to repeatedly narrow down the search subarray, usingMath.floor()
to cut it in half. - Return the index of the element if found, otherwise return
-1
. - Note: Does not account for duplicate values in the array.
const binarySearch = (arr, item) => {
let l = 0,
r = arr.length - 1;
while (l <= r) {
const mid = Math.floor((l + r) / 2);
const guess = arr[mid];
if (guess === item) return mid;
if (guess > item) r = mid - 1;
else l = mid + 1;
}
return -1;
};
binarySearch([1, 2, 3, 4, 5], 1); // 0
binarySearch([1, 2, 3, 4, 5], 5); // 4
binarySearch([1, 2, 3, 4, 5], 6); // -1
bind
- title: bind
- tags: function,object,advanced
Creates a function that invokes fn
with a given context, optionally prepending any additional supplied parameters to the arguments.
- Return a
function
that usesFunction.prototype.apply()
to apply the givencontext
tofn
. - Use the spread operator (
...
) to prepend any additional supplied parameters to the arguments.
const bind = (fn, context, ...boundArgs) => (...args) =>
fn.apply(context, [...boundArgs, ...args]);
function greet(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
const freddy = { user: 'fred' };
const freddyBound = bind(greet, freddy);
console.log(freddyBound('hi', '!')); // 'hi fred!'
bindAll
- title: bindAll
- tags: object,function,intermediate
Binds methods of an object to the object itself, overwriting the existing method.
- Use
Array.prototype.forEach()
to iterate over the givenfns
. - Return a function for each one, using
Function.prototype.apply()
to apply the given context (obj
) tofn
.
const bindAll = (obj, ...fns) =>
fns.forEach(
fn => (
(f = obj[fn]),
(obj[fn] = function() {
return f.apply(obj);
})
)
);
var view = {
label: 'docs',
click: function() {
console.log('clicked ' + this.label);
}
};
bindAll(view, 'click');
document.body.addEventListener('click', view.click);
// Log 'clicked docs' when clicked.
bindKey
- title: bindKey
- tags: function,object,advanced
Creates a function that invokes the method at a given key of an object, optionally prepending any additional supplied parameters to the arguments.
- Return a
function
that usesFunction.prototype.apply()
to bindcontext[fn]
tocontext
. - Use the spread operator (
...
) to prepend any additional supplied parameters to the arguments.
const bindKey = (context, fn, ...boundArgs) => (...args) =>
context[fn].apply(context, [...boundArgs, ...args]);
const freddy = {
user: 'fred',
greet: function(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
};
const freddyBound = bindKey(freddy, 'greet');
console.log(freddyBound('hi', '!')); // 'hi fred!'
binomialCoefficient
- title: binomialCoefficient
- tags: math,algorithm,beginner
Calculates the number of ways to choose k
items from n
items without repetition and without order.
- Use
Number.isNaN()
to check if any of the two values isNaN
. - Check if
k
is less than0
, greater than or equal ton
, equal to1
orn - 1
and return the appropriate result. - Check if
n - k
is less thank
and switch their values accordingly. - Loop from
2
throughk
and calculate the binomial coefficient. - Use
Math.round()
to account for rounding errors in the calculation.
const binomialCoefficient = (n, k) => {
if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
if (k < 0 || k > n) return 0;
if (k === 0 || k === n) return 1;
if (k === 1 || k === n - 1) return n;
if (n - k < k) k = n - k;
let res = n;
for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
return Math.round(res);
};
binomialCoefficient(8, 2); // 28
both
- title: both
- tags: function,logic,beginner unlisted: true
Checks if both of the given functions return true
for a given set of arguments.
- Use the logical and (
&&
) operator on the result of calling the two functions with the suppliedargs
.
const both = (f, g) => (...args) => f(...args) && g(...args);
const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveEven = both(isEven, isPositive);
isPositiveEven(4); // true
isPositiveEven(-2); // false
bottomVisible
- title: bottomVisible
- tags: browser,beginner
Checks if the bottom of the page is visible.
- Use
scrollY
,scrollHeight
andclientHeight
to determine if the bottom of the page is visible.
const bottomVisible = () =>
document.documentElement.clientHeight + window.scrollY >=
(document.documentElement.scrollHeight ||
document.documentElement.clientHeight);
bottomVisible(); // true
btoa
- title: btoa
- tags: node,string,beginner
Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.
- Create a
Buffer
for the given string with binary encoding and useBuffer.toString('base64')
to return the encoded string.
const btoa = str => Buffer.from(str, 'binary').toString('base64');
btoa('foobar'); // 'Zm9vYmFy'
bubbleSort
- title: bubbleSort
- tags: algorithm,array,beginner
Sorts an array of numbers, using the bubble sort algorithm.
- Declare a variable,
swapped
, that indicates if any values were swapped during the current iteration. - Use the spread operator (
...
) to clone the original array,arr
. - Use a
for
loop to iterate over the elements of the cloned array, terminating before the last element. - Use a nested
for
loop to iterate over the segment of the array between0
andi
, swapping any adjacent out of order elements and settingswapped
totrue
. - If
swapped
isfalse
after an iteration, no more changes are needed, so the cloned array is returned.
const bubbleSort = arr => {
let swapped = false;
const a = [...arr];
for (let i = 1; i < a.length - 1; i++) {
swapped = false;
for (let j = 0; j < a.length - i; j++) {
if (a[j + 1] < a[j]) {
[a[j], a[j + 1]] = [a[j + 1], a[j]];
swapped = true;
}
}
if (!swapped) return a;
}
return a;
};
bubbleSort([2, 1, 4, 3]); // [1, 2, 3, 4]
bucketSort
- title: bucketSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the bucket sort algorithm.
- Use
Math.min(),
Math.max()
and the spread operator (...
) to find the minimum and maximum values of the given array. - Use
Array.from()
andMath.floor()
to create the appropriate number ofbuckets
(empty arrays). - Use
Array.prototype.forEach()
to populate each bucket with the appropriate elements from the array. - Use
Array.prototype.reduce()
, the spread operator (...
) andArray.prototype.sort()
to sort each bucket and append it to the result.
const bucketSort = (arr, size = 5) => {
const min = Math.min(...arr);
const max = Math.max(...arr);
const buckets = Array.from(
{ length: Math.floor((max - min) / size) + 1 },
() => []
);
arr.forEach(val => {
buckets[Math.floor((val - min) / size)].push(val);
});
return buckets.reduce((acc, b) => [...acc, ...b.sort((a, b) => a - b)], []);
};
bucketSort([6, 3, 4, 1]); // [1, 3, 4, 6]
byteSize
- title: byteSize
- tags: string,beginner
Returns the length of a string in bytes.
- Convert a given string to a
Blob
Object. - Use
Blob.size
to get the length of the string in bytes.
const byteSize = str => new Blob([str]).size;
byteSize('😀'); // 4
byteSize('Hello World'); // 11
caesarCipher
- title: caesarCipher
- tags: algorithm,string,beginner
Encrypts or decrypts a given string using the Caesar cipher.
- Use the modulo (
%
) operator and the ternary operator (?
) to calculate the correct encryption/decryption key. - Use the spread operator (
...
) andArray.prototype.map()
to iterate over the letters of the given string. - Use
String.prototype.charCodeAt()
andString.fromCharCode()
to convert each letter appropriately, ignoring special characters, spaces etc. - Use
Array.prototype.join()
to combine all the letters into a string. - Pass
true
to the last parameter,decrypt
, to decrypt an encrypted string.
const caesarCipher = (str, shift, decrypt = false) => {
const s = decrypt ? (26 - shift) % 26 : shift;
const n = s > 0 ? s : 26 + (s % 26);
return [...str]
.map((l, i) => {
const c = str.charCodeAt(i);
if (c >= 65 && c <= 90)
return String.fromCharCode(((c - 65 + n) % 26) + 65);
if (c >= 97 && c <= 122)
return String.fromCharCode(((c - 97 + n) % 26) + 97);
return l;
})
.join('');
};
caesarCipher('Hello World!', -3); // 'Ebiil Tloia!'
caesarCipher('Ebiil Tloia!', 23, true); // 'Hello World!'
call
- title: call
- tags: function,advanced
Given a key and a set of arguments, call them when given a context.
- Use a closure to call
key
withargs
for the givencontext
.
const call = (key, ...args) => context => context[key](...args);
Promise.resolve([1, 2, 3])
.then(call('map', x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
.then(map(x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
capitalize
- title: capitalize
- tags: string,intermediate
Capitalizes the first letter of a string.
- Use array destructuring and
String.prototype.toUpperCase()
to capitalize the first letter of the string. - Use
Array.prototype.join('')
to combine the capitalizedfirst
with the...rest
of the characters. - Omit the
lowerRest
argument to keep the rest of the string intact, or set it totrue
to convert to lowercase.
const capitalize = ([first, ...rest], lowerRest = false) =>
first.toUpperCase() +
(lowerRest ? rest.join('').toLowerCase() : rest.join(''));
capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'
capitalizeEveryWord
- title: capitalizeEveryWord
- tags: string,regexp,intermediate
Capitalizes the first letter of every word in a string.
- Use
String.prototype.replace()
to match the first character of each word andString.prototype.toUpperCase()
to capitalize it.
const capitalizeEveryWord = str =>
str.replace(/\b[a-z]/g, char => char.toUpperCase());
capitalizeEveryWord('hello world!'); // 'Hello World!'
cartesianProduct
- title: cartesianProduct
- tags: math,array,beginner
Calculates the cartesian product of two arrays.
- Use
Array.prototype.reduce()
,Array.prototype.map()
and the spread operator (...
) to generate all possible element pairs from the two arrays.
const cartesianProduct = (a, b) =>
a.reduce((p, x) => [...p, ...b.map(y => [x, y])], []);
cartesianProduct(['x', 'y'], [1, 2]);
// [['x', 1], ['x', 2], ['y', 1], ['y', 2]]
castArray
- title: castArray
- tags: type,array,beginner
Casts the provided value as an array if it's not one.
- Use
Array.prototype.isArray()
to determine ifval
is an array and return it as-is or encapsulated in an array accordingly.
const castArray = val => (Array.isArray(val) ? val : [val]);
castArray('foo'); // ['foo']
castArray([1]); // [1]
celsiusToFahrenheit
- title: celsiusToFahrenheit
- tags: math,beginner unlisted: true
Converts Celsius to Fahrenheit.
- Follow the conversion formula
F = 1.8 * C + 32
.
const celsiusToFahrenheit = degrees => 1.8 * degrees + 32;
celsiusToFahrenheit(33); // 91.4
chainAsync
- title: chainAsync
- tags: function,intermediate
Chains asynchronous functions.
- Loop through an array of functions containing asynchronous events, calling
next
when each asynchronous event has completed.
const chainAsync = fns => {
let curr = 0;
const last = fns[fns.length - 1];
const next = () => {
const fn = fns[curr++];
fn === last ? fn() : fn(next);
};
next();
};
chainAsync([
next => {
console.log('0 seconds');
setTimeout(next, 1000);
},
next => {
console.log('1 second');
setTimeout(next, 1000);
},
() => {
console.log('2 second');
}
]);
changeLightness
- title: changeLightness
- tags: string,browser,regexp,intermediate
Changes the lightness value of an hsl()
color string.
- Use
String.prototype.match()
to get an array of 3 strings with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Make sure the lightness is within the valid range (between
0
and100
), usingMath.max()
andMath.min()
. - Use a template literal to create a new
hsl()
string with the updated value.
const changeLightness = (delta, hslStr) => {
const [hue, saturation, lightness] = hslStr.match(/\d+/g).map(Number);
const newLightness = Math.max(
0,
Math.min(100, lightness + parseFloat(delta))
);
return `hsl(${hue}, ${saturation}%, ${newLightness}%)`;
};
changeLightness(10, 'hsl(330, 50%, 50%)'); // 'hsl(330, 50%, 60%)'
changeLightness(-10, 'hsl(330, 50%, 50%)'); // 'hsl(330, 50%, 40%)'
checkProp
- title: checkProp
- tags: function,object,intermediate
Creates a function that will invoke a predicate function for the specified property on a given object.
- Return a curried function, that will invoke
predicate
for the specifiedprop
onobj
and return a boolean.
const checkProp = (predicate, prop) => obj => !!predicate(obj[prop]);
const lengthIs4 = checkProp(l => l === 4, 'length');
lengthIs4([]); // false
lengthIs4([1, 2, 3, 4]); // true
lengthIs4(new Set([1, 2, 3, 4])); // false (Set uses Size, not length)
const session = { user: {} };
const validUserSession = checkProp(u => u.active && !u.disabled, 'user');
validUserSession(session); // false
session.user.active = true;
validUserSession(session); // true
const noLength = checkProp(l => l === undefined, 'length');
noLength([]); // false
noLength({}); // true
noLength(new Set()); // true
chunk
- title: chunk
- tags: array,intermediate
Chunks an array into smaller arrays of a specified size.
- Use
Array.from()
to create a new array, that fits the number of chunks that will be produced. - Use
Array.prototype.slice()
to map each element of the new array to a chunk the length ofsize
. - If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunk = (arr, size) =>
Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
arr.slice(i * size, i * size + size)
);
chunk([1, 2, 3, 4, 5], 2); // [[1, 2], [3, 4], [5]]
chunkIntoN
- title: chunkIntoN
- tags: array,intermediate
Chunks an array into n
smaller arrays.
- Use
Math.ceil()
andArray.prototype.length
to get the size of each chunk. - Use
Array.from()
to create a new array of sizen
. - Use
Array.prototype.slice()
to map each element of the new array to a chunk the length ofsize
. - If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunkIntoN = (arr, n) => {
const size = Math.ceil(arr.length / n);
return Array.from({ length: n }, (v, i) =>
arr.slice(i * size, i * size + size)
);
}
chunkIntoN([1, 2, 3, 4, 5, 6, 7], 4); // [[1, 2], [3, 4], [5, 6], [7]]
clampNumber
- title: clampNumber
- tags: math,beginner
Clamps num
within the inclusive range specified by the boundary values a
and b
.
- If
num
falls within the range, returnnum
. - Otherwise, return the nearest number in the range.
const clampNumber = (num, a, b) =>
Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1
cloneRegExp
- title: cloneRegExp
- tags: type,intermediate
Clones a regular expression.
- Use
new RegExp()
,RegExp.prototype.source
andRegExp.prototype.flags
to clone the given regular expression.
const cloneRegExp = regExp => new RegExp(regExp.source, regExp.flags);
const regExp = /lorem ipsum/gi;
const regExp2 = cloneRegExp(regExp); // regExp !== regExp2
coalesce
- title: coalesce
- tags: type,beginner
Returns the first defined, non-null argument.
- Use
Array.prototype.find()
andArray.prototype.includes()
to find the first value that is not equal toundefined
ornull
.
const coalesce = (...args) => args.find(v => ![undefined, null].includes(v));
coalesce(null, undefined, '', NaN, 'Waldo'); // ''
coalesceFactory
- title: coalesceFactory
- tags: function,type,intermediate
Customizes a coalesce function that returns the first argument which is true based on the given validator.
- Use
Array.prototype.find()
to return the first argument that returnstrue
from the provided argument validation function,valid
.
const coalesceFactory = valid => (...args) => args.find(valid);
const customCoalesce = coalesceFactory(
v => ![null, undefined, '', NaN].includes(v)
);
customCoalesce(undefined, null, NaN, '', 'Waldo'); // 'Waldo'
collectInto
- title: collectInto
- tags: function,array,intermediate
Changes a function that accepts an array into a variadic function.
- Given a function, return a closure that collects all inputs into an array-accepting function.
const collectInto = fn => (...args) => fn(args);
const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)
colorize
- title: colorize
- tags: node,string,intermediate
Adds special characters to text to print in color in the console (combined with console.log()
).
- Use template literals and special characters to add the appropriate color code to the string output.
- For background colors, add a special character that resets the background color at the end of the string.
const colorize = (...args) => ({
black: `\x1b[30m${args.join(' ')}`,
red: `\x1b[31m${args.join(' ')}`,
green: `\x1b[32m${args.join(' ')}`,
yellow: `\x1b[33m${args.join(' ')}`,
blue: `\x1b[34m${args.join(' ')}`,
magenta: `\x1b[35m${args.join(' ')}`,
cyan: `\x1b[36m${args.join(' ')}`,
white: `\x1b[37m${args.join(' ')}`,
bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});
console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite);
// 'foo bar' (first word in yellow letters, second word in green letters, white background for both)
combine
- title: combine
- tags: array,object,intermediate
Combines two arrays of objects, using the specified key to match objects.
- Use
Array.prototype.reduce()
with an object accumulator to combine all objects in both arrays based on the givenprop
. - Use
Object.values()
to convert the resulting object to an array and return it.
const combine = (a, b, prop) =>
Object.values(
[...a, ...b].reduce((acc, v) => {
if (v[prop])
acc[v[prop]] = acc[v[prop]]
? { ...acc[v[prop]], ...v }
: { ...v };
return acc;
}, {})
);
const x = [
{ id: 1, name: 'John' },
{ id: 2, name: 'Maria' }
];
const y = [
{ id: 1, age: 28 },
{ id: 3, age: 26 },
{ age: 3}
];
combine(x, y, 'id');
// [
// { id: 1, name: 'John', age: 28 },
// { id: 2, name: 'Maria' },
// { id: 3, age: 26 }
// ]
compact
- title: compact
- tags: array,beginner
Removes falsy values from an array.
- Use
Array.prototype.filter()
to filter out falsy values (false
,null
,0
,""
,undefined
, andNaN
).
const compact = arr => arr.filter(Boolean);
compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]);
// [ 1, 2, 3, 'a', 's', 34 ]
compactObject
- title: compactObject
- tags: object,array,recursion,advanced
Deeply removes all falsy values from an object or array.
- Use recursion.
- Initialize the iterable data, using
Array.isArray()
,Array.prototype.filter()
andBoolean
for arrays in order to avoid sparse arrays. - Use
Object.keys()
andArray.prototype.reduce()
to iterate over each key with an appropriate initial value. - Use
Boolean
to determine the truthiness of each key's value and add it to the accumulator if it's truthy. - Use
typeof
to determine if a given value is anobject
and call the function again to deeply compact it.
const compactObject = val => {
const data = Array.isArray(val) ? val.filter(Boolean) : val;
return Object.keys(data).reduce(
(acc, key) => {
const value = data[key];
if (Boolean(value))
acc[key] = typeof value === 'object' ? compactObject(value) : value;
return acc;
},
Array.isArray(val) ? [] : {}
);
};
const obj = {
a: null,
b: false,
c: true,
d: 0,
e: 1,
f: '',
g: 'a',
h: [null, false, '', true, 1, 'a'],
i: { j: 0, k: false, l: 'a' }
};
compactObject(obj);
// { c: true, e: 1, g: 'a', h: [ true, 1, 'a' ], i: { l: 'a' } }
compactWhitespace
- title: compactWhitespace
- tags: string,regexp,beginner
Compacts whitespaces in a string.
- Use
String.prototype.replace()
with a regular expression to replace all occurrences of 2 or more whitespace characters with a single space.
const compactWhitespace = str => str.replace(/\s{2,}/g, ' ');
compactWhitespace('Lorem Ipsum'); // 'Lorem Ipsum'
compactWhitespace('Lorem \n Ipsum'); // 'Lorem Ipsum'
complement
- title: complement
- tags: function,logic,beginner
Returns a function that is the logical complement of the given function, fn
.
- Use the logical not (
!
) operator on the result of callingfn
with any suppliedargs
.
const complement = fn => (...args) => !fn(...args);
const isEven = num => num % 2 === 0;
const isOdd = complement(isEven);
isOdd(2); // false
isOdd(3); // true
compose
- title: compose
- tags: function,intermediate
Performs right-to-left function composition.
- Use
Array.prototype.reduce()
to perform right-to-left function composition. - The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.
const compose = (...fns) =>
fns.reduce((f, g) => (...args) => f(g(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(
add5,
multiply
);
multiplyAndAdd5(5, 2); // 15
composeRight
- title: composeRight
- tags: function,intermediate
Performs left-to-right function composition.
- Use
Array.prototype.reduce()
to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const composeRight = (...fns) =>
fns.reduce((f, g) => (...args) => g(f(...args)));
const add = (x, y) => x + y;
const square = x => x * x;
const addAndSquare = composeRight(add, square);
addAndSquare(1, 2); // 9
containsWhitespace
- title: containsWhitespace
- tags: string,regexp,beginner
Checks if the given string contains any whitespace characters.
- Use
RegExp.prototype.test()
with an appropriate regular expression to check if the given string contains any whitespace characters.
const containsWhitespace = str => /\s/.test(str);
containsWhitespace('lorem'); // false
containsWhitespace('lorem ipsum'); // true
converge
- title: converge
- tags: function,intermediate
Accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.
- Use
Array.prototype.map()
andFunction.prototype.apply()
to apply each function to the given arguments. - Use the spread operator (
...
) to callconverger
with the results of all other functions.
const converge = (converger, fns) => (...args) =>
converger(...fns.map(fn => fn.apply(null, args)));
const average = converge((a, b) => a / b, [
arr => arr.reduce((a, v) => a + v, 0),
arr => arr.length
]);
average([1, 2, 3, 4, 5, 6, 7]); // 4
copySign
- title: copySign
- tags: math,beginner
Returns the absolute value of the first number, but the sign of the second.
- Use
Math.sign()
to check if the two numbers have the same sign. - Return
x
if they do,-x
otherwise.
const copySign = (x, y) => Math.sign(x) === Math.sign(y) ? x : -x;
copySign(2, 3); // 2
copySign(2, -3); // -2
copySign(-2, 3); // 2
copySign(-2, -3); // -2
copyToClipboard
- title: copyToClipboard
- tags: browser,string,event,advanced
Copies a string to the clipboard.
Only works as a result of user action (i.e. inside a click
event listener).
- Create a new
<textarea>
element, fill it with the supplied data and add it to the HTML document. - Use
Selection.getRangeAt()
to store the selected range (if any). - Use
Document.execCommand('copy')
to copy to the clipboard. - Remove the
<textarea>
element from the HTML document. - Finally, use
Selection().addRange()
to recover the original selected range (if any). - ⚠️ NOTICE: The same functionality can be easily implemented by using the new asynchronous Clipboard API, which is still experimental but should be used in the future instead of this snippet. Find out more about it here.
const copyToClipboard = str => {
const el = document.createElement('textarea');
el.value = str;
el.setAttribute('readonly', '');
el.style.position = 'absolute';
el.style.left = '-9999px';
document.body.appendChild(el);
const selected =
document.getSelection().rangeCount > 0
? document.getSelection().getRangeAt(0)
: false;
el.select();
document.execCommand('copy');
document.body.removeChild(el);
if (selected) {
document.getSelection().removeAllRanges();
document.getSelection().addRange(selected);
}
};
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.
countBy
- title: countBy
- tags: array,object,intermediate
Groups the elements of an array based on the given function and returns the count of elements in each group.
- Use
Array.prototype.map()
to map the values of an array to a function or property name. - Use
Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const countBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
acc[val] = (acc[val] || 0) + 1;
return acc;
}, {});
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
countBy([{ count: 5 }, { count: 10 }, { count: 5 }], x => x.count)
// {5: 2, 10: 1}
countOccurrences
- title: countOccurrences
- tags: array,intermediate
Counts the occurrences of a value in an array.
- Use
Array.prototype.reduce()
to increment a counter each time the specific value is encountered inside the array.
const countOccurrences = (arr, val) =>
arr.reduce((a, v) => (v === val ? a + 1 : a), 0);
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3
countSubstrings
- title: countSubstrings
- tags: string,algorithm,beginner
Counts the occurrences of a substring in a given string.
- Use
Array.prototype.indexOf()
to look forsearchValue
instr
. - Increment a counter if the value is found and update the index,
i
. - Use a
while
loop that will return as soon as the value returned fromArray.prototype.indexOf()
is-1
.
const countSubstrings = (str, searchValue) => {
let count = 0,
i = 0;
while (true) {
const r = str.indexOf(searchValue, i);
if (r !== -1) [count, i] = [count + 1, r + 1];
else return count;
}
};
countSubstrings('tiktok tok tok tik tok tik', 'tik'); // 3
countSubstrings('tutut tut tut', 'tut'); // 4
countWeekDaysBetween
- title: countWeekDaysBetween
- tags: date,intermediate
Counts the weekdays between two dates.
- Use
Array.from()
to construct an array withlength
equal to the number of days betweenstartDate
andendDate
. - Use
Array.prototype.reduce()
to iterate over the array, checking if each date is a weekday and incrementingcount
. - Update
startDate
with the next day each loop usingDate.prototype.getDate()
andDate.prototype.setDate()
to advance it by one day. - NOTE: Does not take official holidays into account.
const countWeekDaysBetween = (startDate, endDate) =>
Array
.from({ length: (endDate - startDate) / (1000 * 3600 * 24) })
.reduce(count => {
if (startDate.getDay() % 6 !== 0) count++;
startDate = new Date(startDate.setDate(startDate.getDate() + 1));
return count;
}, 0);
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 06, 2020')); // 1
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 14, 2020')); // 7
counter
- title: counter
- tags: browser,advanced
Creates a counter with the specified range, step and duration for the specified selector.
- Check if
step
has the proper sign and change it accordingly. - Use
setInterval()
in combination withMath.abs()
andMath.floor()
to calculate the time between each new text draw. - Use
Document.querySelector()
,Element.innerHTML
to update the value of the selected element. - Omit the fourth argument,
step
, to use a default step of1
. - Omit the fifth argument,
duration
, to use a default duration of2000
ms.
const counter = (selector, start, end, step = 1, duration = 2000) => {
let current = start,
_step = (end - start) * step < 0 ? -step : step,
timer = setInterval(() => {
current += _step;
document.querySelector(selector).innerHTML = current;
if (current >= end) document.querySelector(selector).innerHTML = end;
if (current >= end) clearInterval(timer);
}, Math.abs(Math.floor(duration / (end - start))));
return timer;
};
counter('##my-id', 1, 1000, 5, 2000);
// Creates a 2-second timer for the element with id="my-id"
createDirIfNotExists
- title: createDirIfNotExists
- tags: node,beginner
Creates a directory, if it does not exist.
- Use
fs.existsSync()
to check if the directory exists,fs.mkdirSync()
to create it.
const fs = require('fs');
const createDirIfNotExists = dir => (!fs.existsSync(dir) ? fs.mkdirSync(dir) : undefined);
createDirIfNotExists('test');
// creates the directory 'test', if it doesn't exist
createElement
- title: createElement
- tags: browser,beginner
Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.
- Use
Document.createElement()
to create a new element. - Use
Element.innerHTML
to set its inner HTML to the string supplied as the argument. - Use
ParentNode.firstElementChild
to return the element version of the string.
const createElement = str => {
const el = document.createElement('div');
el.innerHTML = str;
return el.firstElementChild;
};
const el = createElement(
`<div class="container">
<p>Hello!</p>
</div>`
);
console.log(el.className); // 'container'
createEventHub
- title: createEventHub
- tags: browser,event,advanced
Creates a pub/sub (publish–subscribe) event hub with emit
, on
, and off
methods.
- Use
Object.create(null)
to create an emptyhub
object that does not inherit properties fromObject.prototype
. - For
emit
, resolve the array of handlers based on theevent
argument and then run each one withArray.prototype.forEach()
by passing in the data as an argument. - For
on
, create an array for the event if it does not yet exist, then useArray.prototype.push()
to add the handler - to the array.
- For
off
, useArray.prototype.findIndex()
to find the index of the handler in the event array and remove it usingArray.prototype.splice()
.
const createEventHub = () => ({
hub: Object.create(null),
emit(event, data) {
(this.hub[event] || []).forEach(handler => handler(data));
},
on(event, handler) {
if (!this.hub[event]) this.hub[event] = [];
this.hub[event].push(handler);
},
off(event, handler) {
const i = (this.hub[event] || []).findIndex(h => h === handler);
if (i > -1) this.hub[event].splice(i, 1);
if (this.hub[event].length === 0) delete this.hub[event];
}
});
const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;
// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);
// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1
// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);
currentURL
- title: currentURL
- tags: browser,beginner
Returns the current URL.
- Use
Window.location.href
to get the current URL.
const currentURL = () => window.location.href;
currentURL(); // 'https://www.google.com/'
curry
- title: curry
- tags: function,recursion,advanced
Curries a function.
- Use recursion.
- If the number of provided arguments (
args
) is sufficient, call the passed functionfn
. - Otherwise, use
Function.prototype.bind()
to return a curried functionfn
that expects the rest of the arguments. - If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g.
Math.min()
), you can optionally pass the number of arguments to the second parameterarity
.
const curry = (fn, arity = fn.length, ...args) =>
arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);
curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2
cycleGenerator
- title: cycleGenerator
- tags: function,generator,advanced
Creates a generator, looping over the given array indefinitely.
- Use a non-terminating
while
loop, that willyield
a value every timeGenerator.prototype.next()
is called. - Use the module operator (
%
) withArray.prototype.length
to get the next value's index and increment the counter after eachyield
statement.
const cycleGenerator = function* (arr) {
let i = 0;
while (true) {
yield arr[i % arr.length];
i++;
}
};
const binaryCycle = cycleGenerator([0, 1]);
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
dayName
- title: dayName
- tags: date,beginner
Gets the name of the weekday from a Date
object.
- Use
Date.prototype.toLocaleDateString()
with the{ weekday: 'long' }
option to retrieve the weekday. - Use the optional second argument to get a language-specific name or omit it to use the default locale.
const dayName = (date, locale) =>
date.toLocaleDateString(locale, { weekday: 'long' });
dayName(new Date()); // 'Saturday'
dayName(new Date('09/23/2020'), 'de-DE'); // 'Samstag'
dayOfYear
- title: dayOfYear
- tags: date,beginner
Gets the day of the year (number in the range 1-366) from a Date
object.
- Use
new Date()
andDate.prototype.getFullYear()
to get the first day of the year as aDate
object. - Subtract the first day of the year from
date
and divide with the milliseconds in each day to get the result. - Use
Math.floor()
to appropriately round the resulting day count to an integer.
const dayOfYear = date =>
Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);
dayOfYear(new Date()); // 272
daysAgo
- title: daysAgo
- tags: date,beginner
Calculates the date of n
days ago from today as a string representation.
- Use
new Date()
to get the current date,Math.abs()
andDate.prototype.getDate()
to update the date accordingly and set to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const daysAgo = n => {
let d = new Date();
d.setDate(d.getDate() - Math.abs(n));
return d.toISOString().split('T')[0];
};
daysAgo(20); // 2020-09-16 (if current date is 2020-10-06)
daysFromNow
- title: daysFromNow
- tags: date,beginner
Calculates the date of n
days from today as a string representation.
- Use
new Date()
to get the current date,Math.abs()
andDate.prototype.getDate()
to update the date accordingly and set to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const daysFromNow = n => {
let d = new Date();
d.setDate(d.getDate() + Math.abs(n));
return d.toISOString().split('T')[0];
};
daysFromNow(5); // 2020-10-13 (if current date is 2020-10-08)
debounce
- title: debounce
- tags: function,intermediate
Creates a debounced function that delays invoking the provided function until at least ms
milliseconds have elapsed since the last time it was invoked.
- Each time the debounced function is invoked, clear the current pending timeout with
clearTimeout()
and usesetTimeout()
to create a new timeout that delays invoking the function until at leastms
milliseconds has elapsed. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessary arguments. - Omit the second argument,
ms
, to set the timeout at a default of0
ms.
const debounce = (fn, ms = 0) => {
let timeoutId;
return function(...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => fn.apply(this, args), ms);
};
};
window.addEventListener(
'resize',
debounce(() => {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250ms
debouncePromise
- title: debouncePromise
- tags: function,promise,advanced
Creates a debounced function that returns a promise, but delays invoking the provided function until at least ms
milliseconds have elapsed since the last time it was invoked.
All promises returned during this time will return the same data.
- Each time the debounced function is invoked, clear the current pending timeout with
clearTimeout()
and usesetTimeout()
to create a new timeout that delays invoking the function until at leastms
milliseconds has elapsed. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessary arguments. - Create a new
Promise
and add itsresolve
andreject
callbacks to thepending
promises stack. - When
setTimeout
is called, copy the current stack (as it can change between the provided function call and its resolution), clear it and call the provided function. - When the provided function resolves/rejects, resolve/reject all promises in the stack (copied when the function was called) with the returned data.
- Omit the second argument,
ms
, to set the timeout at a default of0
ms.
const debouncePromise = (fn, ms = 0) => {
let timeoutId;
const pending = [];
return (...args) =>
new Promise((res, rej) => {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => {
const currentPending = [...pending];
pending.length = 0;
Promise.resolve(fn.apply(this, args)).then(
data => {
currentPending.forEach(({ resolve }) => resolve(data));
},
error => {
currentPending.forEach(({ reject }) => reject(error));
}
);
}, ms);
pending.push({ resolve: res, reject: rej });
});
};
const fn = arg => new Promise(resolve => {
setTimeout(resolve, 1000, ['resolved', arg]);
});
const debounced = debouncePromise(fn, 200);
debounced('foo').then(console.log);
debounced('bar').then(console.log);
// Will log ['resolved', 'bar'] both times
decapitalize
- title: decapitalize
- tags: string,intermediate
Decapitalizes the first letter of a string.
- Use array destructuring and
String.prototype.toLowerCase()
to decapitalize first letter,...rest
to get array of characters after first letter and thenArray.prototype.join('')
to make it a string again. - Omit the
upperRest
argument to keep the rest of the string intact, or set it totrue
to convert to uppercase.
const decapitalize = ([first, ...rest], upperRest = false) =>
first.toLowerCase() +
(upperRest ? rest.join('').toUpperCase() : rest.join(''));
decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'
deepClone
- title: deepClone
- tags: object,recursion,advanced
Creates a deep clone of an object. Clones primitives, arrays and objects, excluding class instances.
- Use recursion.
- Check if the passed object is
null
and, if so, returnnull
. - Use
Object.assign()
and an empty object ({}
) to create a shallow clone of the original. - Use
Object.keys()
andArray.prototype.forEach()
to determine which key-value pairs need to be deep cloned. - If the object is an
Array
, set theclone
'slength
to that of the original and useArray.from(clone)
to create a clone.
const deepClone = obj => {
if (obj === null) return null;
let clone = Object.assign({}, obj);
Object.keys(clone).forEach(
key =>
(clone[key] =
typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
);
if (Array.isArray(obj)) {
clone.length = obj.length;
return Array.from(clone);
}
return clone;
};
const a = { foo: 'bar', obj: { a: 1, b: 2 } };
const b = deepClone(a); // a !== b, a.obj !== b.obj
deepFlatten
- title: deepFlatten
- tags: array,recursion,intermediate
Deep flattens an array.
- Use recursion.
- Use
Array.prototype.concat()
with an empty array ([]
) and the spread operator (...
) to flatten an array. - Recursively flatten each element that is an array.
const deepFlatten = arr =>
[].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));
deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]
deepFreeze
- title: deepFreeze
- tags: object,recursion,intermediate
Deep freezes an object.
- Use
Object.keys()
to get all the properties of the passed object,Array.prototype.forEach()
to iterate over them. - Call
Object.freeze(obj)
recursively on all properties, applyingdeepFreeze()
as necessary. - Finally, use
Object.freeze()
to freeze the given object.
const deepFreeze = obj => {
Object.keys(obj).forEach(prop => {
if (typeof obj[prop] === 'object') deepFreeze(obj[prop]);
});
return Object.freeze(obj);
};
'use strict';
const val = deepFreeze([1, [2, 3]]);
val[0] = 3; // not allowed
val[1][0] = 4; // not allowed as well
deepGet
- title: deepGet
- tags: object,intermediate
Gets the target value in a nested JSON object, based on the keys
array.
- Compare the keys you want in the nested JSON object as an
Array
. - Use
Array.prototype.reduce()
to get the values in the nested JSON object one by one. - If the key exists in the object, return the target value, otherwise return
null
.
const deepGet = (obj, keys) =>
keys.reduce(
(xs, x) => (xs && xs[x] !== null && xs[x] !== undefined ? xs[x] : null),
obj
);
let index = 2;
const data = {
foo: {
foz: [1, 2, 3],
bar: {
baz: ['a', 'b', 'c']
}
}
};
deepGet(data, ['foo', 'foz', index]); // get 3
deepGet(data, ['foo', 'bar', 'baz', 8, 'foz']); // null
deepMapKeys
- title: deepMapKeys
- tags: object,recursion,advanced
Deep maps an object's keys.
- Creates an object with the same values as the provided object and keys generated by running the provided function for each key.
- Use
Object.keys(obj)
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same values and mapped keys usingfn
.
const deepMapKeys = (obj, fn) =>
Array.isArray(obj)
? obj.map(val => deepMapKeys(val, fn))
: typeof obj === 'object'
? Object.keys(obj).reduce((acc, current) => {
const key = fn(current);
const val = obj[current];
acc[key] =
val !== null && typeof val === 'object' ? deepMapKeys(val, fn) : val;
return acc;
}, {})
: obj;
const obj = {
foo: '1',
nested: {
child: {
withArray: [
{
grandChild: ['hello']
}
]
}
}
};
const upperKeysObj = deepMapKeys(obj, key => key.toUpperCase());
/*
{
"FOO":"1",
"NESTED":{
"CHILD":{
"WITHARRAY":[
{
"GRANDCHILD":[ 'hello' ]
}
]
}
}
}
*/
defaults
- title: defaults
- tags: object,intermediate
Assigns default values for all properties in an object that are undefined
.
- Use
Object.assign()
to create a new empty object and copy the original one to maintain key order. - Use
Array.prototype.reverse()
and the spread operator (...
) to combine the default values from left to right. - Finally, use
obj
again to overwrite properties that originally had a value.
const defaults = (obj, ...defs) =>
Object.assign({}, obj, ...defs.reverse(), obj);
defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }
defer
- title: defer
- tags: function,intermediate
Defers invoking a function until the current call stack has cleared.
- Use
setTimeout()
with a timeout of1
ms to add a new event to the event queue and allow the rendering engine to complete its work. - Use the spread (
...
) operator to supply the function with an arbitrary number of arguments.
const defer = (fn, ...args) => setTimeout(fn, 1, ...args);
// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'
// Example B:
document.querySelector('##someElement').innerHTML = 'Hello';
longRunningFunction();
// Browser will not update the HTML until this has finished
defer(longRunningFunction);
// Browser will update the HTML then run the function
degreesToRads
- title: degreesToRads
- tags: math,beginner
Converts an angle from degrees to radians.
- Use
Math.PI
and the degree to radian formula to convert the angle from degrees to radians.
const degreesToRads = deg => (deg * Math.PI) / 180.0;
degreesToRads(90.0); // ~1.5708
delay
- title: delay
- tags: function,intermediate
Invokes the provided function after ms
milliseconds.
- Use
setTimeout()
to delay execution offn
. - Use the spread (
...
) operator to supply the function with an arbitrary number of arguments.
const delay = (fn, ms, ...args) => setTimeout(fn, ms, ...args);
delay(
function(text) {
console.log(text);
},
1000,
'later'
); // Logs 'later' after one second.
detectDeviceType
- title: detectDeviceType
- tags: browser,regexp,intermediate
Detects whether the page is being viewed on a mobile device or a desktop.
- Use a regular expression to test the
navigator.userAgent
property to figure out if the device is a mobile device or a desktop.
const detectDeviceType = () =>
/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(
navigator.userAgent
)
? 'Mobile'
: 'Desktop';
detectDeviceType(); // 'Mobile' or 'Desktop'
detectLanguage
- title: detectLanguage
- tags: browser,intermediate
Detects the preferred language of the current user.
- Use
NavigationLanguage.language
or the firstNavigationLanguage.languages
if available, otherwise returndefaultLang
. - Omit the second argument,
defaultLang
, to use'en-US'
as the default language code.
const detectLanguage = (defaultLang = 'en-US') =>
navigator.language ||
(Array.isArray(navigator.languages) && navigator.languages[0]) ||
defaultLang;
detectLanguage(); // 'nl-NL'
difference
- title: difference
- tags: array,beginner
Calculates the difference between two arrays, without filtering duplicate values.
- Create a
Set
fromb
to get the unique values inb
. - Use
Array.prototype.filter()
ona
to only keep values not contained inb
, usingSet.prototype.has()
.
const difference = (a, b) => {
const s = new Set(b);
return a.filter(x => !s.has(x));
};
difference([1, 2, 3, 3], [1, 2, 4]); // [3, 3]
differenceBy
- title: differenceBy
- tags: array,intermediate
Returns the difference between two arrays, after applying the provided function to each array element of both.
- Create a
Set
by applyingfn
to each element inb
. - Use
Array.prototype.map()
to applyfn
to each element ina
. - Use
Array.prototype.filter()
in combination withfn
ona
to only keep values not contained inb
, usingSet.prototype.has()
.
const differenceBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.map(fn).filter(el => !s.has(el));
};
differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]
differenceWith
- title: differenceWith
- tags: array,intermediate
Filters out all values from an array for which the comparator function does not return true
.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
to find the appropriate values. - Omit the last argument,
comp
, to use a default strict equality comparator.
const differenceWith = (arr, val, comp = (a, b) => a === b) =>
arr.filter(a => val.findIndex(b => comp(a, b)) === -1);
differenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2]
differenceWith([1, 1.2, 1.3], [1, 1.3, 1.5]); // [1.2]
dig
- title: dig
- tags: object,recursion,intermediate
Gets the target value in a nested JSON object, based on the given key.
- Use the
in
operator to check iftarget
exists inobj
. - If found, return the value of
obj[target]
. - Otherwise use
Object.values(obj)
andArray.prototype.reduce()
to recursively calldig
on each nested object until the first matching key/value pair is found.
const dig = (obj, target) =>
target in obj
? obj[target]
: Object.values(obj).reduce((acc, val) => {
if (acc !== undefined) return acc;
if (typeof val === 'object') return dig(val, target);
}, undefined);
const data = {
level1: {
level2: {
level3: 'some data'
}
}
};
dig(data, 'level3'); // 'some data'
dig(data, 'level4'); // undefined
digitize
- title: digitize
- tags: math,beginner
Converts a number to an array of digits, removing its sign if necessary.
- Use
Math.abs()
to strip the number's sign. - Convert the number to a string, using the spread operator (
...
) to build an array. - Use
Array.prototype.map()
andparseInt()
to transform each value to an integer.
const digitize = n => [...`${Math.abs(n)}`].map(i => parseInt(i));
digitize(123); // [1, 2, 3]
digitize(-123); // [1, 2, 3]
distance
- title: distance
- tags: math,algorithm,beginner
Calculates the distance between two points.
- Use
Math.hypot()
to calculate the Euclidean distance between two points.
const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);
distance(1, 1, 2, 3); // ~2.2361
divmod
- title: divmod
- tags: math,beginner
Returns an array consisting of the quotient and remainder of the given numbers.
- Use
Math.floor()
to get the quotient of the divisionx / y
. - Use the modulo operator (
%
) to get the remainder of the divisionx / y
.
const divmod = (x, y) => [Math.floor(x / y), x % y];
divmod(8, 3); // [2, 2]
divmod(3, 8); // [0, 3]
divmod(5, 5); // [1, 0]
drop
- title: drop
- tags: array,beginner
Creates a new array with n
elements removed from the left.
- Use
Array.prototype.slice()
to remove the specified number of elements from the left. - Omit the last argument,
n
, to use a default value of1
.
const drop = (arr, n = 1) => arr.slice(n);
drop([1, 2, 3]); // [2, 3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []
dropRight
- title: dropRight
- tags: array,beginner
Creates a new array with n
elements removed from the right.
- Use
Array.prototype.slice()
to remove the specified number of elements from the right. - Omit the last argument,
n
, to use a default value of1
.
const dropRight = (arr, n = 1) => arr.slice(0, -n);
dropRight([1, 2, 3]); // [1, 2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []
dropRightWhile
- title: dropRightWhile
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns true
.
Returns the remaining elements in the array.
- Loop through the array, using
Array.prototype.slice()
to drop the last element of the array until the value returned fromfunc
istrue
. - Return the remaining elements.
const dropRightWhile = (arr, func) => {
let rightIndex = arr.length;
while (rightIndex-- && !func(arr[rightIndex]));
return arr.slice(0, rightIndex + 1);
};
dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
dropWhile
- title: dropWhile
- tags: array,intermediate
Removes elements in an array until the passed function returns true
.
Returns the remaining elements in the array.
- Loop through the array, using
Array.prototype.slice()
to drop the first element of the array until the value returned fromfunc
istrue
. - Return the remaining elements.
const dropWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
return arr;
};
dropWhile([1, 2, 3, 4], n => n >= 3); // [3, 4]
either
- title: either
- tags: function,logic,beginner
Checks if at least one function returns true
for a given set of arguments.
- Use the logical or (
||
) operator on the result of calling the two functions with the suppliedargs
.
const either = (f, g) => (...args) => f(...args) || g(...args);
const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveOrEven = either(isPositive, isEven);
isPositiveOrEven(4); // true
isPositiveOrEven(3); // true
elementContains
- title: elementContains
- tags: browser,intermediate
Checks if the parent
element contains the child
element.
- Check that
parent
is not the same element aschild
. - Use
Node.contains()
to check if theparent
element contains thechild
element.
const elementContains = (parent, child) =>
parent !== child && parent.contains(child);
elementContains(
document.querySelector('head'),
document.querySelector('title')
);
// true
elementContains(document.querySelector('body'), document.querySelector('body'));
// false
elementIsFocused
- title: elementIsFocused
- tags: browser,beginner
Checks if the given element is focused.
- Use
Document.activeElement
to determine if the given element is focused.
const elementIsFocused = el => (el === document.activeElement);
elementIsFocused(el); // true if the element is focused
elementIsVisibleInViewport
- title: elementIsVisibleInViewport
- tags: browser,intermediate
Checks if the element specified is visible in the viewport.
- Use
Element.getBoundingClientRect()
and theWindow.inner(Width|Height)
values to determine if a given element is visible in the viewport. - Omit the second argument to determine if the element is entirely visible, or specify
true
to determine if it is partially visible.
const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
const { top, left, bottom, right } = el.getBoundingClientRect();
const { innerHeight, innerWidth } = window;
return partiallyVisible
? ((top > 0 && top < innerHeight) ||
(bottom > 0 && bottom < innerHeight)) &&
((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
: top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)
equals
- title: equals
- tags: object,array,type,advanced
Performs a deep comparison between two values to determine if they are equivalent.
- Check if the two values are identical, if they are both
Date
objects with the same time, usingDate.prototype.getTime()
or if they are both non-object values with an equivalent value (strict comparison). - Check if only one value is
null
orundefined
or if their prototypes differ. - If none of the above conditions are met, use
Object.keys()
to check if both values have the same number of keys. - Use
Array.prototype.every()
to check if every key ina
exists inb
and if they are equivalent by callingequals()
recursively.
const equals = (a, b) => {
if (a === b) return true;
if (a instanceof Date && b instanceof Date)
return a.getTime() === b.getTime();
if (!a || !b || (typeof a !== 'object' && typeof b !== 'object'))
return a === b;
if (a.prototype !== b.prototype) return false;
let keys = Object.keys(a);
if (keys.length !== Object.keys(b).length) return false;
return keys.every(k => equals(a[k], b[k]));
};
equals(
{ a: [2, { e: 3 }], b: [4], c: 'foo' },
{ a: [2, { e: 3 }], b: [4], c: 'foo' }
); // true
equals([1, 2, 3], { 0: 1, 1: 2, 2: 3 }); // true
escapeHTML
- title: escapeHTML
- tags: string,browser,regexp,intermediate
Escapes a string for use in HTML.
- Use
String.prototype.replace()
with a regexp that matches the characters that need to be escaped. - Use the callback function to replace each character instance with its associated escaped character using a dictionary (object).
const escapeHTML = str =>
str.replace(
/[&<>'"]/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
"'": '&##39;',
'"': '"'
}[tag] || tag)
);
escapeHTML('<a href="##">Me & you</a>');
// '<a href="##">Me & you</a>'
escapeRegExp
- title: escapeRegExp
- tags: string,regexp,intermediate
Escapes a string to use in a regular expression.
- Use
String.prototype.replace()
to escape special characters.
const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
escapeRegExp('(test)'); // \\(test\\)
euclideanDistance
- title: euclideanDistance
- tags: math,algorithm,intermediate
Calculates the distance between two points in any number of dimensions.
- Use
Object.keys()
andArray.prototype.map()
to map each coordinate to its difference between the two points. - Use
Math.hypot()
to calculate the Euclidean distance between the two points.
const euclideanDistance = (a, b) =>
Math.hypot(...Object.keys(a).map(k => b[k] - a[k]));
euclideanDistance([1, 1], [2, 3]); // ~2.2361
euclideanDistance([1, 1, 1], [2, 3, 2]); // ~2.4495
everyNth
- title: everyNth
- tags: array,beginner
Returns every nth
element in an array.
- Use
Array.prototype.filter()
to create a new array that contains everynth
element of a given array.
const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]
expandTabs
- title: expandTabs
- tags: string,regexp,beginner
Convert tabs to spaces, where each tab corresponds to count
spaces.
- Use
String.prototype.replace()
with a regular expression andString.prototype.repeat()
to replace each tab character withcount
spaces.
const expandTabs = (str, count) => str.replace(/\t/g, ' '.repeat(count));
expandTabs('\t\tlorem', 3); // ' lorem'
extendHex
- title: extendHex
- tags: string,intermediate
Extends a 3-digit color code to a 6-digit color code.
- Use
Array.prototype.map()
,String.prototype.split()
andArray.prototype.join()
to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form. Array.prototype.slice()
is used to remove##
from string start since it's added once.
const extendHex = shortHex =>
'##' +
shortHex
.slice(shortHex.startsWith('##') ? 1 : 0)
.split('')
.map(x => x + x)
.join('');
extendHex('##03f'); // '##0033ff'
extendHex('05a'); // '##0055aa'
factorial
- title: factorial
- tags: math,algorithm,recursion,beginner
Calculates the factorial of a number.
- Use recursion.
- If
n
is less than or equal to1
, return1
. - Otherwise, return the product of
n
and the factorial ofn - 1
. - Throw a
TypeError
ifn
is a negative number.
const factorial = n =>
n < 0
? (() => {
throw new TypeError('Negative numbers are not allowed!');
})()
: n <= 1
? 1
: n * factorial(n - 1);
factorial(6); // 720
fahrenheitToCelsius
- title: fahrenheitToCelsius
- tags: math,beginner unlisted: true
Converts Fahrenheit to Celsius.
- Follow the conversion formula
C = (F - 32) * 5/9
.
const fahrenheitToCelsius = degrees => (degrees - 32) * 5 / 9;
fahrenheitToCelsius(32); // 0
fibonacci
- title: fibonacci
- tags: math,algorithm,intermediate
Generates an array, containing the Fibonacci sequence, up until the nth term.
- Use
Array.from()
to create an empty array of the specific length, initializing the first two values (0
and1
). - Use
Array.prototype.reduce()
andArray.prototype.concat()
to add values into the array, using the sum of the last two values, except for the first two.
const fibonacci = n =>
Array.from({ length: n }).reduce(
(acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
[]
);
fibonacci(6); // [0, 1, 1, 2, 3, 5]
filterNonUnique
- title: filterNonUnique
- tags: array,beginner
Creates an array with the non-unique values filtered out.
- Use
new Set()
and the spread operator (...
) to create an array of the unique values inarr
. - Use
Array.prototype.filter()
to create an array containing only the unique values.
const filterNonUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]
filterNonUniqueBy
- title: filterNonUniqueBy
- tags: array,intermediate
Creates an array with the non-unique values filtered out, based on a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.every()
to create an array containing only the unique values, based on the comparator function,fn
. - The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterNonUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));
filterNonUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id === b.id
); // [ { id: 2, value: 'c' } ]
filterUnique
- title: filterUnique
- tags: array,beginner
Creates an array with the unique values filtered out.
- Use
new Set()
and the spread operator (...
) to create an array of the unique values inarr
. - Use
Array.prototype.filter()
to create an array containing only the non-unique values.
const filterUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) !== arr.lastIndexOf(i));
filterUnique([1, 2, 2, 3, 4, 4, 5]); // [2, 4]
filterUniqueBy
- title: filterUniqueBy
- tags: array,intermediate
Creates an array with the unique values filtered out, based on a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.every()
to create an array containing only the non-unique values, based on the comparator function,fn
. - The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.some((x, j) => (i !== j) === fn(v, x, i, j)));
filterUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 3, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 0, value: 'e' } ]
findKey
- title: findKey
- tags: object,intermediate
Finds the first key that satisfies the provided testing function.
Otherwise undefined
is returned.
- Use
Object.keys(obj)
to get all the properties of the object,Array.prototype.find()
to test each key-value pair usingfn
. - The callback receives three arguments - the value, the key and the object.
const findKey = (obj, fn) =>
Object.keys(obj).find(key => fn(obj[key], key, obj));
findKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
x => x['active']
); // 'barney'
findKeys
- title: findKeys
- tags: object,beginner
Finds all the keys in the provided object that match the given value.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.filter()
to test each key-value pair and return all keys that are equal to the given value.
const findKeys = (obj, val) =>
Object.keys(obj).filter(key => obj[key] === val);
const ages = {
Leo: 20,
Zoey: 21,
Jane: 20,
};
findKeys(ages, 20); // [ 'Leo', 'Jane' ]
findLast
- title: findLast
- tags: array,beginner
Finds the last element for which the provided function returns a truthy value.
- Use
Array.prototype.filter()
to remove elements for whichfn
returns falsy values. - Use
Array.prototype.pop()
to get the last element in the filtered array.
const findLast = (arr, fn) => arr.filter(fn).pop();
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3
findLastIndex
- title: findLastIndex
- tags: array,intermediate
Finds the index of the last element for which the provided function returns a truthy value.
- Use
Array.prototype.map()
to map each element to an array with its index and value. - Use
Array.prototype.filter()
to remove elements for whichfn
returns falsy values - Use
Array.prototype.pop()
to get the last element in the filtered array. - Return
-1
if there are no matching elements.
const findLastIndex = (arr, fn) =>
(arr
.map((val, i) => [i, val])
.filter(([i, val]) => fn(val, i, arr))
.pop() || [-1])[0];
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)
findLastKey
- title: findLastKey
- tags: object,intermediate
Finds the last key that satisfies the provided testing function.
Otherwise undefined
is returned.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.reverse()
to reverse the order andArray.prototype.find()
to test the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const findLastKey = (obj, fn) =>
Object.keys(obj)
.reverse()
.find(key => fn(obj[key], key, obj));
findLastKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
x => x['active']
); // 'pebbles'
flatten
- title: flatten
- tags: array,recursion,intermediate
Flattens an array up to the specified depth.
- Use recursion, decrementing
depth
by1
for each level of depth. - Use
Array.prototype.reduce()
andArray.prototype.concat()
to merge elements or arrays. - Base case, for
depth
equal to1
stops recursion. - Omit the second argument,
depth
, to flatten only to a depth of1
(single flatten).
const flatten = (arr, depth = 1) =>
arr.reduce(
(a, v) =>
a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v),
[]
);
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]
flattenObject
- title: flattenObject
- tags: object,recursion,advanced
Flattens an object with the paths for keys.
- Use recursion.
- Use
Object.keys(obj)
combined withArray.prototype.reduce()
to convert every leaf node to a flattened path node. - If the value of a key is an object, the function calls itself with the appropriate
prefix
to create the path usingObject.assign()
. - Otherwise, it adds the appropriate prefixed key-value pair to the accumulator object.
- You should always omit the second argument,
prefix
, unless you want every key to have a prefix.
const flattenObject = (obj, prefix = '') =>
Object.keys(obj).reduce((acc, k) => {
const pre = prefix.length ? `${prefix}.` : '';
if (
typeof obj[k] === 'object' &&
obj[k] !== null &&
Object.keys(obj[k]).length > 0
)
Object.assign(acc, flattenObject(obj[k], pre + k));
else acc[pre + k] = obj[k];
return acc;
}, {});
flattenObject({ a: { b: { c: 1 } }, d: 1 }); // { 'a.b.c': 1, d: 1 }
flip
- title: flip
- tags: function,intermediate
Takes a function as an argument, then makes the first argument the last.
- Use argument destructuring and a closure with variadic arguments.
- Splice the first argument, using the spread operator (
...
), to make it the last before applying the rest.
const flip = fn => (first, ...rest) => fn(...rest, first);
let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b
forEachRight
- title: forEachRight
- tags: array,intermediate
Executes a provided function once for each array element, starting from the array's last element.
- Use
Array.prototype.slice()
to clone the given array andArray.prototype.reverse()
to reverse it. - Use
Array.prototype.forEach()
to iterate over the reversed array.
const forEachRight = (arr, callback) =>
arr
.slice()
.reverse()
.forEach(callback);
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'
forOwn
- title: forOwn
- tags: object,intermediate
Iterates over all own properties of an object, running a callback for each one.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.forEach()
to run the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const forOwn = (obj, fn) =>
Object.keys(obj).forEach(key => fn(obj[key], key, obj));
forOwn({ foo: 'bar', a: 1 }, v => console.log(v)); // 'bar', 1
forOwnRight
- title: forOwnRight
- tags: object,intermediate
Iterates over all own properties of an object in reverse, running a callback for each one.
- Use
Object.keys(obj)
to get all the properties of the object,Array.prototype.reverse()
to reverse their order. - Use
Array.prototype.forEach()
to run the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const forOwnRight = (obj, fn) =>
Object.keys(obj)
.reverse()
.forEach(key => fn(obj[key], key, obj));
forOwnRight({ foo: 'bar', a: 1 }, v => console.log(v)); // 1, 'bar'
formToObject
- title: formToObject
- tags: browser,object,intermediate
Encodes a set of form elements as an object
.
- Use the
Foata
constructor to convert the HTMLform
toFoata
andArray.from()
to convert to an array. - Collect the object from the array using
Array.prototype.reduce()
.
const formToObject = form =>
Array.from(new Foata(form)).reduce(
(acc, [key, value]) => ({
...acc,
[key]: value
}),
{}
);
formToObject(document.querySelector('##form'));
// { email: 'test@email.com', name: 'Test Name' }
formatDuration
- title: formatDuration
- tags: date,math,string,intermediate
Returns the human-readable format of the given number of milliseconds.
- Divide
ms
with the appropriate values to obtain the appropriate values forday
,hour
,minute
,second
andmillisecond
. - Use
Object.entries()
withArray.prototype.filter()
to keep only non-zero values. - Use
Array.prototype.map()
to create the string for each value, pluralizing appropriately. - Use
String.prototype.join(', ')
to combine the values into a string.
const formatDuration = ms => {
if (ms < 0) ms = -ms;
const time = {
day: Math.floor(ms / 86400000),
hour: Math.floor(ms / 3600000) % 24,
minute: Math.floor(ms / 60000) % 60,
second: Math.floor(ms / 1000) % 60,
millisecond: Math.floor(ms) % 1000
};
return Object.entries(time)
.filter(val => val[1] !== 0)
.map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
.join(', ');
};
formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574);
// '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'
formatNumber
- title: formatNumber
- tags: string,math,beginner
Formats a number using the local number format order.
- Use
Number.prototype.toLocaleString()
to convert a number to using the local number format separators.
const formatNumber = num => num.toLocaleString();
formatNumber(123456); // '123,456' in `en-US`
formatNumber(15675436903); // '15.675.436.903' in `de-DE`
frequencies
- title: frequencies
- tags: array,object,intermediate
Creates an object with the unique values of an array as keys and their frequencies as the values.
- Use
Array.prototype.reduce()
to map unique values to an object's keys, adding to existing keys every time the same value is encountered.
const frequencies = arr =>
arr.reduce((a, v) => {
a[v] = a[v] ? a[v] + 1 : 1;
return a;
}, {});
frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // { a: 4, b: 2, c: 1 }
frequencies([...'ball']); // { b: 1, a: 1, l: 2 }
fromCamelCase
- title: fromCamelCase
- tags: string,intermediate
Converts a string from camelcase.
- Use
String.prototype.replace()
to break the string into words and add aseparator
between them. - Omit the second argument to use a default
separator
of_
.
const fromCamelCase = (str, separator = '_') =>
str
.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
.replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
.toLowerCase();
fromCamelCase('someDatabaseFieldName', ' '); // 'some database field name'
fromCamelCase('someLabelThatNeedsToBeDecamelized', '-');
// 'some-label-that-needs-to-be-decamelized'
fromCamelCase('someJavascriptProperty', '_'); // 'some_javascript_property'
fromCamelCase('JSONToCSV', '.'); // 'json.to.csv'
fromTimestamp
- title: fromTimestamp
- tags: date,beginner
Creates a Date
object from a Unix timestamp.
- Convert the timestamp to milliseconds by multiplying with
1000
. - Use
new Date()
to create a newDate
object.
const fromTimestamp = timestamp => new Date(timestamp * 1000);
fromTimestamp(1602162242); // 2020-10-08T13:04:02.000Z
frozenSet
- title: frozenSet
- tags: array,intermediate
Creates a frozen Set
object.
- Use the
new Set()
constructor to create a newSet
object fromiterable
. - Set the
add
,delete
andclear
methods of the newly created object toundefined
, so that they cannot be used, practically freezing the object.
const frozenSet = iterable => {
const s = new Set(iterable);
s.add = undefined;
s.delete = undefined;
s.clear = undefined;
return s;
};
frozenSet([1, 2, 3, 1, 2]);
// Set { 1, 2, 3, add: undefined, delete: undefined, clear: undefined }
fullscreen
- title: fullscreen
- tags: browser,intermediate
Opens or closes an element in fullscreen mode.
- Use
Document.querySelector()
andElement.requestFullscreen()
to open the given element in fullscreen. - Use
Document.exitFullscreen()
to exit fullscreen mode. - Omit the second argument,
el
, to usebody
as the default element. - Omit the first element,
mode
, to open the element in fullscreen mode by default.
const fullscreen = (mode = true, el = 'body') =>
mode
? document.querySelector(el).requestFullscreen()
: document.exitFullscreen();
fullscreen(); // Opens `body` in fullscreen mode
fullscreen(false); // Exits fullscreen mode
functionName
- title: functionName
- tags: function,beginner
Logs the name of a function.
- Use
console.debug()
and thename
property of the passed function to log the function's name to thedebug
channel of the console. - Return the given function
fn
.
const functionName = fn => (console.debug(fn.name), fn);
let m = functionName(Math.max)(5, 6);
// max (logged in debug channel of console)
// m = 6
functions
- title: functions
- tags: object,function,advanced
Gets an array of function property names from own (and optionally inherited) enumerable properties of an object.
- Use
Object.keys(obj)
to iterate over the object's own properties. - If
inherited
istrue
, useObject.getPrototypeOf(obj)
to also get the object's inherited properties. - Use
Array.prototype.filter()
to keep only those properties that are functions. - Omit the second argument,
inherited
, to not include inherited properties by default.
const functions = (obj, inherited = false) =>
(inherited
? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
: Object.keys(obj)
).filter(key => typeof obj[key] === 'function');
function Foo() {
this.a = () => 1;
this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']
gcd
- title: gcd
- tags: math,algorithm,recursion,intermediate
Calculates the greatest common divisor between two or more numbers/arrays.
- The inner
_gcd
function uses recursion. - Base case is when
y
equals0
. In this case, returnx
. - Otherwise, return the GCD of
y
and the remainder of the divisionx/y
.
const gcd = (...arr) => {
const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
return [...arr].reduce((a, b) => _gcd(a, b));
};
gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4
generateItems
- title: generateItems
- tags: array,function,intermediate
Generates an array with the given amount of items, using the given function.
- Use
Array.from()
to create an empty array of the specific length, callingfn
with the index of each newly created element. - The callback takes one argument - the index of each element.
const generateItems = (n, fn) => Array.from({ length: n }, (_, i) => fn(i));
generateItems(10, Math.random);
// [0.21, 0.08, 0.40, 0.96, 0.96, 0.24, 0.19, 0.96, 0.42, 0.70]
generatorToArray
- title: generatorToArray
- tags: function,array,generator,beginner
Converts the output of a generator function to an array.
- Use the spread operator (
...
) to convert the output of the generator function to an array.
const generatorToArray = gen => [...gen];
const s = new Set([1, 2, 1, 3, 1, 4]);
generatorToArray(s.entries()); // [[ 1, 1 ], [ 2, 2 ], [ 3, 3 ], [ 4, 4 ]]
geometricProgression
- title: geometricProgression
- tags: math,algorithm,intermediate
Initializes an array containing the numbers in the specified range where start
and end
are inclusive and the ratio between two terms is step
.
Returns an error if step
equals 1
.
- Use
Array.from()
,Math.log()
andMath.floor()
to create an array of the desired length,Array.prototype.map()
to fill with the desired values in a range. - Omit the second argument,
start
, to use a default value of1
. - Omit the third argument,
step
, to use a default value of2
.
const geometricProgression = (end, start = 1, step = 2) =>
Array.from({
length: Math.floor(Math.log(end / start) / Math.log(step)) + 1,
}).map((_, i) => start * step ** i);
geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]
get
- title: get
- tags: object,regexp,intermediate
Retrieves a set of properties indicated by the given selectors from an object.
- Use
Array.prototype.map()
for each selector,String.prototype.replace()
to replace square brackets with dots. - Use
String.prototype.split('.')
to split each selector. - Use
Array.prototype.filter()
to remove empty values andArray.prototype.reduce()
to get the value indicated by each selector.
const get = (from, ...selectors) =>
[...selectors].map(s =>
s
.replace(/\[([^\[\]]*)\]/g, '.$1.')
.split('.')
.filter(t => t !== '')
.reduce((prev, cur) => prev && prev[cur], from)
);
const obj = {
selector: { to: { val: 'val to select' } },
target: [1, 2, { a: 'test' }],
};
get(obj, 'selector.to.val', 'target[0]', 'target[2].a');
// ['val to select', 1, 'test']
getAncestors
- title: getAncestors
- tags: browser,beginner
Returns all the ancestors of an element from the document root to the given element.
- Use
Node.parentNode
and awhile
loop to move up the ancestor tree of the element. - Use
Array.prototype.unshift()
to add each new ancestor to the start of the array.
const getAncestors = el => {
let ancestors = [];
while (el) {
ancestors.unshift(el);
el = el.parentNode;
}
return ancestors;
};
getAncestors(document.querySelector('nav'));
// [document, html, body, header, nav]
getBaseURL
- title: getBaseURL
- tags: string,browser,regexp,beginner
Gets the current URL without any parameters or fragment identifiers.
- Use
String.prototype.replace()
with an appropriate regular expression to remove everything after either'?'
or'##'
, if found.
const getBaseURL = url => url.replace(/[?##].*$/, '');
getBaseURL('http://url.com/page?name=Adam&surname=Smith');
// 'http://url.com/page'
getColonTimeFrate
- title: getColonTimeFrate
- tags: date,string,beginner
Returns a string of the form HH:MM:SS
from a Date
object.
- Use
Date.prototype.toTimeString()
andString.prototype.slice()
to get theHH:MM:SS
part of a givenDate
object.
const getColonTimeFrate = date => date.toTimeString().slice(0, 8);
getColonTimeFrate(new Date()); // '08:38:00'
getDaysDiffBetweenDates
- title: getDaysDiffBetweenDates
- tags: date,intermediate
Calculates the difference (in days) between two dates.
- Subtract the two
Date
object and divide by the number of milliseconds in a day to get the difference (in days) between them.
const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
(dateFinal - dateInitial) / (1000 * 3600 * 24);
getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9
getElementsBiggerThanViewport
- title: getElementsBiggerThanViewport
- tags: browser,intermediate
Returns an array of HTML elements whose width is larger than that of the viewport's.
- Use
HTMLElement.offsetWidth
to get the width of thedocument
. - Use
Array.prototype.filter()
on the result ofDocument.querySelectorAll()
to check the width of all elements in the document.
const getElementsBiggerThanViewport = () => {
const docWidth = document.documentElement.offsetWidth;
return [...document.querySelectorAll('*')].filter(
el => el.offsetWidth > docWidth
);
};
getElementsBiggerThanViewport(); // <div id="ultra-wide-item" />
getImages
- title: getImages
- tags: browser,intermediate
Fetches all images from within an element and puts them into an array.
- Use
Element.getElementsByTagName()
to get all<img>
elements inside the provided element. - Use
Array.prototype.map()
to map everysrc
attribute of each<img>
element. - If
includeDuplicates
isfalse
, create a newSet
to eliminate duplicates and return it after spreading into an array. - Omit the second argument,
includeDuplicates
, to discard duplicates by default.
const getImages = (el, includeDuplicates = false) => {
const images = [...el.getElementsByTagName('img')].map(img =>
img.getAttribute('src')
);
return includeDuplicates ? images : [...new Set(images)];
};
getImages(document, true); // ['image1.jpg', 'image2.png', 'image1.png', '...']
getImages(document, false); // ['image1.jpg', 'image2.png', '...']
getMeridiemSuffixOfInteger
- title: getMeridiemSuffixOfInteger
- tags: date,beginner
Converts an integer to a suffixed string, adding am
or pm
based on its value.
- Use the modulo operator (
%
) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.
const getMeridiemSuffixOfInteger = num =>
num === 0 || num === 24
? 12 + 'am'
: num === 12
? 12 + 'pm'
: num < 12
? (num % 12) + 'am'
: (num % 12) + 'pm';
getMeridiemSuffixOfInteger(0); // '12am'
getMeridiemSuffixOfInteger(11); // '11am'
getMeridiemSuffixOfInteger(13); // '1pm'
getMeridiemSuffixOfInteger(25); // '1pm'
getMonthsDiffBetweenDates
- title: getMonthsDiffBetweenDates
- tags: date,intermediate
Calculates the difference (in months) between two dates.
- Use
Date.prototype.getFullYear()
andDate.prototype.getMonth()
to calculate the difference (in months) between twoDate
objects.
const getMonthsDiffBetweenDates = (dateInitial, dateFinal) =>
Math.max(
(dateFinal.getFullYear() - dateInitial.getFullYear()) * 12 +
dateFinal.getMonth() -
dateInitial.getMonth(),
0
);
getMonthsDiffBetweenDates(new Date('2017-12-13'), new Date('2018-04-29')); // 4
getParentsUntil
- title: getParentsUntil
- tags: browser,intermediate
Finds all the ancestors of an element up until the element matched by the specified selector.
- Use
Node.parentNode
and awhile
loop to move up the ancestor tree of the element. - Use
Array.prototype.unshift()
to add each new ancestor to the start of the array. - Use
Element.matches()
to check if the current element matches the specifiedselector
.
const getParentsUntil = (el, selector) => {
let parents = [],
_el = el.parentNode;
while (_el && typeof _el.matches === 'function') {
parents.unshift(_el);
if (_el.matches(selector)) return parents;
else _el = _el.parentNode;
}
return [];
};
getParentsUntil(document.querySelector('##home-link'), 'header');
// [header, nav, ul, li]
getProtocol
- title: getProtocol
- tags: browser,beginner
Gets the protocol being used on the current page.
- Use
Window.location.protocol
to get the protocol (http:
orhttps:
) of the current page.
const getProtocol = () => window.location.protocol;
getProtocol(); // 'https:'
getScrollPosition
- title: getScrollPosition
- tags: browser,intermediate
Returns the scroll position of the current page.
- Use
Window.pageXOffset
andWindow.pageYOffset
if they are defined, otherwiseElement.scrollLeft
andElement.scrollTop
. - Omit the single argument,
el
, to use a default value ofwindow
.
const getScrollPosition = (el = window) => ({
x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});
getScrollPosition(); // {x: 0, y: 200}
getSelectedText
- title: getSelectedText
- tags: browser,beginner
Gets the currently selected text.
- Use
Window.getSelection()
andSelection.toString()
to get the currently selected text.
const getSelectedText = () => window.getSelection().toString();
getSelectedText(); // 'Lorem ipsum'
getSiblings
- title: getSiblings
- tags: browser,intermediate
Returns an array containing all the siblings of the given element.
- Use
Node.parentNode
andNode.childNodes
to get aNodeList
of all the elements contained in the element's parent. - Use the spread operator (
...
) andArray.prototype.filter()
to convert to an array and remove the given element from it.
const getSiblings = el =>
[...el.parentNode.childNodes].filter(node => node !== el);
getSiblings(document.querySelector('head')); // ['body']
getStyle
- title: getStyle
- tags: browser,css,beginner
Retrieves the value of a CSS rule for the specified element.
- Use
Window.getComputedStyle()
to get the value of the CSS rule for the specified element.
const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];
getStyle(document.querySelector('p'), 'font-size'); // '16px'
getTimestamp
- title: getTimestamp
- tags: date,beginner
Gets the Unix timestamp from a Date
object.
- Use
Date.prototype.getTime()
to get the timestamp in milliseconds and divide by1000
to get the timestamp in seconds. - Use
Math.floor()
to appropriately round the resulting timestamp to an integer. - Omit the argument,
date
, to use the current date.
const getTimestamp = (date = new Date()) => Math.floor(date.getTime() / 1000);
getTimestamp(); // 1602162242
getType
- title: getType
- tags: type,beginner
Returns the native type of a value.
- Return
'undefined'
or'null'
if the value isundefined
ornull
. - Otherwise, use
Object.prototype.constructor.name
to get the name of the constructor.
const getType = v =>
(v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name);
getType(new Set([1, 2, 3])); // 'Set'
getURLParameters
- title: getURLParameters
- tags: browser,string,regexp,intermediate
Creates an object containing the parameters of the current URL.
- Use
String.prototype.match()
with an appropriate regular expression to get all key-value pairs. - Use
Array.prototype.reduce()
to map and combine them into a single object. - Pass
location.search
as the argument to apply to the currenturl
.
const getURLParameters = url =>
(url.match(/([^?=&]+)(=([^&]*))/g) || []).reduce(
(a, v) => (
(a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a
),
{}
);
getURLParameters('google.com'); // {}
getURLParameters('http://url.com/page?name=Adam&surname=Smith');
// {name: 'Adam', surname: 'Smith'}
getVerticalOffset
- title: getVerticalOffset
- tags: browser,beginner
Finds the distance from a given element to the top of the document.
- Use a
while
loop andHTMLElement.offsetParent
to move up the offset parents of the given element. - Add
HTMLElement.offsetTop
for each element and return the result.
const getVerticalOffset = el => {
let offset = el.offsetTop,
_el = el;
while (_el.offsetParent) {
_el = _el.offsetParent;
offset += _el.offsetTop;
}
return offset;
};
getVerticalOffset('.my-element'); // 120
groupBy
- title: groupBy
- tags: array,object,intermediate
Groups the elements of an array based on the given function.
- Use
Array.prototype.map()
to map the values of the array to a function or property name. - Use
Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const groupBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val, i) => {
acc[val] = (acc[val] || []).concat(arr[i]);
return acc;
}, {});
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}
hammingDistance
- title: hammingDistance
- tags: math,algorithm,intermediate
Calculates the Hamming distance between two values.
- Use the XOR operator (
^
) to find the bit difference between the two numbers. - Convert to a binary string using
Number.prototype.toString(2)
. - Count and return the number of
1
s in the string, usingString.prototype.match(/1/g)
.
const hammingDistance = (num1, num2) =>
((num1 ^ num2).toString(2).match(/1/g) || '').length;
hammingDistance(2, 3); // 1
hasClass
- title: hasClass
- tags: browser,css,beginner
Checks if the given element has the specified class.
- Use
Element.classList
andDOMTokenList.contains()
to check if the element has the specified class.
const hasClass = (el, className) => el.classList.contains(className);
hasClass(document.querySelector('p.special'), 'special'); // true
hasDuplicates
- title: hasDuplicates
- tags: array,beginner
Checks if there are duplicate values in a flat array.
- Use
Set()
to get the unique values in the array. - Use
Set.prototype.size
andArray.prototype.length
to check if the count of the unique values is the same as elements in the original array.
const hasDuplicates = arr => new Set(arr).size !== arr.length;
hasDuplicates([0, 1, 1, 2]); // true
hasDuplicates([0, 1, 2, 3]); // false
hasFlags
- title: hasFlags
- tags: node,intermediate
Checks if the current process's arguments contain the specified flags.
- Use
Array.prototype.every()
andArray.prototype.includes()
to check ifprocess.argv
contains all the specified flags. - Use a regular expression to test if the specified flags are prefixed with
-
or--
and prefix them accordingly.
const hasFlags = (...flags) =>
flags.every(flag =>
process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag)
);
// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // false
hasKey
- title: hasKey
- tags: object,intermediate
Checks if the target value exists in a JSON object.
- Check if
keys
is non-empty and useArray.prototype.every()
to sequentially check its keys to internal depth of the object,obj
. - Use
Object.prototype.hasOwnProperty()
to check ifobj
does not have the current key or is not an object, stop propagation and returnfalse
. - Otherwise assign the key's value to
obj
to use on the next iteration. - Return
false
beforehand if given key list is empty.
const hasKey = (obj, keys) => {
return (
keys.length > 0 &&
keys.every(key => {
if (typeof obj !== 'object' || !obj.hasOwnProperty(key)) return false;
obj = obj[key];
return true;
})
);
};
let obj = {
a: 1,
b: { c: 4 },
'b.d': 5
};
hasKey(obj, ['a']); // true
hasKey(obj, ['b']); // true
hasKey(obj, ['b', 'c']); // true
hasKey(obj, ['b.d']); // true
hasKey(obj, ['d']); // false
hasKey(obj, ['c']); // false
hasKey(obj, ['b', 'f']); // false
hashBrowser
- title: hashBrowser
- tags: browser,promise,advanced
Creates a hash for a value using the SHA-256 algorithm. Returns a promise.
- Use the SubtleCrypto API to create a hash for the given value.
- Create a new
TextEncoder
and use it to encodeval
, passing its value toSubtleCrypto.digest()
to generate a digest of the given data. - Use
DataView.prototype.getUint32()
to read data from the resolvedArrayBuffer
. - Add the data to an array using
Array.prototype.push()
after converting it to its hexadecimal representation usingNumber.prototype.toString(16)
. - Finally, use
Array.prototype.join()
to combine values in the array ofhexes
into a string.
const hashBrowser = val =>
crypto.subtle
.digest('SHA-256', new TextEncoder('utf-8').encode(val))
.then(h => {
let hexes = [],
view = new DataView(h);
for (let i = 0; i < view.byteLength; i += 4)
hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
return hexes.join('');
});
hashBrowser(
JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })
).then(console.log);
// '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'
hashNode
- title: hashNode
- tags: node,promise,advanced
Creates a hash for a value using the SHA-256 algorithm. Returns a promise.
- Use
crypto.createHash()
to create aHash
object with the appropriate algorithm. - Use
hash.update()
to add the data fromval
to theHash
,hash.digest()
to calculate the digest of the data. - Use
setTimeout()
to prevent blocking on a long operation, and return aPromise
to give it a familiar interface.
const crypto = require('crypto');
const hashNode = val =>
new Promise(resolve =>
setTimeout(
() => resolve(crypto.createHash('sha256').update(val).digest('hex')),
0
)
);
hashNode(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(
console.log
);
// '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'
haveSameContents
- title: haveSameContents
- tags: array,intermediate
Checks if two arrays contain the same elements regardless of order.
- Use a
for...of
loop over aSet
created from the values of both arrays. - Use
Array.prototype.filter()
to compare the amount of occurrences of each distinct value in both arrays. - Return
false
if the counts do not match for any element,true
otherwise.
const haveSameContents = (a, b) => {
for (const v of new Set([...a, ...b]))
if (a.filter(e => e === v).length !== b.filter(e => e === v).length)
return false;
return true;
};
haveSameContents([1, 2, 4], [2, 4, 1]); // true
head
- title: head
- tags: array,beginner
Returns the head of an array.
- Check if
arr
is truthy and has alength
property. - Use
arr[0]
if possible to return the first element, otherwise returnundefined
.
const head = arr => (arr && arr.length ? arr[0] : undefined);
head([1, 2, 3]); // 1
head([]); // undefined
head(null); // undefined
head(undefined); // undefined
heapsort
- title: heapsort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the heapsort algorithm.
- Use recursion.
- Use the spread operator (
...
) to clone the original array,arr
. - Use closures to declare a variable,
l
, and a functionheapify
. - Use a
for
loop andMath.floor()
in combination withheapify
to create a max heap from the array. - Use a
for
loop to repeatedly narrow down the considered range, usingheapify
and swapping values as necessary in order to sort the cloned array.
const heapsort = arr => {
const a = [...arr];
let l = a.length;
const heapify = (a, i) => {
const left = 2 * i + 1;
const right = 2 * i + 2;
let max = i;
if (left < l && a[left] > a[max]) max = left;
if (right < l && a[right] > a[max]) max = right;
if (max !== i) {
[a[max], a[i]] = [a[i], a[max]];
heapify(a, max);
}
};
for (let i = Math.floor(l / 2); i >= 0; i -= 1) heapify(a, i);
for (i = a.length - 1; i > 0; i--) {
[a[0], a[i]] = [a[i], a[0]];
l--;
heapify(a, 0);
}
return a;
};
heapsort([6, 3, 4, 1]); // [1, 3, 4, 6]
hexToRGB
- title: hexToRGB
- tags: string,math,advanced
Converts a color code to an rgb()
or rgba()
string if alpha value is provided.
- Use bitwise right-shift operator and mask bits with
&
(and) operator to convert a hexadecimal color code (with or without prefixed with##
) to a string with the RGB values. - If it's 3-digit color code, first convert to 6-digit version.
- If an alpha value is provided alongside 6-digit hex, give
rgba()
string in return.
const hexToRGB = hex => {
let alpha = false,
h = hex.slice(hex.startsWith('##') ? 1 : 0);
if (h.length === 3) h = [...h].map(x => x + x).join('');
else if (h.length === 8) alpha = true;
h = parseInt(h, 16);
return (
'rgb' +
(alpha ? 'a' : '') +
'(' +
(h >>> (alpha ? 24 : 16)) +
', ' +
((h & (alpha ? 0x00ff0000 : 0x00ff00)) >>> (alpha ? 16 : 8)) +
', ' +
((h & (alpha ? 0x0000ff00 : 0x0000ff)) >>> (alpha ? 8 : 0)) +
(alpha ? `, ${h & 0x000000ff}` : '') +
')'
);
};
hexToRGB('##27ae60ff'); // 'rgba(39, 174, 96, 255)'
hexToRGB('27ae60'); // 'rgb(39, 174, 96)'
hexToRGB('##fff'); // 'rgb(255, 255, 255)'
hide
- title: hide
- tags: browser,css,beginner
Hides all the elements specified.
- Use
NodeList.prototype.forEach()
to applydisplay: none
to each element specified.
const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));
hide(document.querySelectorAll('img')); // Hides all <img> elements on the page
httpDelete
- title: httpDelete
- tags: browser,intermediate
Makes a DELETE
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aDELETE
request to the givenurl
. - Handle the
onload
event, by running the providedcallback
function. - Handle the
onerror
event, by running the providederr
function. - Omit the third argument,
err
to log the request to the console's error stream by default.
const httpDelete = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('DELETE', url, true);
request.onload = () => callback(request);
request.onerror = () => err(request);
request.send();
};
httpDelete('https://jsonplaceholder.typicode.com/posts/1', request => {
console.log(request.responseText);
}); // Logs: {}
httpGet
- title: httpGet
- tags: browser,intermediate
Makes a GET
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aGET
request to the givenurl
. - Handle the
onload
event, by calling the givencallback
theresponseText
. - Handle the
onerror
event, by running the providederr
function. - Omit the third argument,
err
, to log errors to the console'serror
stream by default.
const httpGet = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('GET', url, true);
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send();
};
httpGet(
'https://jsonplaceholder.typicode.com/posts/1',
console.log
); /*
Logs: {
"userId": 1,
"id": 1,
"title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
"body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}
*/
httpPost
- title: httpPost
- tags: browser,intermediate
Makes a POST
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aPOST
request to the givenurl
. - Set the value of an
HTTP
request header withsetRequestHeader
method. - Handle the
onload
event, by calling the givencallback
theresponseText
. - Handle the
onerror
event, by running the providederr
function. - Omit the fourth argument,
err
, to log errors to the console'serror
stream by default.
const httpPost = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('POST', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send(data);
};
const newPost = {
userId: 1,
id: 1337,
- title: 'Foo',
body: 'bar bar bar'
};
const data = JSON.stringify(newPost);
httpPost(
'https://jsonplaceholder.typicode.com/posts',
data,
console.log
); /*
Logs: {
"userId": 1,
"id": 1337,
"title": "Foo",
"body": "bar bar bar"
}
*/
httpPost(
'https://jsonplaceholder.typicode.com/posts',
null, // does not send a body
console.log
); /*
Logs: {
"id": 101
}
*/
httpPut
- title: httpPut
- tags: browser,intermediate
Makes a PUT
request to the passed URL.
- Use
XMLHttpRequest
web api to make aPUT
request to the givenurl
. - Set the value of an
HTTP
request header withsetRequestHeader
method. - Handle the
onload
event, by running the providedcallback
function. - Handle the
onerror
event, by running the providederr
function. - Omit the last argument,
err
to log the request to the console's error stream by default.
const httpPut = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('PUT', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request);
request.onerror = () => err(request);
request.send(data);
};
const password = 'fooBaz';
const data = JSON.stringify({
id: 1,
- title: 'foo',
body: 'bar',
userId: 1
});
httpPut('https://jsonplaceholder.typicode.com/posts/1', data, request => {
console.log(request.responseText);
}); /*
Logs: {
id: 1,
- title: 'foo',
body: 'bar',
userId: 1
}
*/
httpsRedirect
- title: httpsRedirect
- tags: browser,intermediate
Redirects the page to HTTPS if it's currently in HTTP.
- Use
location.protocol
to get the protocol currently being used. - If it's not HTTPS, use
location.replace()
to replace the existing page with the HTTPS version of the page. - Use
location.href
to get the full address, split it withString.prototype.split()
and remove the protocol part of the URL. - Note that pressing the back button doesn't take it back to the HTTP page as its replaced in the history.
const httpsRedirect = () => {
if (location.protocol !== 'https:')
location.replace('https://' + location.href.split('//')[1]);
};
httpsRedirect();
// If you are on http://mydomain.com, you are redirected to https://mydomain.com
hz
- title: hz
- tags: function,intermediate unlisted: true
Measures the number of times a function is executed per second (hz
/hertz
).
- Use
performance.now()
to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the functioniterations
times. - Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed.
- Omit the second argument,
iterations
, to use the default of 100 iterations.
const hz = (fn, iterations = 100) => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return (1000 * iterations) / (performance.now() - before);
};
const numbers = Array(10000).fill().map((_, i) => i);
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
let sum = 0;
for (let i = 0; i < numbers.length; i++) sum += numbers[i];
return sum;
};
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784
inRange
- title: inRange
- tags: math,beginner
Checks if the given number falls within the given range.
- Use arithmetic comparison to check if the given number is in the specified range.
- If the second argument,
end
, is not specified, the range is considered to be from0
tostart
.
const inRange = (n, start, end = null) => {
if (end && start > end) [end, start] = [start, end];
return end == null ? n >= 0 && n < start : n >= start && n < end;
};
inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // false
includesAll
- title: includesAll
- tags: array,beginner
Checks if all the elements in values
are included in arr
.
- Use
Array.prototype.every()
andArray.prototype.includes()
to check if all elements ofvalues
are included inarr
.
const includesAll = (arr, values) => values.every(v => arr.includes(v));
includesAll([1, 2, 3, 4], [1, 4]); // true
includesAll([1, 2, 3, 4], [1, 5]); // false
includesAny
- title: includesAny
- tags: array,beginner
Checks if at least one element of values
is included in arr
.
- Use
Array.prototype.some()
andArray.prototype.includes()
to check if at least one element ofvalues
is included inarr
.
const includesAny = (arr, values) => values.some(v => arr.includes(v));
includesAny([1, 2, 3, 4], [2, 9]); // true
includesAny([1, 2, 3, 4], [8, 9]); // false
indentString
- title: indentString
- tags: string,beginner
Indents each line in the provided string.
- Use
String.prototype.replace()
and a regular expression to add the character specified byindent
count
times at the start of each line. - Omit the third argument,
indent
, to use a default indentation character of' '
.
const indentString = (str, count, indent = ' ') =>
str.replace(/^/gm, indent.repeat(count));
indentString('Lorem\nIpsum', 2); // ' Lorem\n Ipsum'
indentString('Lorem\nIpsum', 2, '_'); // '__Lorem\n__Ipsum'
indexOfAll
- title: indexOfAll
- tags: array,intermediate
Finds all indexes of val
in an array.
If val
never occurs, returns an empty array.
- Use
Array.prototype.reduce()
to loop over elements and store indexes for matching elements.
const indexOfAll = (arr, val) =>
arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3]
indexOfAll([1, 2, 3], 4); // []
indexOfSubstrings
- title: indexOfSubstrings
- tags: string,algorithm,generator,intermediate
Finds all the indexes of a substring in a given string.
- Use
Array.prototype.indexOf()
to look forsearchValue
instr
. - Use
yield
to return the index if the value is found and update the index,i
. - Use a
while
loop that will terminate the generator as soon as the value returned fromArray.prototype.indexOf()
is-1
.
const indexOfSubstrings = function* (str, searchValue) {
let i = 0;
while (true) {
const r = str.indexOf(searchValue, i);
if (r !== -1) {
yield r;
i = r + 1;
} else return;
}
};
[...indexOfSubstrings('tiktok tok tok tik tok tik', 'tik')]; // [0, 15, 23]
[...indexOfSubstrings('tutut tut tut', 'tut')]; // [0, 2, 6, 10]
[...indexOfSubstrings('hello', 'hi')]; // []
initial
- title: initial
- tags: array,beginner
Returns all the elements of an array except the last one.
- Use
Array.prototype.slice(0, -1)
to return all but the last element of the array.
const initial = arr => arr.slice(0, -1);
initial([1, 2, 3]); // [1, 2]
initialize2DArray
- title: initialize2DArray
- tags: array,intermediate
Initializes a 2D array of given width and height and value.
- Use
Array.from()
andArray.prototype.map()
to generateh
rows where each is a new array of sizew
. - Use
Array.prototype.fill()
to initialize all items with valueval
. - Omit the last argument,
val
, to use a default value ofnull
.
const initialize2DArray = (w, h, val = null) =>
Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));
initialize2DArray(2, 2, 0); // [[0, 0], [0, 0]]
initializeArrayWithRange
- title: initializeArrayWithRange
- tags: array,intermediate
Initializes an array containing the numbers in the specified range where start
and end
are inclusive with their common difference step
.
- Use
Array.from()
to create an array of the desired length. - Use
(end - start + 1)/step
and a map function to fill the array with the desired values in the given range. - Omit the second argument,
start
, to use a default value of0
. - Omit the last argument,
step
, to use a default value of1
.
const initializeArrayWithRange = (end, start = 0, step = 1) =>
Array.from(
{ length: Math.ceil((end - start + 1) / step) },
(_, i) => i * step + start
);
initializeArrayWithRange(5); // [0, 1, 2, 3, 4, 5]
initializeArrayWithRange(7, 3); // [3, 4, 5, 6, 7]
initializeArrayWithRange(9, 0, 2); // [0, 2, 4, 6, 8]
initializeArrayWithRangeRight
- title: initializeArrayWithRangeRight
- tags: array,intermediate
Initializes an array containing the numbers in the specified range (in reverse) where start
and end
are inclusive with their common difference step
.
- Use
Array.from(Math.ceil((end+1-start)/step))
to create an array of the desired length(the amounts of elements is equal to(end-start)/step
or(end+1-start)/step
for inclusive end),Array.prototype.map()
to fill with the desired values in a range. - Omit the second argument,
start
, to use a default value of0
. - Omit the last argument,
step
, to use a default value of1
.
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
(v, i, arr) => (arr.length - i - 1) * step + start
);
initializeArrayWithRangeRight(5); // [5, 4, 3, 2, 1, 0]
initializeArrayWithRangeRight(7, 3); // [7, 6, 5, 4, 3]
initializeArrayWithRangeRight(9, 0, 2); // [8, 6, 4, 2, 0]
initializeArrayWithValues
- title: initializeArrayWithValues
- tags: array,intermediate
Initializes and fills an array with the specified values.
- Use
Array.from()
to create an array of the desired length,Array.prototype.fill()
to fill it with the desired values. - Omit the last argument,
val
, to use a default value of0
.
const initializeArrayWithValues = (n, val = 0) =>
Array.from({ length: n }).fill(val);
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]
initializeNDArray
- title: initializeNDArray
- tags: array,recursion,intermediate
Create a n-dimensional array with given value.
- Use recursion.
- Use
Array.from()
,Array.prototype.map()
to generate rows where each is a new array initialized usinginitializeNDArray()
.
const initializeNDArray = (val, ...args) =>
args.length === 0
? val
: Array.from({ length: args[0] }).map(() =>
initializeNDArray(val, ...args.slice(1))
);
initializeNDArray(1, 3); // [1, 1, 1]
initializeNDArray(5, 2, 2, 2); // [[[5, 5], [5, 5]], [[5, 5], [5, 5]]]
injectCSS
- title: injectCSS
- tags: browser,css,intermediate
Injects the given CSS code into the current document
- Use
Document.createElement()
to create a newstyle
element and set its type totext/css
. - Use
Element.innerText
to set the value to the given CSS string. - Use
Document.head
andElement.appendChild()
to append the new element to the document head. - Return the newly created
style
element.
const injectCSS = css => {
let el = document.createElement('style');
el.type = 'text/css';
el.innerText = css;
document.head.appendChild(el);
return el;
};
injectCSS('body { background-color: ##000 }');
// '<style type="text/css">body { background-color: ##000 }</style>'
insertAfter
- title: insertAfter
- tags: browser,beginner
Inserts an HTML string after the end of the specified element.
- Use
Element.insertAdjacentHTML()
with a position of'afterend'
to parsehtmlString
and insert it after the end ofel
.
const insertAfter = (el, htmlString) =>
el.insertAdjacentHTML('afterend', htmlString);
insertAfter(document.getElementById('myId'), '<p>after</p>');
// <div id="myId">...</div> <p>after</p>
insertAt
- title: insertAt
- tags: array,intermediate
Mutates the original array to insert the given values after the specified index.
- Use
Array.prototype.splice()
with an appropriate index and a delete count of0
, spreading the given values to be inserted.
const insertAt = (arr, i, ...v) => {
arr.splice(i + 1, 0, ...v);
return arr;
};
let myArray = [1, 2, 3, 4];
insertAt(myArray, 2, 5); // myArray = [1, 2, 3, 5, 4]
let otherArray = [2, 10];
insertAt(otherArray, 0, 4, 6, 8); // otherArray = [2, 4, 6, 8, 10]
insertBefore
- title: insertBefore
- tags: browser,beginner
Inserts an HTML string before the start of the specified element.
- Use
Element.insertAdjacentHTML()
with a position of'beforebegin'
to parsehtmlString
and insert it before the start ofel
.
const insertBefore = (el, htmlString) =>
el.insertAdjacentHTML('beforebegin', htmlString);
insertBefore(document.getElementById('myId'), '<p>before</p>');
// <p>before</p> <div id="myId">...</div>
insertionSort
- title: insertionSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the insertion sort algorithm.
- Use
Array.prototype.reduce()
to iterate over all the elements in the given array. - If the
length
of the accumulator is0
, add the current element to it. - Use
Array.prototype.some()
to iterate over the results in the accumulator until the correct position is found. - Use
Array.prototype.splice()
to insert the current element into the accumulator.
const insertionSort = arr =>
arr.reduce((acc, x) => {
if (!acc.length) return [x];
acc.some((y, j) => {
if (x <= y) {
acc.splice(j, 0, x);
return true;
}
if (x > y && j === acc.length - 1) {
acc.splice(j + 1, 0, x);
return true;
}
return false;
});
return acc;
}, []);
insertionSort([6, 3, 4, 1]); // [1, 3, 4, 6]
intersection
- title: intersection
- tags: array,intermediate
Returns the elements that exist in both arrays, filtering duplicate values.
- Create a
Set
fromb
, then useArray.prototype.filter()
ona
to only keep values contained inb
.
const intersection = (a, b) => {
const s = new Set(b);
return [...new Set(a)].filter(x => s.has(x));
};
intersection([1, 2, 3], [4, 3, 2]); // [2, 3]
intersectionBy
- title: intersectionBy
- tags: array,intermediate
Returns the elements that exist in both arrays, after applying the provided function to each array element of both.
- Create a
Set
by applyingfn
to all elements inb
. - Use
Array.prototype.filter()
ona
to only keep elements, which produce values contained inb
whenfn
is applied to them.
const intersectionBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return [...new Set(a)].filter(x => s.has(fn(x)));
};
intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]
intersectionBy(
[{ - title: 'Apple' }, { - title: 'Orange' }],
[{ - title: 'Orange' }, { - title: 'Melon' }],
x => x.title
); // [{ - title: 'Orange' }]
intersectionWith
- title: intersectionWith
- tags: array,intermediate
Returns the elements that exist in both arrays, using a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
in combination with the provided comparator to determine intersecting values.
const intersectionWith = (a, b, comp) =>
a.filter(x => b.findIndex(y => comp(x, y)) !== -1);
intersectionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1.5, 3, 0]
invertKeyValues
- title: invertKeyValues
- tags: object,advanced
Inverts the key-value pairs of an object, without mutating it.
- Use
Object.keys()
andArray.prototype.reduce()
to invert the key-value pairs of an object and apply the function provided (if any). - Omit the second argument,
fn
, to get the inverted keys without applying a function to them. - The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.
const invertKeyValues = (obj, fn) =>
Object.keys(obj).reduce((acc, key) => {
const val = fn ? fn(obj[key]) : obj[key];
acc[val] = acc[val] || [];
acc[val].push(key);
return acc;
}, {});
invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value);
// { group1: [ 'a', 'c' ], group2: [ 'b' ] }
is
- title: is
- tags: type,array,intermediate
Checks if the provided value is of the specified type.
- Ensure the value is not
undefined
ornull
usingArray.prototype.includes()
. - Compare the
constructor
property on the value withtype
to check if the provided value is of the specifiedtype
.
const is = (type, val) => ![, null].includes(val) && val.constructor === type;
is(Array, [1]); // true
is(ArrayBuffer, new ArrayBuffer()); // true
is(Map, new Map()); // true
is(RegExp, /./g); // true
is(Set, new Set()); // true
is(WeakMap, new WeakMap()); // true
is(WeakSet, new WeakSet()); // true
is(String, ''); // true
is(String, new String('')); // true
is(Number, 1); // true
is(Number, new Number(1)); // true
is(Boolean, true); // true
is(Boolean, new Boolean(true)); // true
isAbsoluteURL
- title: isAbsoluteURL
- tags: string,browser,regexp,intermediate
Checks if the given string is an absolute URL.
- Use
RegExp.prototype.test()
to test if the string is an absolute URL.
const isAbsoluteURL = str => /^[a-z][a-z0-9+.-]*:/.test(str);
isAbsoluteURL('https://google.com'); // true
isAbsoluteURL('ftp://www.myserver.net'); // true
isAbsoluteURL('/foo/bar'); // false
isAfterDate
- title: isAfterDate
- tags: date,beginner
Checks if a date is after another date.
- Use the greater than operator (
>
) to check if the first date comes after the second one.
const isAfterDate = (dateA, dateB) => dateA > dateB;
isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // true
isAlpha
- title: isAlpha
- tags: string,regexp,beginner
Checks if a string contains only alpha characters.
- Use
RegExp.prototype.test()
to check if the given string matches against the alphabetic regexp pattern.
const isAlpha = str => /^[a-zA-Z]*$/.test(str);
isAlpha('sampleInput'); // true
isAlpha('this Will fail'); // false
isAlpha('123'); // false
isAlphaNumeric
- title: isAlphaNumeric
- tags: string,regexp,beginner
Checks if a string contains only alphanumeric characters.
- Use
RegExp.prototype.test()
to check if the input string matches against the alphanumeric regexp pattern.
const isAlphaNumeric = str => /^[a-z0-9]+$/gi.test(str);
isAlphaNumeric('hello123'); // true
isAlphaNumeric('123'); // true
isAlphaNumeric('hello 123'); // false (space character is not alphanumeric)
isAlphaNumeric('##$hello'); // false
isAnagram
- title: isAnagram
- tags: string,regexp,intermediate
Checks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).
- Use
String.prototype.toLowerCase()
andString.prototype.replace()
with an appropriate regular expression to remove unnecessary characters. - Use
String.prototype.split('')
,Array.prototype.sort()
andArray.prototype.join('')
on both strings to normalize them, then check if their normalized forms are equal.
const isAnagram = (str1, str2) => {
const normalize = str =>
str
.toLowerCase()
.replace(/[^a-z0-9]/gi, '')
.split('')
.sort()
.join('');
return normalize(str1) === normalize(str2);
};
isAnagram('iceman', 'cinema'); // true
isArrayLike
- title: isArrayLike
- tags: type,array,intermediate
Checks if the provided argument is array-like (i.e. is iterable).
- Check if the provided argument is not
null
and that itsSymbol.iterator
property is a function.
const isArrayLike = obj =>
obj != null && typeof obj[Symbol.iterator] === 'function';
isArrayLike([1, 2, 3]); // true
isArrayLike(document.querySelectorAll('.className')); // true
isArrayLike('abc'); // true
isArrayLike(null); // false
isAsyncFunction
- title: isAsyncFunction
- tags: type,function,intermediate
Checks if the given argument is an async
function.
- Use
Object.prototype.toString()
andFunction.prototype.call()
and check if the result is'[object AsyncFunction]'
.
const isAsyncFunction = val =>
Object.prototype.toString.call(val) === '[object AsyncFunction]';
isAsyncFunction(function() {}); // false
isAsyncFunction(async function() {}); // true
isBeforeDate
- title: isBeforeDate
- tags: date,beginner
Checks if a date is before another date.
- Use the less than operator (
<
) to check if the first date comes before the second one.
const isBeforeDate = (dateA, dateB) => dateA < dateB;
isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // true
isBetweenDates
- title: isBetweenDates
- tags: date,beginner
Checks if a date is between two other dates.
- Use the greater than (
>
) and less than (<
) operators to check ifdate
is betweendateStart
anddateEnd
.
const isBetweenDates = (dateStart, dateEnd, date) =>
date > dateStart && date < dateEnd;
isBetweenDates(
new Date(2010, 11, 20),
new Date(2010, 11, 30),
new Date(2010, 11, 19)
); // false
isBetweenDates(
new Date(2010, 11, 20),
new Date(2010, 11, 30),
new Date(2010, 11, 25)
); // true
isBoolean
- title: isBoolean
- tags: type,beginner
Checks if the given argument is a native boolean element.
- Use
typeof
to check if a value is classified as a boolean primitive.
const isBoolean = val => typeof val === 'boolean';
isBoolean(null); // false
isBoolean(false); // true
isBrowser
- title: isBrowser
- tags: browser,node,intermediate
Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.
- Use
Array.prototype.includes()
on thetypeof
values of bothwindow
anddocument
(globals usually only available in a browser environment unless they were explicitly defined), which will returntrue
if one of them isundefined
. typeof
allows globals to be checked for existence without throwing aReferenceError
.- If both of them are not
undefined
, then the current environment is assumed to be a browser.
const isBrowser = () => ![typeof window, typeof document].includes('undefined');
isBrowser(); // true (browser)
isBrowser(); // false (Node)
isBrowserTabFocused
- title: isBrowserTabFocused
- tags: browser,beginner
Checks if the browser tab of the page is focused.
- Use the
Document.hidden
property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.
const isBrowserTabFocused = () => !document.hidden;
isBrowserTabFocused(); // true
isContainedIn
- title: isContainedIn
- tags: array,intermediate
Checks if the elements of the first array are contained in the second one regardless of order.
- Use a
for...of
loop over aSet
created from the first array. - Use
Array.prototype.some()
to check if all distinct values are contained in the second array. - Use
Array.prototype.filter()
to compare the number of occurrences of each distinct value in both arrays. - Return
false
if the count of any element is greater in the first array than the second one,true
otherwise.
const isContainedIn = (a, b) => {
for (const v of new Set(a)) {
if (
!b.some(e => e === v) ||
a.filter(e => e === v).length > b.filter(e => e === v).length
)
return false;
}
return true;
};
isContainedIn([1, 4], [2, 4, 1]); // true
isDateValid
- title: isDateValid
- tags: date,intermediate
Checks if a valid date object can be created from the given values.
- Use the spread operator (
...
) to pass the array of arguments to theDate
constructor. - Use
Date.prototype.valueOf()
andNumber.isNaN()
to check if a validDate
object can be created from the given values.
const isDateValid = (...val) => !Number.isNaN(new Date(...val).valueOf());
isDateValid('December 17, 1995 03:24:00'); // true
isDateValid('1995-12-17T03:24:00'); // true
isDateValid('1995-12-17 T03:24:00'); // false
isDateValid('Duck'); // false
isDateValid(1995, 11, 17); // true
isDateValid(1995, 11, 17, 'Duck'); // false
isDateValid({}); // false
isDeepFrozen
- title: isDeepFrozen
- tags: object,recursion,intermediate
Checks if an object is deeply frozen.
- Use recursion.
- Use
Object.isFrozen()
on the given object. - Use
Object.keys()
,Array.prototype.every()
to check that all keys are either deeply frozen objects or non-object values.
const isDeepFrozen = obj =>
Object.isFrozen(obj) &&
Object.keys(obj).every(
prop => typeof obj[prop] !== 'object' || isDeepFrozen(obj[prop])
);
const x = Object.freeze({ a: 1 });
const y = Object.freeze({ b: { c: 2 } });
isDeepFrozen(x); // true
isDeepFrozen(y); // false
isDisjoint
- title: isDisjoint
- tags: array,intermediate
Checks if the two iterables are disjointed (have no common values).
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that the two iterables have no common values.
const isDisjoint = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sA].every(v => !sB.has(v));
};
isDisjoint(new Set([1, 2]), new Set([3, 4])); // true
isDisjoint(new Set([1, 2]), new Set([1, 3])); // false
isDivisible
- title: isDivisible
- tags: math,beginner
Checks if the first numeric argument is divisible by the second one.
- Use the modulo operator (
%
) to check if the remainder is equal to0
.
const isDivisible = (dividend, divisor) => dividend % divisor === 0;
isDivisible(6, 3); // true
isDuplexStream
- title: isDuplexStream
- tags: node,type,intermediate
Checks if the given argument is a duplex (readable and writable) stream.
- Check if the value is different from
null
. - Use
typeof
to check if a value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_read
,_write
and_readableState
,_writableState
properties arefunction
andobject
respectively.
const isDuplexStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';
const Stream = require('stream');
isDuplexStream(new Stream.Duplex()); // true
isEmpty
- title: isEmpty
- tags: type,array,object,string,beginner
Checks if the a value is an empty object/collection, has no enumerable properties or is any type that is not considered a collection.
- Check if the provided value is
null
or if itslength
is equal to0
.
const isEmpty = val => val == null || !(Object.keys(val) || val).length;
isEmpty([]); // true
isEmpty({}); // true
isEmpty(''); // true
isEmpty([1, 2]); // false
isEmpty({ a: 1, b: 2 }); // false
isEmpty('text'); // false
isEmpty(123); // true - type is not considered a collection
isEmpty(true); // true - type is not considered a collection
isEven
- title: isEven
- tags: math,beginner
Checks if the given number is even.
- Checks whether a number is odd or even using the modulo (
%
) operator. - Returns
true
if the number is even,false
if the number is odd.
const isEven = num => num % 2 === 0;
isEven(3); // false
isFunction
- title: isFunction
- tags: type,function,beginner
Checks if the given argument is a function.
- Use
typeof
to check if a value is classified as a function primitive.
const isFunction = val => typeof val === 'function';
isFunction('x'); // false
isFunction(x => x); // true
isGeneratorFunction
- title: isGeneratorFunction
- tags: type,function,intermediate
Checks if the given argument is a generator function.
- Use
Object.prototype.toString()
andFunction.prototype.call()
and check if the result is'[object GeneratorFunction]'
.
const isGeneratorFunction = val =>
Object.prototype.toString.call(val) === '[object GeneratorFunction]';
isGeneratorFunction(function() {}); // false
isGeneratorFunction(function*() {}); // true
isISOString
- title: isISOString
- tags: date,intermediate
Checks if the given string is valid in the simplified extended ISO format (ISO 8601).
- Use
new Date()
to create a date object from the given string. - Use
Date.prototype.valueOf()
andNumber.isNaN()
to check if the produced date object is valid. - Use
Date.prototype.toISOString()
to compare the ISO formatted string representation of the date with the original string.
const isISOString = val => {
const d = new Date(val);
return !Number.isNaN(d.valueOf()) && d.toISOString() === val;
};
isISOString('2020-10-12T10:10:10.000Z'); // true
isISOString('2020-10-12'); // false
isLeapYear
- title: isLeapYear
- tags: date,beginner
Checks if the given year
is a leap year.
- Use
new Date()
, setting the date to February 29th of the givenyear
. - Use
Date.prototype.getMonth()
to check if the month is equal to1
.
const isLeapYear = year => new Date(year, 1, 29).getMonth() === 1;
isLeapYear(2019); // false
isLeapYear(2020); // true
isLocalStorageEnabled
- title: isLocalStorageEnabled
- tags: browser,intermediate
Checks if localStorage
is enabled.
- Use a
try...catch
block to returntrue
if all operations complete successfully,false
otherwise. - Use
Storage.setItem()
andStorage.removeItem()
to test storing and deleting a value inwindow.localStorage
.
const isLocalStorageEnabled = () => {
try {
const key = `__storage__test`;
window.localStorage.setItem(key, null);
window.localStorage.removeItem(key);
return true;
} catch (e) {
return false;
}
};
isLocalStorageEnabled(); // true, if localStorage is accessible
isLowerCase
- title: isLowerCase
- tags: string,beginner
Checks if a string is lower case.
- Convert the given string to lower case, using
String.prototype.toLowerCase()
and compare it to the original.
const isLowerCase = str => str === str.toLowerCase();
isLowerCase('abc'); // true
isLowerCase('a3@$'); // true
isLowerCase('Ab4'); // false
isNegativeZero
- title: isNegativeZero
- tags: math,intermediate
Checks if the given value is equal to negative zero (-0
).
- Check whether a passed value is equal to
0
and if1
divided by the value equals-Infinity
.
const isNegativeZero = val => val === 0 && 1 / val === -Infinity;
isNegativeZero(-0); // true
isNegativeZero(0); // false
isNil
- title: isNil
- tags: type,beginner
Checks if the specified value is null
or undefined
.
- Use the strict equality operator to check if the value of
val
is equal tonull
orundefined
.
const isNil = val => val === undefined || val === null;
isNil(null); // true
isNil(undefined); // true
isNil(''); // false
isNode
- title: isNode
- tags: node,browser,intermediate
Determines if the current runtime environment is Node.js.
- Use the
process
global object that provides information about the current Node.js process. - Check if
process
is defined andprocess.versions
,process.versions.node
are notnull
.
const isNode = () =>
typeof process !== 'undefined' &&
process.versions !== null &&
process.versions.node !== null;
isNode(); // true (Node)
isNode(); // false (browser)
isNull
- title: isNull
- tags: type,beginner
Checks if the specified value is null
.
- Use the strict equality operator to check if the value of
val
is equal tonull
.
const isNull = val => val === null;
isNull(null); // true
isNumber
- title: isNumber
- tags: type,math,beginner
Checks if the given argument is a number.
- Use
typeof
to check if a value is classified as a number primitive. - To safeguard against
NaN
, check ifval === val
(asNaN
has atypeof
equal tonumber
and is the only value not equal to itself).
const isNumber = val => typeof val === 'number' && val === val;
isNumber(1); // true
isNumber('1'); // false
isNumber(NaN); // false
isObject
- title: isObject
- tags: type,object,beginner
Checks if the passed value is an object or not.
- Uses the
Object
constructor to create an object wrapper for the given value. - If the value is
null
orundefined
, create and return an empty object. - Otherwise, return an object of a type that corresponds to the given value.
const isObject = obj => obj === Object(obj);
isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // false
isObjectLike
- title: isObjectLike
- tags: type,object,beginner
Checks if a value is object-like.
- Check if the provided value is not
null
and itstypeof
is equal to'object'
.
const isObjectLike = val => val !== null && typeof val === 'object';
isObjectLike({}); // true
isObjectLike([1, 2, 3]); // true
isObjectLike(x => x); // false
isObjectLike(null); // false
isOdd
- title: isOdd
- tags: math,beginner
Checks if the given number is odd.
- Check whether a number is odd or even using the modulo (
%
) operator. - Return
true
if the number is odd,false
if the number is even.
const isOdd = num => num % 2 === 1;
isOdd(3); // true
isPlainObject
- title: isPlainObject
- tags: type,object,intermediate
Checks if the provided value is an object created by the Object constructor.
- Check if the provided value is truthy.
- Use
typeof
to check if it is an object andObject.prototype.constructor
to make sure the constructor is equal toObject
.
const isPlainObject = val =>
!!val && typeof val === 'object' && val.constructor === Object;
isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // false
isPowerOfTen
- title: isPowerOfTen
- tags: math,beginner
Checks if the given number is a power of 10
.
- Use
Math.log10()
and the modulo operator (%
) to determine ifn
is a power of10
.
const isPowerOfTen = n => Math.log10(n) % 1 === 0;
isPowerOfTen(1); // true
isPowerOfTen(10); // true
isPowerOfTen(20); // false
isPowerOfTwo
- title: isPowerOfTwo
- tags: math,beginner
Checks if the given number is a power of 2
.
- Use the bitwise binary AND operator (
&
) to determine ifn
is a power of2
. - Additionally, check that
n
is not falsy.
const isPowerOfTwo = n => !!n && (n & (n - 1)) == 0;
isPowerOfTwo(0); // false
isPowerOfTwo(1); // true
isPowerOfTwo(8); // true
isPrime
- title: isPrime
- tags: math,algorithm,beginner
Checks if the provided integer is a prime number.
- Check numbers from
2
to the square root of the given number. - Return
false
if any of them divides the given number, else returntrue
, unless the number is less than2
.
const isPrime = num => {
const boundary = Math.floor(Math.sqrt(num));
for (let i = 2; i <= boundary; i++) if (num % i === 0) return false;
return num >= 2;
};
isPrime(11); // true
isPrimitive
- title: isPrimitive
- tags: type,intermediate
Checks if the passed value is primitive or not.
- Create an object from
val
and compare it withval
to determine if the passed value is primitive (i.e. not equal to the created object).
const isPrimitive = val => Object(val) !== val;
isPrimitive(null); // true
isPrimitive(undefined); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // false
isPrimitive({}); // false
isPromiseLike
- title: isPromiseLike
- tags: type,function,promise,intermediate
Checks if an object looks like a Promise
.
- Check if the object is not
null
, itstypeof
matches eitherobject
orfunction
and if it has a.then
property, which is also afunction
.
const isPromiseLike = obj =>
obj !== null &&
(typeof obj === 'object' || typeof obj === 'function') &&
typeof obj.then === 'function';
isPromiseLike({
then: function() {
return '';
}
}); // true
isPromiseLike(null); // false
isPromiseLike({}); // false
isReadableStream
- title: isReadableStream
- tags: node,type,intermediate
Checks if the given argument is a readable stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_read
and_readableState
properties arefunction
andobject
respectively.
const isReadableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object';
const fs = require('fs');
isReadableStream(fs.createReadStream('test.txt')); // true
isSameDate
- title: isSameDate
- tags: date,beginner
Checks if a date is the same as another date.
- Use
Date.prototype.toISOString()
and strict equality checking (===
) to check if the first date is the same as the second one.
const isSameDate = (dateA, dateB) =>
dateA.toISOString() === dateB.toISOString();
isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // true
isSessionStorageEnabled
- title: isSessionStorageEnabled
- tags: browser,intermediate
Checks if sessionStorage
is enabled.
- Use a
try...catch
block to returntrue
if all operations complete successfully,false
otherwise. - Use
Storage.setItem()
andStorage.removeItem()
to test storing and deleting a value inwindow.sessionStorage
.
const isSessionStorageEnabled = () => {
try {
const key = `__storage__test`;
window.sessionStorage.setItem(key, null);
window.sessionStorage.removeItem(key);
return true;
} catch (e) {
return false;
}
};
isSessionStorageEnabled(); // true, if sessionStorage is accessible
isSorted
- title: isSorted
- tags: array,intermediate
Checks if a numeric array is sorted.
- Calculate the ordering
direction
for the first pair of adjacent array elements. - Return
0
if the given array is empty, only has one element or thedirection
changes for any pair of adjacent array elements. - Use
Math.sign()
to covert the final value ofdirection
to-1
(descending order) or1
(ascending order).
const isSorted = arr => {
if (arr.length <= 1) return 0;
const direction = arr[1] - arr[0];
for (let i = 2; i < arr.length; i++) {
if ((arr[i] - arr[i - 1]) * direction < 0) return 0;
}
return Math.sign(direction);
};
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0
isSorted([4]); // 0
isStream
- title: isStream
- tags: node,type,intermediate
Checks if the given argument is a stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
.
const isStream = val =>
val !== null && typeof val === 'object' && typeof val.pipe === 'function';
const fs = require('fs');
isStream(fs.createReadStream('test.txt')); // true
isString
- title: isString
- tags: type,string,beginner
Checks if the given argument is a string. Only works for string primitives.
- Use
typeof
to check if a value is classified as a string primitive.
const isString = val => typeof val === 'string';
isString('10'); // true
isSymbol
- title: isSymbol
- tags: type,beginner
Checks if the given argument is a symbol.
- Use
typeof
to check if a value is classified as a symbol primitive.
const isSymbol = val => typeof val === 'symbol';
isSymbol(Symbol('x')); // true
isTravisCI
- title: isTravisCI
- tags: node,intermediate
Checks if the current environment is Travis CI.
- Check if the current environment has the
TRAVIS
andCI
environment variables (reference).
const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;
isTravisCI(); // true (if code is running on Travis CI)
isUndefined
- title: isUndefined
- tags: type,beginner
Checks if the specified value is undefined
.
- Use the strict equality operator to check if
val
is equal toundefined
.
const isUndefined = val => val === undefined;
isUndefined(undefined); // true
isUpperCase
- title: isUpperCase
- tags: string,beginner
Checks if a string is upper case.
- Convert the given string to upper case, using
String.prototype.toUpperCase()
and compare it to the original.
const isUpperCase = str => str === str.toUpperCase();
isUpperCase('ABC'); // true
isUpperCase('A3@$'); // true
isUpperCase('aB4'); // false
isValidJSON
- title: isValidJSON
- tags: type,intermediate
Checks if the provided string is a valid JSON.
- Use
JSON.parse()
and atry... catch
block to check if the provided string is a valid JSON.
const isValidJSON = str => {
try {
JSON.parse(str);
return true;
} catch (e) {
return false;
}
};
isValidJSON('{"name":"Adam","age":20}'); // true
isValidJSON('{"name":"Adam",age:"20"}'); // false
isValidJSON(null); // true
isWeekday
- title: isWeekday
- tags: date,beginner
Checks if the given date is a weekday.
- Use
Date.prototype.getDay()
to check weekday by using a modulo operator (%
). - Omit the argument,
d
, to use the current date as default.
const isWeekday = (d = new Date()) => d.getDay() % 6 !== 0;
isWeekday(); // true (if current date is 2019-07-19)
isWeekend
- title: isWeekend
- tags: date,beginner
Checks if the given date is a weekend.
- Use
Date.prototype.getDay()
to check weekend by using a modulo operator (%
). - Omit the argument,
d
, to use the current date as default.
const isWeekend = (d = new Date()) => d.getDay() % 6 === 0;
isWeekend(); // 2018-10-19 (if current date is 2018-10-18)
isWritableStream
- title: isWritableStream
- tags: node,type,intermediate
Checks if the given argument is a writable stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_write
and_writableState
properties arefunction
andobject
respectively.
const isWritableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';
const fs = require('fs');
isWritableStream(fs.createWriteStream('test.txt')); // true
javascript
JAVASCRIPT SNIPPETS
CSVToArray
- title: CSVToArray
- tags: string,array,intermediate
Converts a comma-separated values (CSV) string to a 2D array.
- Use
Array.prototype.slice()
andArray.prototype.indexOf('\n')
to remove the first row (title row) ifomitFirstRow
istrue
. - Use
String.prototype.split('\n')
to create a string for each row, thenString.prototype.split(delimiter)
to separate the values in each row. - Omit the second argument,
delimiter
, to use a default delimiter of,
. - Omit the third argument,
omitFirstRow
, to include the first row (title row) of the CSV string.
const CSVToArray = (data, delimiter = ',', omitFirstRow = false) =>
data
.slice(omitFirstRow ? data.indexOf('\n') + 1 : 0)
.split('\n')
.map(v => v.split(delimiter));
CSVToArray('a,b\nc,d'); // [['a', 'b'], ['c', 'd']];
CSVToArray('a;b\nc;d', ';'); // [['a', 'b'], ['c', 'd']];
CSVToArray('col1,col2\na,b\nc,d', ',', true); // [['a', 'b'], ['c', 'd']];
CSVToJSON
- title: CSVToJSON
- tags: string,object,advanced
Converts a comma-separated values (CSV) string to a 2D array of objects. The first row of the string is used as the title row.
- Use
Array.prototype.slice()
andArray.prototype.indexOf('\n')
andString.prototype.split(delimiter)
to separate the first row (title row) into values. - Use
String.prototype.split('\n')
to create a string for each row, thenArray.prototype.map()
andString.prototype.split(delimiter)
to separate the values in each row. - Use
Array.prototype.reduce()
to create an object for each row's values, with the keys parsed from the title row. - Omit the second argument,
delimiter
, to use a default delimiter of,
.
const CSVToJSON = (data, delimiter = ',') => {
const titles = data.slice(0, data.indexOf('\n')).split(delimiter);
return data
.slice(data.indexOf('\n') + 1)
.split('\n')
.map(v => {
const values = v.split(delimiter);
return titles.reduce(
(obj, title, index) => ((obj[title] = values[index]), obj),
{}
);
});
};
CSVToJSON('col1,col2\na,b\nc,d');
// [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
CSVToJSON('col1;col2\na;b\nc;d', ';');
// [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
HSBToRGB
- title: HSBToRGB
- tags: math,intermediate
Converts a HSB color tuple to RGB format.
- Use the HSB to RGB conversion formula to convert to the appropriate format.
- The range of the input parameters is H: [0, 360], S: [0, 100], B: [0, 100].
- The range of all output values is [0, 255].
const HSBToRGB = (h, s, b) => {
s /= 100;
b /= 100;
const k = (n) => (n + h / 60) % 6;
const f = (n) => b * (1 - s * Math.max(0, Math.min(k(n), 4 - k(n), 1)));
return [255 * f(5), 255 * f(3), 255 * f(1)];
};
HSBToRGB(18, 81, 99); // [252.45, 109.31084999999996, 47.965499999999984]
HSLToRGB
- title: HSLToRGB
- tags: math,intermediate
Converts a HSL color tuple to RGB format.
- Use the HSL to RGB conversion formula to convert to the appropriate format.
- The range of the input parameters is H: [0, 360], S: [0, 100], L: [0, 100].
- The range of all output values is [0, 255].
const HSLToRGB = (h, s, l) => {
s /= 100;
l /= 100;
const k = n => (n + h / 30) % 12;
const a = s * Math.min(l, 1 - l);
const f = n =>
l - a * Math.max(-1, Math.min(k(n) - 3, Math.min(9 - k(n), 1)));
return [255 * f(0), 255 * f(8), 255 * f(4)];
};
HSLToRGB(13, 100, 11); // [56.1, 12.155, 0]
JSONToFile
- title: JSONToFile
- tags: node,intermediate
Writes a JSON object to a file.
- Use
fs.writeFileSync()
, template literals andJSON.stringify()
to write ajson
object to a.json
file.
const fs = require('fs');
const JSONToFile = (obj, filename) =>
fs.writeFileSync(`${filename}.json`, JSON.stringify(obj, null, 2));
JSONToFile({ test: 'is passed' }, 'testJsonFile');
// writes the object to 'testJsonFile.json'
JSONtoCSV
- title: JSONtoCSV
- tags: array,string,object,advanced
Converts an array of objects to a comma-separated values (CSV) string that contains only the columns
specified.
- Use
Array.prototype.join(delimiter)
to combine all the names incolumns
to create the first row. - Use
Array.prototype.map()
andArray.prototype.reduce()
to create a row for each object, substituting non-existent values with empty strings and only mapping values incolumns
. - Use
Array.prototype.join('\n')
to combine all rows into a string. - Omit the third argument,
delimiter
, to use a default delimiter of,
.
const JSONtoCSV = (arr, columns, delimiter = ',') =>
[
columns.join(delimiter),
...arr.map(obj =>
columns.reduce(
(acc, key) =>
`${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
''
)
),
].join('\n');
JSONtoCSV(
[{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
['a', 'b']
); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV(
[{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
['a', 'b'],
';'
); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'
RGBToHSB
- title: RGBToHSB
- tags: math,intermediate
Converts a RGB color tuple to HSB format.
- Use the RGB to HSB conversion formula to convert to the appropriate format.
- The range of all input parameters is [0, 255].
- The range of the resulting values is H: [0, 360], S: [0, 100], B: [0, 100].
const RGBToHSB = (r, g, b) => {
r /= 255;
g /= 255;
b /= 255;
const v = Math.max(r, g, b),
n = v - Math.min(r, g, b);
const h =
n && v === r ? (g - b) / n : v === g ? 2 + (b - r) / n : 4 + (r - g) / n;
return [60 * (h < 0 ? h + 6 : h), v && (n / v) * 100, v * 100];
};
RGBToHSB(252, 111, 48);
// [18.529411764705856, 80.95238095238095, 98.82352941176471]
RGBToHSL
- title: RGBToHSL
- tags: math,intermediate
Converts a RGB color tuple to HSL format.
- Use the RGB to HSL conversion formula to convert to the appropriate format.
- The range of all input parameters is [0, 255].
- The range of the resulting values is H: [0, 360], S: [0, 100], L: [0, 100].
const RGBToHSL = (r, g, b) => {
r /= 255;
g /= 255;
b /= 255;
const l = Math.max(r, g, b);
const s = l - Math.min(r, g, b);
const h = s
? l === r
? (g - b) / s
: l === g
? 2 + (b - r) / s
: 4 + (r - g) / s
: 0;
return [
60 * h < 0 ? 60 * h + 360 : 60 * h,
100 * (s ? (l <= 0.5 ? s / (2 * l - s) : s / (2 - (2 * l - s))) : 0),
(100 * (2 * l - s)) / 2,
];
};
RGBToHSL(45, 23, 11); // [21.17647, 60.71428, 10.98039]
RGBToHex
- title: RGBToHex
- tags: string,math,intermediate
Converts the values of RGB components to a hexadecimal color code.
- Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (
<<
) andNumber.prototype.toString(16)
. - Use
String.prototype.padStart(6, '0')
to get a 6-digit hexadecimal value.
const RGBToHex = (r, g, b) =>
((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');
RGBToHex(255, 165, 1); // 'ffa501'
URLJoin
- title: URLJoin
- tags: string,regexp,advanced
Joins all given URL segments together, then normalizes the resulting URL.
- Use
String.prototype.join('/')
to combine URL segments. - Use a series of
String.prototype.replace()
calls with various regexps to normalize the resulting URL (remove double slashes, add proper slashes for protocol, remove slashes before parameters, combine parameters with'&'
and normalize first parameter delimiter).
const URLJoin = (...args) =>
args
.join('/')
.replace(/[\/]+/g, '/')
.replace(/^(.+):\//, '$1://')
.replace(/^file:/, 'file:/')
.replace(/\/(\?|&|##[^!])/g, '$1')
.replace(/\?/g, '&')
.replace('&', '?');
URLJoin('http://www.google.com', 'a', '/b/cd', '?foo=123', '?bar=foo');
// 'http://www.google.com/a/b/cd?foo=123&bar=foo'
UUIDGeneratorBrowser
- title: UUIDGeneratorBrowser
- tags: browser,random,intermediate
Generates a UUID in a browser.
- Use
Crypto.getRandomValues()
to generate a UUID, compliant with RFC4122 version 4. - Use
Number.prototype.toString(16)
to convert it to a proper UUID.
const UUIDGeneratorBrowser = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(
c ^
(crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))
).toString(16)
);
UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'
UUIDGeneratorNode
- title: UUIDGeneratorNode
- tags: node,random,intermediate
Generates a UUID in Node.JS.
- Use
crypto.randomBytes()
to generate a UUID, compliant with RFC4122 version 4. - Use
Number.prototype.toString(16)
to convert it to a proper UUID.
const crypto = require('crypto');
const UUIDGeneratorNode = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
);
UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'
accumulate
- title: accumulate
- tags: math,array,intermediate
Creates an array of partial sums.
- Use
Array.prototype.reduce()
, initialized with an empty array accumulator to iterate overnums
. - Use
Array.prototype.slice(-1)
, the spread operator (...
) and the unary+
operator to add each value to the accumulator array containing the previous sums.
const accumulate = (...nums) =>
nums.reduce((acc, n) => [...acc, n + +acc.slice(-1)], []);
accumulate(1, 2, 3, 4); // [1, 3, 6, 10]
accumulate(...[1, 2, 3, 4]); // [1, 3, 6, 10]
addClass
- title: addClass
- tags: browser,beginner
Adds a class to an HTML element.
- Use
Element.classList
andDOMTokenList.add()
to add the specified class to the element.
const addClass = (el, className) => el.classList.add(className);
addClass(document.querySelector('p'), 'special');
// The paragraph will now have the 'special' class
addDaysToDate
- title: addDaysToDate
- tags: date,intermediate
Calculates the date of n
days from the given date, returning its string representation.
- Use
new Date()
to create a date object from the first argument. - Use
Date.prototype.getDate()
andDate.prototype.setDate()
to addn
days to the given date. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const addDaysToDate = (date, n) => {
const d = new Date(date);
d.setDate(d.getDate() + n);
return d.toISOString().split('T')[0];
};
addDaysToDate('2020-10-15', 10); // '2020-10-25'
addDaysToDate('2020-10-15', -10); // '2020-10-05'
addMinutesToDate
- title: addMinutesToDate
- tags: date,intermediate
Calculates the date of n
minutes from the given date, returning its string representation.
- Use
new Date()
to create a date object from the first argument. - Use
Date.prototype.getTime()
andDate.prototype.setTime()
to addn
minutes to the given date. - Use
Date.prototype.toISOString()
,String.prototype.split()
andString.prototype.replace()
to return a string inyyyy-mm-dd HH:MM:SS
format.
const addMinutesToDate = (date, n) => {
const d = new Date(date);
d.setTime(d.getTime() + n * 60000);
return d.toISOString().split('.')[0].replace('T',' ');
};
addMinutesToDate('2020-10-19 12:00:00', 10); // '2020-10-19 12:10:00'
addMinutesToDate('2020-10-19', -10); // '2020-10-18 23:50:00'
addMultipleEvents
- title: addMultipleListeners
- tags: browser,event,intermediate
Adds multiple event listeners with the same handler to an element.
- Use
Array.prototype.forEach()
andEventTarget.addEventListener()
to add multiple event listeners with an assigned callback function to an element.
const addMultipleListeners = (el, types, listener, options, useCapture) => {
types.forEach(type =>
el.addEventListener(type, listener, options, useCapture)
);
};
addMultipleListeners(
document.querySelector('.my-element'),
['click', 'mousedown'],
() => { console.log('hello!') }
);
addStyles
- title: addStyles
- tags: browser,beginner
Adds the provided styles to the given element.
- Use
Object.assign()
andElementCSSInlineStyle.style
to merge the providedstyles
object into the style of the given element.
const addStyles = (el, styles) => Object.assign(el.style, styles);
addStyles(document.getElementById('my-element'), {
background: 'red',
color: '##ffff00',
fontSize: '3rem'
});
addWeekDays
- title: addWeekDays
- tags: date,intermediate
Calculates the date after adding the given number of business days.
- Use
Array.from()
to construct an array withlength
equal to thecount
of business days to be added. - Use
Array.prototype.reduce()
to iterate over the array, starting fromstartDate
and incrementing, usingDate.prototype.getDate()
andDate.prototype.setDate()
. - If the current
date
is on a weekend, update it again by adding either one day or two days to make it a weekday. - NOTE: Does not take official holidays into account.
const addWeekDays = (startDate, count) =>
Array.from({ length: count }).reduce(date => {
date = new Date(date.setDate(date.getDate() + 1));
if (date.getDay() % 6 === 0)
date = new Date(date.setDate(date.getDate() + (date.getDay() / 6 + 1)));
return date;
}, startDate);
addWeekDays(new Date('Oct 09, 2020'), 5); // 'Oct 16, 2020'
addWeekDays(new Date('Oct 12, 2020'), 5); // 'Oct 19, 2020'
all
- title: all
- tags: array,beginner
Checks if the provided predicate function returns true
for all elements in a collection.
- Use
Array.prototype.every()
to test if all elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const all = (arr, fn = Boolean) => arr.every(fn);
all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // true
allEqual
- title: allEqual
- tags: array,beginner
Checks if all elements in an array are equal.
- Use
Array.prototype.every()
to check if all the elements of the array are the same as the first one. - Elements in the array are compared using the strict comparison operator, which does not account for
NaN
self-inequality.
const allEqual = arr => arr.every(val => val === arr[0]);
allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // true
allEqualBy
- title: allEqualBy
- tags: array,intermediate
Checks if all elements in an array are equal, based on the provided mapping function.
- Apply
fn
to the first element ofarr
. - Use
Array.prototype.every()
to check iffn
returns the same value for all elements in the array as it did for the first one. - Elements in the array are compared using the strict comparison operator, which does not account for
NaN
self-inequality.
const allEqualBy = (arr, fn) => {
const eql = fn(arr[0]);
return arr.every(val => fn(val) === eql);
};
allEqualBy([1.1, 1.2, 1.3], Math.round); // true
allEqualBy([1.1, 1.3, 1.6], Math.round); // false
allUnique
- title: allUnique
- tags: array,beginner
Checks if all elements in an array are unique.
- Create a new
Set
from the mapped values to keep only unique occurrences. - Use
Array.prototype.length
andSet.prototype.size
to compare the length of the unique values to the original array.
const allUnique = arr => arr.length === new Set(arr).size;
allUnique([1, 2, 3, 4]); // true
allUnique([1, 1, 2, 3]); // false
allUniqueBy
- title: allUniqueBy
- tags: array,intermediate
Checks if all elements in an array are unique, based on the provided mapping function.
- Use
Array.prototype.map()
to applyfn
to all elements inarr
. - Create a new
Set
from the mapped values to keep only unique occurrences. - Use
Array.prototype.length
andSet.prototype.size
to compare the length of the unique mapped values to the original array.
const allUniqueBy = (arr, fn) => arr.length === new Set(arr.map(fn)).size;
allUniqueBy([1.2, 2.4, 2.9], Math.round); // true
allUniqueBy([1.2, 2.3, 2.4], Math.round); // false
and
- title: and
- tags: math,logic,beginner unlisted: true
Checks if both arguments are true
.
- Use the logical and (
&&
) operator on the two given values.
const and = (a, b) => a && b;
and(true, true); // true
and(true, false); // false
and(false, false); // false
any
- title: any
- tags: array,beginner
Checks if the provided predicate function returns true
for at least one element in a collection.
- Use
Array.prototype.some()
to test if any elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const any = (arr, fn = Boolean) => arr.some(fn);
any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // true
aperture
- title: aperture
- tags: array,intermediate
Creates an array of n
-tuples of consecutive elements.
- Use
Array.prototype.slice()
andArray.prototype.map()
to create an array of appropriate length. - Populate the array with
n
-tuples of consecutive elements fromarr
. - If
n
is greater than the length ofarr
, return an empty array.
const aperture = (n, arr) =>
n > arr.length
? []
: arr.slice(n - 1).map((v, i) => arr.slice(i, i + n));
aperture(2, [1, 2, 3, 4]); // [[1, 2], [2, 3], [3, 4]]
aperture(3, [1, 2, 3, 4]); // [[1, 2, 3], [2, 3, 4]]
aperture(5, [1, 2, 3, 4]); // []
approximatelyEqual
- title: approximatelyEqual
- tags: math,beginner
Checks if two numbers are approximately equal to each other.
- Use
Math.abs()
to compare the absolute difference of the two values toepsilon
. - Omit the third argument,
epsilon
, to use a default value of0.001
.
const approximatelyEqual = (v1, v2, epsilon = 0.001) =>
Math.abs(v1 - v2) < epsilon;
approximatelyEqual(Math.PI / 2.0, 1.5708); // true
arithmeticProgression
- title: arithmeticProgression
- tags: math,algorithm,beginner
Creates an array of numbers in the arithmetic progression, starting with the given positive integer and up to the specified limit.
- Use
Array.from()
to create an array of the desired length,lim/n
, and a map function to fill it with the desired values in the given range.
const arithmeticProgression = (n, lim) =>
Array.from({ length: Math.ceil(lim / n) }, (_, i) => (i + 1) * n );
arithmeticProgression(5, 25); // [5, 10, 15, 20, 25]
arrayToCSV
- title: arrayToCSV
- tags: array,string,intermediate
Converts a 2D array to a comma-separated values (CSV) string.
- Use
Array.prototype.map()
andArray.prototype.join(delimiter)
to combine individual 1D arrays (rows) into strings. - Use
Array.prototype.join('\n')
to combine all rows into a CSV string, separating each row with a newline. - Omit the second argument,
delimiter
, to use a default delimiter of,
.
const arrayToCSV = (arr, delimiter = ',') =>
arr
.map(v =>
v.map(x => (isNaN(x) ? `"${x.replace(/"/g, '""')}"` : x)).join(delimiter)
)
.join('\n');
arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'
arrayToCSV([['a', '"b" great'], ['c', 3.1415]]);
// '"a","""b"" great"\n"c",3.1415'
arrayToHTMLList
- title: arrayToHTMLList
- tags: browser,array,intermediate
Converts the given array elements into <li>
tags and appends them to the list of the given id.
- Use
Array.prototype.map()
andDocument.querySelector()
to create a list of html tags.
const arrayToHTMLList = (arr, listID) =>
document.querySelector(`##${listID}`).innerHTML += arr
.map(item => `<li>${item}</li>`)
.join('');
arrayToHTMLList(['item 1', 'item 2'], 'myListID');
ary
- title: ary
- tags: function,advanced
Creates a function that accepts up to n
arguments, ignoring any additional arguments.
- Call the provided function,
fn
, with up ton
arguments, usingArray.prototype.slice(0, n)
and the spread operator (...
).
const ary = (fn, n) => (...args) => fn(...args.slice(0, n));
const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [6, 4, 8], [10]].map(x => firstTwoMax(...x)); // [6, 6, 10]
atob
- title: atob
- tags: node,string,beginner
Decodes a string of data which has been encoded using base-64 encoding.
- Create a
Buffer
for the given string with base-64 encoding and useBuffer.toString('binary')
to return the decoded string.
const atob = str => Buffer.from(str, 'base64').toString('binary');
atob('Zm9vYmFy'); // 'foobar'
attempt
- title: attempt
- tags: function,intermediate
Attempts to invoke a function with the provided arguments, returning either the result or the caught error object.
- Use a
try... catch
block to return either the result of the function or an appropriate error. - If the caught object is not an
Error
, use it to create a newError
.
const attempt = (fn, ...args) => {
try {
return fn(...args);
} catch (e) {
return e instanceof Error ? e : new Error(e);
}
};
var elements = attempt(function(selector) {
return document.querySelectorAll(selector);
}, '>_>');
if (elements instanceof Error) elements = []; // elements = []
average
- title: average
- tags: math,array,beginner
Calculates the average of two or more numbers.
- Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
. - Divide the resulting array by its length.
const average = (...nums) =>
nums.reduce((acc, val) => acc + val, 0) / nums.length;
average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2
averageBy
- title: averageBy
- tags: math,array,intermediate
Calculates the average of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
. - Divide the resulting array by its length.
const averageBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val) => acc + val, 0) / arr.length;
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5
bifurcate
- title: bifurcate
- tags: array,intermediate
Splits values into two groups, based on the result of the given filter
array.
- Use
Array.prototype.reduce()
andArray.prototype.push()
to add elements to groups, based onfilter
. - If
filter
has a truthy value for any element, add it to the first group, otherwise add it to the second group.
const bifurcate = (arr, filter) =>
arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [
[],
[],
]);
bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]);
// [ ['beep', 'boop', 'bar'], ['foo'] ]
bifurcateBy
- title: bifurcateBy
- tags: array,intermediate
Splits values into two groups, based on the result of the given filtering function.
- Use
Array.prototype.reduce()
andArray.prototype.push()
to add elements to groups, based on the value returned byfn
for each element. - If
fn
returns a truthy value for any element, add it to the first group, otherwise add it to the second group.
const bifurcateBy = (arr, fn) =>
arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [
[],
[],
]);
bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b');
// [ ['beep', 'boop', 'bar'], ['foo'] ]
binary
- title: binary
- tags: function,intermediate
Creates a function that accepts up to two arguments, ignoring any additional arguments.
- Call the provided function,
fn
, with the first two arguments given.
const binary = fn => (a, b) => fn(a, b);
['2', '1', '0'].map(binary(Math.max)); // [2, 1, 2]
binarySearch
- title: binarySearch
- tags: algorithm,array,beginner
Finds the index of a given element in a sorted array using the binary search algorithm.
- Declare the left and right search boundaries,
l
andr
, initialized to0
and thelength
of the array respectively. - Use a
while
loop to repeatedly narrow down the search subarray, usingMath.floor()
to cut it in half. - Return the index of the element if found, otherwise return
-1
. - Note: Does not account for duplicate values in the array.
const binarySearch = (arr, item) => {
let l = 0,
r = arr.length - 1;
while (l <= r) {
const mid = Math.floor((l + r) / 2);
const guess = arr[mid];
if (guess === item) return mid;
if (guess > item) r = mid - 1;
else l = mid + 1;
}
return -1;
};
binarySearch([1, 2, 3, 4, 5], 1); // 0
binarySearch([1, 2, 3, 4, 5], 5); // 4
binarySearch([1, 2, 3, 4, 5], 6); // -1
bind
- title: bind
- tags: function,object,advanced
Creates a function that invokes fn
with a given context, optionally prepending any additional supplied parameters to the arguments.
- Return a
function
that usesFunction.prototype.apply()
to apply the givencontext
tofn
. - Use the spread operator (
...
) to prepend any additional supplied parameters to the arguments.
const bind = (fn, context, ...boundArgs) => (...args) =>
fn.apply(context, [...boundArgs, ...args]);
function greet(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
const freddy = { user: 'fred' };
const freddyBound = bind(greet, freddy);
console.log(freddyBound('hi', '!')); // 'hi fred!'
bindAll
- title: bindAll
- tags: object,function,intermediate
Binds methods of an object to the object itself, overwriting the existing method.
- Use
Array.prototype.forEach()
to iterate over the givenfns
. - Return a function for each one, using
Function.prototype.apply()
to apply the given context (obj
) tofn
.
const bindAll = (obj, ...fns) =>
fns.forEach(
fn => (
(f = obj[fn]),
(obj[fn] = function() {
return f.apply(obj);
})
)
);
var view = {
label: 'docs',
click: function() {
console.log('clicked ' + this.label);
}
};
bindAll(view, 'click');
document.body.addEventListener('click', view.click);
// Log 'clicked docs' when clicked.
bindKey
- title: bindKey
- tags: function,object,advanced
Creates a function that invokes the method at a given key of an object, optionally prepending any additional supplied parameters to the arguments.
- Return a
function
that usesFunction.prototype.apply()
to bindcontext[fn]
tocontext
. - Use the spread operator (
...
) to prepend any additional supplied parameters to the arguments.
const bindKey = (context, fn, ...boundArgs) => (...args) =>
context[fn].apply(context, [...boundArgs, ...args]);
const freddy = {
user: 'fred',
greet: function(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
};
const freddyBound = bindKey(freddy, 'greet');
console.log(freddyBound('hi', '!')); // 'hi fred!'
binomialCoefficient
- title: binomialCoefficient
- tags: math,algorithm,beginner
Calculates the number of ways to choose k
items from n
items without repetition and without order.
- Use
Number.isNaN()
to check if any of the two values isNaN
. - Check if
k
is less than0
, greater than or equal ton
, equal to1
orn - 1
and return the appropriate result. - Check if
n - k
is less thank
and switch their values accordingly. - Loop from
2
throughk
and calculate the binomial coefficient. - Use
Math.round()
to account for rounding errors in the calculation.
const binomialCoefficient = (n, k) => {
if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
if (k < 0 || k > n) return 0;
if (k === 0 || k === n) return 1;
if (k === 1 || k === n - 1) return n;
if (n - k < k) k = n - k;
let res = n;
for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
return Math.round(res);
};
binomialCoefficient(8, 2); // 28
both
- title: both
- tags: function,logic,beginner unlisted: true
Checks if both of the given functions return true
for a given set of arguments.
- Use the logical and (
&&
) operator on the result of calling the two functions with the suppliedargs
.
const both = (f, g) => (...args) => f(...args) && g(...args);
const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveEven = both(isEven, isPositive);
isPositiveEven(4); // true
isPositiveEven(-2); // false
bottomVisible
- title: bottomVisible
- tags: browser,beginner
Checks if the bottom of the page is visible.
- Use
scrollY
,scrollHeight
andclientHeight
to determine if the bottom of the page is visible.
const bottomVisible = () =>
document.documentElement.clientHeight + window.scrollY >=
(document.documentElement.scrollHeight ||
document.documentElement.clientHeight);
bottomVisible(); // true
btoa
- title: btoa
- tags: node,string,beginner
Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.
- Create a
Buffer
for the given string with binary encoding and useBuffer.toString('base64')
to return the encoded string.
const btoa = str => Buffer.from(str, 'binary').toString('base64');
btoa('foobar'); // 'Zm9vYmFy'
bubbleSort
- title: bubbleSort
- tags: algorithm,array,beginner
Sorts an array of numbers, using the bubble sort algorithm.
- Declare a variable,
swapped
, that indicates if any values were swapped during the current iteration. - Use the spread operator (
...
) to clone the original array,arr
. - Use a
for
loop to iterate over the elements of the cloned array, terminating before the last element. - Use a nested
for
loop to iterate over the segment of the array between0
andi
, swapping any adjacent out of order elements and settingswapped
totrue
. - If
swapped
isfalse
after an iteration, no more changes are needed, so the cloned array is returned.
const bubbleSort = arr => {
let swapped = false;
const a = [...arr];
for (let i = 1; i < a.length - 1; i++) {
swapped = false;
for (let j = 0; j < a.length - i; j++) {
if (a[j + 1] < a[j]) {
[a[j], a[j + 1]] = [a[j + 1], a[j]];
swapped = true;
}
}
if (!swapped) return a;
}
return a;
};
bubbleSort([2, 1, 4, 3]); // [1, 2, 3, 4]
bucketSort
- title: bucketSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the bucket sort algorithm.
- Use
Math.min(),
Math.max()
and the spread operator (...
) to find the minimum and maximum values of the given array. - Use
Array.from()
andMath.floor()
to create the appropriate number ofbuckets
(empty arrays). - Use
Array.prototype.forEach()
to populate each bucket with the appropriate elements from the array. - Use
Array.prototype.reduce()
, the spread operator (...
) andArray.prototype.sort()
to sort each bucket and append it to the result.
const bucketSort = (arr, size = 5) => {
const min = Math.min(...arr);
const max = Math.max(...arr);
const buckets = Array.from(
{ length: Math.floor((max - min) / size) + 1 },
() => []
);
arr.forEach(val => {
buckets[Math.floor((val - min) / size)].push(val);
});
return buckets.reduce((acc, b) => [...acc, ...b.sort((a, b) => a - b)], []);
};
bucketSort([6, 3, 4, 1]); // [1, 3, 4, 6]
byteSize
- title: byteSize
- tags: string,beginner
Returns the length of a string in bytes.
- Convert a given string to a
Blob
Object. - Use
Blob.size
to get the length of the string in bytes.
const byteSize = str => new Blob([str]).size;
byteSize('😀'); // 4
byteSize('Hello World'); // 11
caesarCipher
- title: caesarCipher
- tags: algorithm,string,beginner
Encrypts or decrypts a given string using the Caesar cipher.
- Use the modulo (
%
) operator and the ternary operator (?
) to calculate the correct encryption/decryption key. - Use the spread operator (
...
) andArray.prototype.map()
to iterate over the letters of the given string. - Use
String.prototype.charCodeAt()
andString.fromCharCode()
to convert each letter appropriately, ignoring special characters, spaces etc. - Use
Array.prototype.join()
to combine all the letters into a string. - Pass
true
to the last parameter,decrypt
, to decrypt an encrypted string.
const caesarCipher = (str, shift, decrypt = false) => {
const s = decrypt ? (26 - shift) % 26 : shift;
const n = s > 0 ? s : 26 + (s % 26);
return [...str]
.map((l, i) => {
const c = str.charCodeAt(i);
if (c >= 65 && c <= 90)
return String.fromCharCode(((c - 65 + n) % 26) + 65);
if (c >= 97 && c <= 122)
return String.fromCharCode(((c - 97 + n) % 26) + 97);
return l;
})
.join('');
};
caesarCipher('Hello World!', -3); // 'Ebiil Tloia!'
caesarCipher('Ebiil Tloia!', 23, true); // 'Hello World!'
call
- title: call
- tags: function,advanced
Given a key and a set of arguments, call them when given a context.
- Use a closure to call
key
withargs
for the givencontext
.
const call = (key, ...args) => context => context[key](...args);
Promise.resolve([1, 2, 3])
.then(call('map', x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
.then(map(x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
capitalize
- title: capitalize
- tags: string,intermediate
Capitalizes the first letter of a string.
- Use array destructuring and
String.prototype.toUpperCase()
to capitalize the first letter of the string. - Use
Array.prototype.join('')
to combine the capitalizedfirst
with the...rest
of the characters. - Omit the
lowerRest
argument to keep the rest of the string intact, or set it totrue
to convert to lowercase.
const capitalize = ([first, ...rest], lowerRest = false) =>
first.toUpperCase() +
(lowerRest ? rest.join('').toLowerCase() : rest.join(''));
capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'
capitalizeEveryWord
- title: capitalizeEveryWord
- tags: string,regexp,intermediate
Capitalizes the first letter of every word in a string.
- Use
String.prototype.replace()
to match the first character of each word andString.prototype.toUpperCase()
to capitalize it.
const capitalizeEveryWord = str =>
str.replace(/\b[a-z]/g, char => char.toUpperCase());
capitalizeEveryWord('hello world!'); // 'Hello World!'
cartesianProduct
- title: cartesianProduct
- tags: math,array,beginner
Calculates the cartesian product of two arrays.
- Use
Array.prototype.reduce()
,Array.prototype.map()
and the spread operator (...
) to generate all possible element pairs from the two arrays.
const cartesianProduct = (a, b) =>
a.reduce((p, x) => [...p, ...b.map(y => [x, y])], []);
cartesianProduct(['x', 'y'], [1, 2]);
// [['x', 1], ['x', 2], ['y', 1], ['y', 2]]
castArray
- title: castArray
- tags: type,array,beginner
Casts the provided value as an array if it's not one.
- Use
Array.prototype.isArray()
to determine ifval
is an array and return it as-is or encapsulated in an array accordingly.
const castArray = val => (Array.isArray(val) ? val : [val]);
castArray('foo'); // ['foo']
castArray([1]); // [1]
celsiusToFahrenheit
- title: celsiusToFahrenheit
- tags: math,beginner unlisted: true
Converts Celsius to Fahrenheit.
- Follow the conversion formula
F = 1.8 * C + 32
.
const celsiusToFahrenheit = degrees => 1.8 * degrees + 32;
celsiusToFahrenheit(33); // 91.4
chainAsync
- title: chainAsync
- tags: function,intermediate
Chains asynchronous functions.
- Loop through an array of functions containing asynchronous events, calling
next
when each asynchronous event has completed.
const chainAsync = fns => {
let curr = 0;
const last = fns[fns.length - 1];
const next = () => {
const fn = fns[curr++];
fn === last ? fn() : fn(next);
};
next();
};
chainAsync([
next => {
console.log('0 seconds');
setTimeout(next, 1000);
},
next => {
console.log('1 second');
setTimeout(next, 1000);
},
() => {
console.log('2 second');
}
]);
changeLightness
- title: changeLightness
- tags: string,browser,regexp,intermediate
Changes the lightness value of an hsl()
color string.
- Use
String.prototype.match()
to get an array of 3 strings with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Make sure the lightness is within the valid range (between
0
and100
), usingMath.max()
andMath.min()
. - Use a template literal to create a new
hsl()
string with the updated value.
const changeLightness = (delta, hslStr) => {
const [hue, saturation, lightness] = hslStr.match(/\d+/g).map(Number);
const newLightness = Math.max(
0,
Math.min(100, lightness + parseFloat(delta))
);
return `hsl(${hue}, ${saturation}%, ${newLightness}%)`;
};
changeLightness(10, 'hsl(330, 50%, 50%)'); // 'hsl(330, 50%, 60%)'
changeLightness(-10, 'hsl(330, 50%, 50%)'); // 'hsl(330, 50%, 40%)'
checkProp
- title: checkProp
- tags: function,object,intermediate
Creates a function that will invoke a predicate function for the specified property on a given object.
- Return a curried function, that will invoke
predicate
for the specifiedprop
onobj
and return a boolean.
const checkProp = (predicate, prop) => obj => !!predicate(obj[prop]);
const lengthIs4 = checkProp(l => l === 4, 'length');
lengthIs4([]); // false
lengthIs4([1, 2, 3, 4]); // true
lengthIs4(new Set([1, 2, 3, 4])); // false (Set uses Size, not length)
const session = { user: {} };
const validUserSession = checkProp(u => u.active && !u.disabled, 'user');
validUserSession(session); // false
session.user.active = true;
validUserSession(session); // true
const noLength = checkProp(l => l === undefined, 'length');
noLength([]); // false
noLength({}); // true
noLength(new Set()); // true
chunk
- title: chunk
- tags: array,intermediate
Chunks an array into smaller arrays of a specified size.
- Use
Array.from()
to create a new array, that fits the number of chunks that will be produced. - Use
Array.prototype.slice()
to map each element of the new array to a chunk the length ofsize
. - If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunk = (arr, size) =>
Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
arr.slice(i * size, i * size + size)
);
chunk([1, 2, 3, 4, 5], 2); // [[1, 2], [3, 4], [5]]
chunkIntoN
- title: chunkIntoN
- tags: array,intermediate
Chunks an array into n
smaller arrays.
- Use
Math.ceil()
andArray.prototype.length
to get the size of each chunk. - Use
Array.from()
to create a new array of sizen
. - Use
Array.prototype.slice()
to map each element of the new array to a chunk the length ofsize
. - If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunkIntoN = (arr, n) => {
const size = Math.ceil(arr.length / n);
return Array.from({ length: n }, (v, i) =>
arr.slice(i * size, i * size + size)
);
}
chunkIntoN([1, 2, 3, 4, 5, 6, 7], 4); // [[1, 2], [3, 4], [5, 6], [7]]
clampNumber
- title: clampNumber
- tags: math,beginner
Clamps num
within the inclusive range specified by the boundary values a
and b
.
- If
num
falls within the range, returnnum
. - Otherwise, return the nearest number in the range.
const clampNumber = (num, a, b) =>
Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1
cloneRegExp
- title: cloneRegExp
- tags: type,intermediate
Clones a regular expression.
- Use
new RegExp()
,RegExp.prototype.source
andRegExp.prototype.flags
to clone the given regular expression.
const cloneRegExp = regExp => new RegExp(regExp.source, regExp.flags);
const regExp = /lorem ipsum/gi;
const regExp2 = cloneRegExp(regExp); // regExp !== regExp2
coalesce
- title: coalesce
- tags: type,beginner
Returns the first defined, non-null argument.
- Use
Array.prototype.find()
andArray.prototype.includes()
to find the first value that is not equal toundefined
ornull
.
const coalesce = (...args) => args.find(v => ![undefined, null].includes(v));
coalesce(null, undefined, '', NaN, 'Waldo'); // ''
coalesceFactory
- title: coalesceFactory
- tags: function,type,intermediate
Customizes a coalesce function that returns the first argument which is true based on the given validator.
- Use
Array.prototype.find()
to return the first argument that returnstrue
from the provided argument validation function,valid
.
const coalesceFactory = valid => (...args) => args.find(valid);
const customCoalesce = coalesceFactory(
v => ![null, undefined, '', NaN].includes(v)
);
customCoalesce(undefined, null, NaN, '', 'Waldo'); // 'Waldo'
collectInto
- title: collectInto
- tags: function,array,intermediate
Changes a function that accepts an array into a variadic function.
- Given a function, return a closure that collects all inputs into an array-accepting function.
const collectInto = fn => (...args) => fn(args);
const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)
colorize
- title: colorize
- tags: node,string,intermediate
Adds special characters to text to print in color in the console (combined with console.log()
).
- Use template literals and special characters to add the appropriate color code to the string output.
- For background colors, add a special character that resets the background color at the end of the string.
const colorize = (...args) => ({
black: `\x1b[30m${args.join(' ')}`,
red: `\x1b[31m${args.join(' ')}`,
green: `\x1b[32m${args.join(' ')}`,
yellow: `\x1b[33m${args.join(' ')}`,
blue: `\x1b[34m${args.join(' ')}`,
magenta: `\x1b[35m${args.join(' ')}`,
cyan: `\x1b[36m${args.join(' ')}`,
white: `\x1b[37m${args.join(' ')}`,
bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});
console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite);
// 'foo bar' (first word in yellow letters, second word in green letters, white background for both)
combine
- title: combine
- tags: array,object,intermediate
Combines two arrays of objects, using the specified key to match objects.
- Use
Array.prototype.reduce()
with an object accumulator to combine all objects in both arrays based on the givenprop
. - Use
Object.values()
to convert the resulting object to an array and return it.
const combine = (a, b, prop) =>
Object.values(
[...a, ...b].reduce((acc, v) => {
if (v[prop])
acc[v[prop]] = acc[v[prop]]
? { ...acc[v[prop]], ...v }
: { ...v };
return acc;
}, {})
);
const x = [
{ id: 1, name: 'John' },
{ id: 2, name: 'Maria' }
];
const y = [
{ id: 1, age: 28 },
{ id: 3, age: 26 },
{ age: 3}
];
combine(x, y, 'id');
// [
// { id: 1, name: 'John', age: 28 },
// { id: 2, name: 'Maria' },
// { id: 3, age: 26 }
// ]
compact
- title: compact
- tags: array,beginner
Removes falsy values from an array.
- Use
Array.prototype.filter()
to filter out falsy values (false
,null
,0
,""
,undefined
, andNaN
).
const compact = arr => arr.filter(Boolean);
compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]);
// [ 1, 2, 3, 'a', 's', 34 ]
compactObject
- title: compactObject
- tags: object,array,recursion,advanced
Deeply removes all falsy values from an object or array.
- Use recursion.
- Initialize the iterable data, using
Array.isArray()
,Array.prototype.filter()
andBoolean
for arrays in order to avoid sparse arrays. - Use
Object.keys()
andArray.prototype.reduce()
to iterate over each key with an appropriate initial value. - Use
Boolean
to determine the truthiness of each key's value and add it to the accumulator if it's truthy. - Use
typeof
to determine if a given value is anobject
and call the function again to deeply compact it.
const compactObject = val => {
const data = Array.isArray(val) ? val.filter(Boolean) : val;
return Object.keys(data).reduce(
(acc, key) => {
const value = data[key];
if (Boolean(value))
acc[key] = typeof value === 'object' ? compactObject(value) : value;
return acc;
},
Array.isArray(val) ? [] : {}
);
};
const obj = {
a: null,
b: false,
c: true,
d: 0,
e: 1,
f: '',
g: 'a',
h: [null, false, '', true, 1, 'a'],
i: { j: 0, k: false, l: 'a' }
};
compactObject(obj);
// { c: true, e: 1, g: 'a', h: [ true, 1, 'a' ], i: { l: 'a' } }
compactWhitespace
- title: compactWhitespace
- tags: string,regexp,beginner
Compacts whitespaces in a string.
- Use
String.prototype.replace()
with a regular expression to replace all occurrences of 2 or more whitespace characters with a single space.
const compactWhitespace = str => str.replace(/\s{2,}/g, ' ');
compactWhitespace('Lorem Ipsum'); // 'Lorem Ipsum'
compactWhitespace('Lorem \n Ipsum'); // 'Lorem Ipsum'
complement
- title: complement
- tags: function,logic,beginner
Returns a function that is the logical complement of the given function, fn
.
- Use the logical not (
!
) operator on the result of callingfn
with any suppliedargs
.
const complement = fn => (...args) => !fn(...args);
const isEven = num => num % 2 === 0;
const isOdd = complement(isEven);
isOdd(2); // false
isOdd(3); // true
compose
- title: compose
- tags: function,intermediate
Performs right-to-left function composition.
- Use
Array.prototype.reduce()
to perform right-to-left function composition. - The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.
const compose = (...fns) =>
fns.reduce((f, g) => (...args) => f(g(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(
add5,
multiply
);
multiplyAndAdd5(5, 2); // 15
composeRight
- title: composeRight
- tags: function,intermediate
Performs left-to-right function composition.
- Use
Array.prototype.reduce()
to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const composeRight = (...fns) =>
fns.reduce((f, g) => (...args) => g(f(...args)));
const add = (x, y) => x + y;
const square = x => x * x;
const addAndSquare = composeRight(add, square);
addAndSquare(1, 2); // 9
containsWhitespace
- title: containsWhitespace
- tags: string,regexp,beginner
Checks if the given string contains any whitespace characters.
- Use
RegExp.prototype.test()
with an appropriate regular expression to check if the given string contains any whitespace characters.
const containsWhitespace = str => /\s/.test(str);
containsWhitespace('lorem'); // false
containsWhitespace('lorem ipsum'); // true
converge
- title: converge
- tags: function,intermediate
Accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.
- Use
Array.prototype.map()
andFunction.prototype.apply()
to apply each function to the given arguments. - Use the spread operator (
...
) to callconverger
with the results of all other functions.
const converge = (converger, fns) => (...args) =>
converger(...fns.map(fn => fn.apply(null, args)));
const average = converge((a, b) => a / b, [
arr => arr.reduce((a, v) => a + v, 0),
arr => arr.length
]);
average([1, 2, 3, 4, 5, 6, 7]); // 4
copySign
- title: copySign
- tags: math,beginner
Returns the absolute value of the first number, but the sign of the second.
- Use
Math.sign()
to check if the two numbers have the same sign. - Return
x
if they do,-x
otherwise.
const copySign = (x, y) => Math.sign(x) === Math.sign(y) ? x : -x;
copySign(2, 3); // 2
copySign(2, -3); // -2
copySign(-2, 3); // 2
copySign(-2, -3); // -2
copyToClipboard
- title: copyToClipboard
- tags: browser,string,event,advanced
Copies a string to the clipboard.
Only works as a result of user action (i.e. inside a click
event listener).
- Create a new
<textarea>
element, fill it with the supplied data and add it to the HTML document. - Use
Selection.getRangeAt()
to store the selected range (if any). - Use
Document.execCommand('copy')
to copy to the clipboard. - Remove the
<textarea>
element from the HTML document. - Finally, use
Selection().addRange()
to recover the original selected range (if any). - ⚠️ NOTICE: The same functionality can be easily implemented by using the new asynchronous Clipboard API, which is still experimental but should be used in the future instead of this snippet. Find out more about it here.
const copyToClipboard = str => {
const el = document.createElement('textarea');
el.value = str;
el.setAttribute('readonly', '');
el.style.position = 'absolute';
el.style.left = '-9999px';
document.body.appendChild(el);
const selected =
document.getSelection().rangeCount > 0
? document.getSelection().getRangeAt(0)
: false;
el.select();
document.execCommand('copy');
document.body.removeChild(el);
if (selected) {
document.getSelection().removeAllRanges();
document.getSelection().addRange(selected);
}
};
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.
countBy
- title: countBy
- tags: array,object,intermediate
Groups the elements of an array based on the given function and returns the count of elements in each group.
- Use
Array.prototype.map()
to map the values of an array to a function or property name. - Use
Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const countBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
acc[val] = (acc[val] || 0) + 1;
return acc;
}, {});
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
countBy([{ count: 5 }, { count: 10 }, { count: 5 }], x => x.count)
// {5: 2, 10: 1}
countOccurrences
- title: countOccurrences
- tags: array,intermediate
Counts the occurrences of a value in an array.
- Use
Array.prototype.reduce()
to increment a counter each time the specific value is encountered inside the array.
const countOccurrences = (arr, val) =>
arr.reduce((a, v) => (v === val ? a + 1 : a), 0);
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3
countSubstrings
- title: countSubstrings
- tags: string,algorithm,beginner
Counts the occurrences of a substring in a given string.
- Use
Array.prototype.indexOf()
to look forsearchValue
instr
. - Increment a counter if the value is found and update the index,
i
. - Use a
while
loop that will return as soon as the value returned fromArray.prototype.indexOf()
is-1
.
const countSubstrings = (str, searchValue) => {
let count = 0,
i = 0;
while (true) {
const r = str.indexOf(searchValue, i);
if (r !== -1) [count, i] = [count + 1, r + 1];
else return count;
}
};
countSubstrings('tiktok tok tok tik tok tik', 'tik'); // 3
countSubstrings('tutut tut tut', 'tut'); // 4
countWeekDaysBetween
- title: countWeekDaysBetween
- tags: date,intermediate
Counts the weekdays between two dates.
- Use
Array.from()
to construct an array withlength
equal to the number of days betweenstartDate
andendDate
. - Use
Array.prototype.reduce()
to iterate over the array, checking if each date is a weekday and incrementingcount
. - Update
startDate
with the next day each loop usingDate.prototype.getDate()
andDate.prototype.setDate()
to advance it by one day. - NOTE: Does not take official holidays into account.
const countWeekDaysBetween = (startDate, endDate) =>
Array
.from({ length: (endDate - startDate) / (1000 * 3600 * 24) })
.reduce(count => {
if (startDate.getDay() % 6 !== 0) count++;
startDate = new Date(startDate.setDate(startDate.getDate() + 1));
return count;
}, 0);
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 06, 2020')); // 1
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 14, 2020')); // 7
counter
- title: counter
- tags: browser,advanced
Creates a counter with the specified range, step and duration for the specified selector.
- Check if
step
has the proper sign and change it accordingly. - Use
setInterval()
in combination withMath.abs()
andMath.floor()
to calculate the time between each new text draw. - Use
Document.querySelector()
,Element.innerHTML
to update the value of the selected element. - Omit the fourth argument,
step
, to use a default step of1
. - Omit the fifth argument,
duration
, to use a default duration of2000
ms.
const counter = (selector, start, end, step = 1, duration = 2000) => {
let current = start,
_step = (end - start) * step < 0 ? -step : step,
timer = setInterval(() => {
current += _step;
document.querySelector(selector).innerHTML = current;
if (current >= end) document.querySelector(selector).innerHTML = end;
if (current >= end) clearInterval(timer);
}, Math.abs(Math.floor(duration / (end - start))));
return timer;
};
counter('##my-id', 1, 1000, 5, 2000);
// Creates a 2-second timer for the element with id="my-id"
createDirIfNotExists
- title: createDirIfNotExists
- tags: node,beginner
Creates a directory, if it does not exist.
- Use
fs.existsSync()
to check if the directory exists,fs.mkdirSync()
to create it.
const fs = require('fs');
const createDirIfNotExists = dir => (!fs.existsSync(dir) ? fs.mkdirSync(dir) : undefined);
createDirIfNotExists('test');
// creates the directory 'test', if it doesn't exist
createElement
- title: createElement
- tags: browser,beginner
Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.
- Use
Document.createElement()
to create a new element. - Use
Element.innerHTML
to set its inner HTML to the string supplied as the argument. - Use
ParentNode.firstElementChild
to return the element version of the string.
const createElement = str => {
const el = document.createElement('div');
el.innerHTML = str;
return el.firstElementChild;
};
const el = createElement(
`<div class="container">
<p>Hello!</p>
</div>`
);
console.log(el.className); // 'container'
createEventHub
- title: createEventHub
- tags: browser,event,advanced
Creates a pub/sub (publish–subscribe) event hub with emit
, on
, and off
methods.
- Use
Object.create(null)
to create an emptyhub
object that does not inherit properties fromObject.prototype
. - For
emit
, resolve the array of handlers based on theevent
argument and then run each one withArray.prototype.forEach()
by passing in the data as an argument. - For
on
, create an array for the event if it does not yet exist, then useArray.prototype.push()
to add the handler - to the array.
- For
off
, useArray.prototype.findIndex()
to find the index of the handler in the event array and remove it usingArray.prototype.splice()
.
const createEventHub = () => ({
hub: Object.create(null),
emit(event, data) {
(this.hub[event] || []).forEach(handler => handler(data));
},
on(event, handler) {
if (!this.hub[event]) this.hub[event] = [];
this.hub[event].push(handler);
},
off(event, handler) {
const i = (this.hub[event] || []).findIndex(h => h === handler);
if (i > -1) this.hub[event].splice(i, 1);
if (this.hub[event].length === 0) delete this.hub[event];
}
});
const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;
// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);
// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1
// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);
currentURL
- title: currentURL
- tags: browser,beginner
Returns the current URL.
- Use
Window.location.href
to get the current URL.
const currentURL = () => window.location.href;
currentURL(); // 'https://www.google.com/'
curry
- title: curry
- tags: function,recursion,advanced
Curries a function.
- Use recursion.
- If the number of provided arguments (
args
) is sufficient, call the passed functionfn
. - Otherwise, use
Function.prototype.bind()
to return a curried functionfn
that expects the rest of the arguments. - If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g.
Math.min()
), you can optionally pass the number of arguments to the second parameterarity
.
const curry = (fn, arity = fn.length, ...args) =>
arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);
curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2
cycleGenerator
- title: cycleGenerator
- tags: function,generator,advanced
Creates a generator, looping over the given array indefinitely.
- Use a non-terminating
while
loop, that willyield
a value every timeGenerator.prototype.next()
is called. - Use the module operator (
%
) withArray.prototype.length
to get the next value's index and increment the counter after eachyield
statement.
const cycleGenerator = function* (arr) {
let i = 0;
while (true) {
yield arr[i % arr.length];
i++;
}
};
const binaryCycle = cycleGenerator([0, 1]);
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
dayName
- title: dayName
- tags: date,beginner
Gets the name of the weekday from a Date
object.
- Use
Date.prototype.toLocaleDateString()
with the{ weekday: 'long' }
option to retrieve the weekday. - Use the optional second argument to get a language-specific name or omit it to use the default locale.
const dayName = (date, locale) =>
date.toLocaleDateString(locale, { weekday: 'long' });
dayName(new Date()); // 'Saturday'
dayName(new Date('09/23/2020'), 'de-DE'); // 'Samstag'
dayOfYear
- title: dayOfYear
- tags: date,beginner
Gets the day of the year (number in the range 1-366) from a Date
object.
- Use
new Date()
andDate.prototype.getFullYear()
to get the first day of the year as aDate
object. - Subtract the first day of the year from
date
and divide with the milliseconds in each day to get the result. - Use
Math.floor()
to appropriately round the resulting day count to an integer.
const dayOfYear = date =>
Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);
dayOfYear(new Date()); // 272
daysAgo
- title: daysAgo
- tags: date,beginner
Calculates the date of n
days ago from today as a string representation.
- Use
new Date()
to get the current date,Math.abs()
andDate.prototype.getDate()
to update the date accordingly and set to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const daysAgo = n => {
let d = new Date();
d.setDate(d.getDate() - Math.abs(n));
return d.toISOString().split('T')[0];
};
daysAgo(20); // 2020-09-16 (if current date is 2020-10-06)
daysFromNow
- title: daysFromNow
- tags: date,beginner
Calculates the date of n
days from today as a string representation.
- Use
new Date()
to get the current date,Math.abs()
andDate.prototype.getDate()
to update the date accordingly and set to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const daysFromNow = n => {
let d = new Date();
d.setDate(d.getDate() + Math.abs(n));
return d.toISOString().split('T')[0];
};
daysFromNow(5); // 2020-10-13 (if current date is 2020-10-08)
debounce
- title: debounce
- tags: function,intermediate
Creates a debounced function that delays invoking the provided function until at least ms
milliseconds have elapsed since the last time it was invoked.
- Each time the debounced function is invoked, clear the current pending timeout with
clearTimeout()
and usesetTimeout()
to create a new timeout that delays invoking the function until at leastms
milliseconds has elapsed. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessary arguments. - Omit the second argument,
ms
, to set the timeout at a default of0
ms.
const debounce = (fn, ms = 0) => {
let timeoutId;
return function(...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => fn.apply(this, args), ms);
};
};
window.addEventListener(
'resize',
debounce(() => {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250ms
debouncePromise
- title: debouncePromise
- tags: function,promise,advanced
Creates a debounced function that returns a promise, but delays invoking the provided function until at least ms
milliseconds have elapsed since the last time it was invoked.
All promises returned during this time will return the same data.
- Each time the debounced function is invoked, clear the current pending timeout with
clearTimeout()
and usesetTimeout()
to create a new timeout that delays invoking the function until at leastms
milliseconds has elapsed. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessary arguments. - Create a new
Promise
and add itsresolve
andreject
callbacks to thepending
promises stack. - When
setTimeout
is called, copy the current stack (as it can change between the provided function call and its resolution), clear it and call the provided function. - When the provided function resolves/rejects, resolve/reject all promises in the stack (copied when the function was called) with the returned data.
- Omit the second argument,
ms
, to set the timeout at a default of0
ms.
const debouncePromise = (fn, ms = 0) => {
let timeoutId;
const pending = [];
return (...args) =>
new Promise((res, rej) => {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => {
const currentPending = [...pending];
pending.length = 0;
Promise.resolve(fn.apply(this, args)).then(
data => {
currentPending.forEach(({ resolve }) => resolve(data));
},
error => {
currentPending.forEach(({ reject }) => reject(error));
}
);
}, ms);
pending.push({ resolve: res, reject: rej });
});
};
const fn = arg => new Promise(resolve => {
setTimeout(resolve, 1000, ['resolved', arg]);
});
const debounced = debouncePromise(fn, 200);
debounced('foo').then(console.log);
debounced('bar').then(console.log);
// Will log ['resolved', 'bar'] both times
decapitalize
- title: decapitalize
- tags: string,intermediate
Decapitalizes the first letter of a string.
- Use array destructuring and
String.prototype.toLowerCase()
to decapitalize first letter,...rest
to get array of characters after first letter and thenArray.prototype.join('')
to make it a string again. - Omit the
upperRest
argument to keep the rest of the string intact, or set it totrue
to convert to uppercase.
const decapitalize = ([first, ...rest], upperRest = false) =>
first.toLowerCase() +
(upperRest ? rest.join('').toUpperCase() : rest.join(''));
decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'
deepClone
- title: deepClone
- tags: object,recursion,advanced
Creates a deep clone of an object. Clones primitives, arrays and objects, excluding class instances.
- Use recursion.
- Check if the passed object is
null
and, if so, returnnull
. - Use
Object.assign()
and an empty object ({}
) to create a shallow clone of the original. - Use
Object.keys()
andArray.prototype.forEach()
to determine which key-value pairs need to be deep cloned. - If the object is an
Array
, set theclone
'slength
to that of the original and useArray.from(clone)
to create a clone.
const deepClone = obj => {
if (obj === null) return null;
let clone = Object.assign({}, obj);
Object.keys(clone).forEach(
key =>
(clone[key] =
typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
);
if (Array.isArray(obj)) {
clone.length = obj.length;
return Array.from(clone);
}
return clone;
};
const a = { foo: 'bar', obj: { a: 1, b: 2 } };
const b = deepClone(a); // a !== b, a.obj !== b.obj
deepFlatten
- title: deepFlatten
- tags: array,recursion,intermediate
Deep flattens an array.
- Use recursion.
- Use
Array.prototype.concat()
with an empty array ([]
) and the spread operator (...
) to flatten an array. - Recursively flatten each element that is an array.
const deepFlatten = arr =>
[].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));
deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]
deepFreeze
- title: deepFreeze
- tags: object,recursion,intermediate
Deep freezes an object.
- Use
Object.keys()
to get all the properties of the passed object,Array.prototype.forEach()
to iterate over them. - Call
Object.freeze(obj)
recursively on all properties, applyingdeepFreeze()
as necessary. - Finally, use
Object.freeze()
to freeze the given object.
const deepFreeze = obj => {
Object.keys(obj).forEach(prop => {
if (typeof obj[prop] === 'object') deepFreeze(obj[prop]);
});
return Object.freeze(obj);
};
'use strict';
const val = deepFreeze([1, [2, 3]]);
val[0] = 3; // not allowed
val[1][0] = 4; // not allowed as well
deepGet
- title: deepGet
- tags: object,intermediate
Gets the target value in a nested JSON object, based on the keys
array.
- Compare the keys you want in the nested JSON object as an
Array
. - Use
Array.prototype.reduce()
to get the values in the nested JSON object one by one. - If the key exists in the object, return the target value, otherwise return
null
.
const deepGet = (obj, keys) =>
keys.reduce(
(xs, x) => (xs && xs[x] !== null && xs[x] !== undefined ? xs[x] : null),
obj
);
let index = 2;
const data = {
foo: {
foz: [1, 2, 3],
bar: {
baz: ['a', 'b', 'c']
}
}
};
deepGet(data, ['foo', 'foz', index]); // get 3
deepGet(data, ['foo', 'bar', 'baz', 8, 'foz']); // null
deepMapKeys
- title: deepMapKeys
- tags: object,recursion,advanced
Deep maps an object's keys.
- Creates an object with the same values as the provided object and keys generated by running the provided function for each key.
- Use
Object.keys(obj)
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same values and mapped keys usingfn
.
const deepMapKeys = (obj, fn) =>
Array.isArray(obj)
? obj.map(val => deepMapKeys(val, fn))
: typeof obj === 'object'
? Object.keys(obj).reduce((acc, current) => {
const key = fn(current);
const val = obj[current];
acc[key] =
val !== null && typeof val === 'object' ? deepMapKeys(val, fn) : val;
return acc;
}, {})
: obj;
const obj = {
foo: '1',
nested: {
child: {
withArray: [
{
grandChild: ['hello']
}
]
}
}
};
const upperKeysObj = deepMapKeys(obj, key => key.toUpperCase());
/*
{
"FOO":"1",
"NESTED":{
"CHILD":{
"WITHARRAY":[
{
"GRANDCHILD":[ 'hello' ]
}
]
}
}
}
*/
defaults
- title: defaults
- tags: object,intermediate
Assigns default values for all properties in an object that are undefined
.
- Use
Object.assign()
to create a new empty object and copy the original one to maintain key order. - Use
Array.prototype.reverse()
and the spread operator (...
) to combine the default values from left to right. - Finally, use
obj
again to overwrite properties that originally had a value.
const defaults = (obj, ...defs) =>
Object.assign({}, obj, ...defs.reverse(), obj);
defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }
defer
- title: defer
- tags: function,intermediate
Defers invoking a function until the current call stack has cleared.
- Use
setTimeout()
with a timeout of1
ms to add a new event to the event queue and allow the rendering engine to complete its work. - Use the spread (
...
) operator to supply the function with an arbitrary number of arguments.
const defer = (fn, ...args) => setTimeout(fn, 1, ...args);
// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'
// Example B:
document.querySelector('##someElement').innerHTML = 'Hello';
longRunningFunction();
// Browser will not update the HTML until this has finished
defer(longRunningFunction);
// Browser will update the HTML then run the function
degreesToRads
- title: degreesToRads
- tags: math,beginner
Converts an angle from degrees to radians.
- Use
Math.PI
and the degree to radian formula to convert the angle from degrees to radians.
const degreesToRads = deg => (deg * Math.PI) / 180.0;
degreesToRads(90.0); // ~1.5708
delay
- title: delay
- tags: function,intermediate
Invokes the provided function after ms
milliseconds.
- Use
setTimeout()
to delay execution offn
. - Use the spread (
...
) operator to supply the function with an arbitrary number of arguments.
const delay = (fn, ms, ...args) => setTimeout(fn, ms, ...args);
delay(
function(text) {
console.log(text);
},
1000,
'later'
); // Logs 'later' after one second.
detectDeviceType
- title: detectDeviceType
- tags: browser,regexp,intermediate
Detects whether the page is being viewed on a mobile device or a desktop.
- Use a regular expression to test the
navigator.userAgent
property to figure out if the device is a mobile device or a desktop.
const detectDeviceType = () =>
/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(
navigator.userAgent
)
? 'Mobile'
: 'Desktop';
detectDeviceType(); // 'Mobile' or 'Desktop'
detectLanguage
- title: detectLanguage
- tags: browser,intermediate
Detects the preferred language of the current user.
- Use
NavigationLanguage.language
or the firstNavigationLanguage.languages
if available, otherwise returndefaultLang
. - Omit the second argument,
defaultLang
, to use'en-US'
as the default language code.
const detectLanguage = (defaultLang = 'en-US') =>
navigator.language ||
(Array.isArray(navigator.languages) && navigator.languages[0]) ||
defaultLang;
detectLanguage(); // 'nl-NL'
difference
- title: difference
- tags: array,beginner
Calculates the difference between two arrays, without filtering duplicate values.
- Create a
Set
fromb
to get the unique values inb
. - Use
Array.prototype.filter()
ona
to only keep values not contained inb
, usingSet.prototype.has()
.
const difference = (a, b) => {
const s = new Set(b);
return a.filter(x => !s.has(x));
};
difference([1, 2, 3, 3], [1, 2, 4]); // [3, 3]
differenceBy
- title: differenceBy
- tags: array,intermediate
Returns the difference between two arrays, after applying the provided function to each array element of both.
- Create a
Set
by applyingfn
to each element inb
. - Use
Array.prototype.map()
to applyfn
to each element ina
. - Use
Array.prototype.filter()
in combination withfn
ona
to only keep values not contained inb
, usingSet.prototype.has()
.
const differenceBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.map(fn).filter(el => !s.has(el));
};
differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]
differenceWith
- title: differenceWith
- tags: array,intermediate
Filters out all values from an array for which the comparator function does not return true
.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
to find the appropriate values. - Omit the last argument,
comp
, to use a default strict equality comparator.
const differenceWith = (arr, val, comp = (a, b) => a === b) =>
arr.filter(a => val.findIndex(b => comp(a, b)) === -1);
differenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2]
differenceWith([1, 1.2, 1.3], [1, 1.3, 1.5]); // [1.2]
dig
- title: dig
- tags: object,recursion,intermediate
Gets the target value in a nested JSON object, based on the given key.
- Use the
in
operator to check iftarget
exists inobj
. - If found, return the value of
obj[target]
. - Otherwise use
Object.values(obj)
andArray.prototype.reduce()
to recursively calldig
on each nested object until the first matching key/value pair is found.
const dig = (obj, target) =>
target in obj
? obj[target]
: Object.values(obj).reduce((acc, val) => {
if (acc !== undefined) return acc;
if (typeof val === 'object') return dig(val, target);
}, undefined);
const data = {
level1: {
level2: {
level3: 'some data'
}
}
};
dig(data, 'level3'); // 'some data'
dig(data, 'level4'); // undefined
digitize
- title: digitize
- tags: math,beginner
Converts a number to an array of digits, removing its sign if necessary.
- Use
Math.abs()
to strip the number's sign. - Convert the number to a string, using the spread operator (
...
) to build an array. - Use
Array.prototype.map()
andparseInt()
to transform each value to an integer.
const digitize = n => [...`${Math.abs(n)}`].map(i => parseInt(i));
digitize(123); // [1, 2, 3]
digitize(-123); // [1, 2, 3]
distance
- title: distance
- tags: math,algorithm,beginner
Calculates the distance between two points.
- Use
Math.hypot()
to calculate the Euclidean distance between two points.
const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);
distance(1, 1, 2, 3); // ~2.2361
divmod
- title: divmod
- tags: math,beginner
Returns an array consisting of the quotient and remainder of the given numbers.
- Use
Math.floor()
to get the quotient of the divisionx / y
. - Use the modulo operator (
%
) to get the remainder of the divisionx / y
.
const divmod = (x, y) => [Math.floor(x / y), x % y];
divmod(8, 3); // [2, 2]
divmod(3, 8); // [0, 3]
divmod(5, 5); // [1, 0]
drop
- title: drop
- tags: array,beginner
Creates a new array with n
elements removed from the left.
- Use
Array.prototype.slice()
to remove the specified number of elements from the left. - Omit the last argument,
n
, to use a default value of1
.
const drop = (arr, n = 1) => arr.slice(n);
drop([1, 2, 3]); // [2, 3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []
dropRight
- title: dropRight
- tags: array,beginner
Creates a new array with n
elements removed from the right.
- Use
Array.prototype.slice()
to remove the specified number of elements from the right. - Omit the last argument,
n
, to use a default value of1
.
const dropRight = (arr, n = 1) => arr.slice(0, -n);
dropRight([1, 2, 3]); // [1, 2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []
dropRightWhile
- title: dropRightWhile
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns true
.
Returns the remaining elements in the array.
- Loop through the array, using
Array.prototype.slice()
to drop the last element of the array until the value returned fromfunc
istrue
. - Return the remaining elements.
const dropRightWhile = (arr, func) => {
let rightIndex = arr.length;
while (rightIndex-- && !func(arr[rightIndex]));
return arr.slice(0, rightIndex + 1);
};
dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
dropWhile
- title: dropWhile
- tags: array,intermediate
Removes elements in an array until the passed function returns true
.
Returns the remaining elements in the array.
- Loop through the array, using
Array.prototype.slice()
to drop the first element of the array until the value returned fromfunc
istrue
. - Return the remaining elements.
const dropWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
return arr;
};
dropWhile([1, 2, 3, 4], n => n >= 3); // [3, 4]
either
- title: either
- tags: function,logic,beginner
Checks if at least one function returns true
for a given set of arguments.
- Use the logical or (
||
) operator on the result of calling the two functions with the suppliedargs
.
const either = (f, g) => (...args) => f(...args) || g(...args);
const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveOrEven = either(isPositive, isEven);
isPositiveOrEven(4); // true
isPositiveOrEven(3); // true
elementContains
- title: elementContains
- tags: browser,intermediate
Checks if the parent
element contains the child
element.
- Check that
parent
is not the same element aschild
. - Use
Node.contains()
to check if theparent
element contains thechild
element.
const elementContains = (parent, child) =>
parent !== child && parent.contains(child);
elementContains(
document.querySelector('head'),
document.querySelector('title')
);
// true
elementContains(document.querySelector('body'), document.querySelector('body'));
// false
elementIsFocused
- title: elementIsFocused
- tags: browser,beginner
Checks if the given element is focused.
- Use
Document.activeElement
to determine if the given element is focused.
const elementIsFocused = el => (el === document.activeElement);
elementIsFocused(el); // true if the element is focused
elementIsVisibleInViewport
- title: elementIsVisibleInViewport
- tags: browser,intermediate
Checks if the element specified is visible in the viewport.
- Use
Element.getBoundingClientRect()
and theWindow.inner(Width|Height)
values to determine if a given element is visible in the viewport. - Omit the second argument to determine if the element is entirely visible, or specify
true
to determine if it is partially visible.
const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
const { top, left, bottom, right } = el.getBoundingClientRect();
const { innerHeight, innerWidth } = window;
return partiallyVisible
? ((top > 0 && top < innerHeight) ||
(bottom > 0 && bottom < innerHeight)) &&
((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
: top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)
equals
- title: equals
- tags: object,array,type,advanced
Performs a deep comparison between two values to determine if they are equivalent.
- Check if the two values are identical, if they are both
Date
objects with the same time, usingDate.prototype.getTime()
or if they are both non-object values with an equivalent value (strict comparison). - Check if only one value is
null
orundefined
or if their prototypes differ. - If none of the above conditions are met, use
Object.keys()
to check if both values have the same number of keys. - Use
Array.prototype.every()
to check if every key ina
exists inb
and if they are equivalent by callingequals()
recursively.
const equals = (a, b) => {
if (a === b) return true;
if (a instanceof Date && b instanceof Date)
return a.getTime() === b.getTime();
if (!a || !b || (typeof a !== 'object' && typeof b !== 'object'))
return a === b;
if (a.prototype !== b.prototype) return false;
let keys = Object.keys(a);
if (keys.length !== Object.keys(b).length) return false;
return keys.every(k => equals(a[k], b[k]));
};
equals(
{ a: [2, { e: 3 }], b: [4], c: 'foo' },
{ a: [2, { e: 3 }], b: [4], c: 'foo' }
); // true
equals([1, 2, 3], { 0: 1, 1: 2, 2: 3 }); // true
escapeHTML
- title: escapeHTML
- tags: string,browser,regexp,intermediate
Escapes a string for use in HTML.
- Use
String.prototype.replace()
with a regexp that matches the characters that need to be escaped. - Use the callback function to replace each character instance with its associated escaped character using a dictionary (object).
const escapeHTML = str =>
str.replace(
/[&<>'"]/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
"'": '&##39;',
'"': '"'
}[tag] || tag)
);
escapeHTML('<a href="##">Me & you</a>');
// '<a href="##">Me & you</a>'
escapeRegExp
- title: escapeRegExp
- tags: string,regexp,intermediate
Escapes a string to use in a regular expression.
- Use
String.prototype.replace()
to escape special characters.
const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
escapeRegExp('(test)'); // \\(test\\)
euclideanDistance
- title: euclideanDistance
- tags: math,algorithm,intermediate
Calculates the distance between two points in any number of dimensions.
- Use
Object.keys()
andArray.prototype.map()
to map each coordinate to its difference between the two points. - Use
Math.hypot()
to calculate the Euclidean distance between the two points.
const euclideanDistance = (a, b) =>
Math.hypot(...Object.keys(a).map(k => b[k] - a[k]));
euclideanDistance([1, 1], [2, 3]); // ~2.2361
euclideanDistance([1, 1, 1], [2, 3, 2]); // ~2.4495
everyNth
- title: everyNth
- tags: array,beginner
Returns every nth
element in an array.
- Use
Array.prototype.filter()
to create a new array that contains everynth
element of a given array.
const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]
expandTabs
- title: expandTabs
- tags: string,regexp,beginner
Convert tabs to spaces, where each tab corresponds to count
spaces.
- Use
String.prototype.replace()
with a regular expression andString.prototype.repeat()
to replace each tab character withcount
spaces.
const expandTabs = (str, count) => str.replace(/\t/g, ' '.repeat(count));
expandTabs('\t\tlorem', 3); // ' lorem'
extendHex
- title: extendHex
- tags: string,intermediate
Extends a 3-digit color code to a 6-digit color code.
- Use
Array.prototype.map()
,String.prototype.split()
andArray.prototype.join()
to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form. Array.prototype.slice()
is used to remove##
from string start since it's added once.
const extendHex = shortHex =>
'##' +
shortHex
.slice(shortHex.startsWith('##') ? 1 : 0)
.split('')
.map(x => x + x)
.join('');
extendHex('##03f'); // '##0033ff'
extendHex('05a'); // '##0055aa'
factorial
- title: factorial
- tags: math,algorithm,recursion,beginner
Calculates the factorial of a number.
- Use recursion.
- If
n
is less than or equal to1
, return1
. - Otherwise, return the product of
n
and the factorial ofn - 1
. - Throw a
TypeError
ifn
is a negative number.
const factorial = n =>
n < 0
? (() => {
throw new TypeError('Negative numbers are not allowed!');
})()
: n <= 1
? 1
: n * factorial(n - 1);
factorial(6); // 720
fahrenheitToCelsius
- title: fahrenheitToCelsius
- tags: math,beginner unlisted: true
Converts Fahrenheit to Celsius.
- Follow the conversion formula
C = (F - 32) * 5/9
.
const fahrenheitToCelsius = degrees => (degrees - 32) * 5 / 9;
fahrenheitToCelsius(32); // 0
fibonacci
- title: fibonacci
- tags: math,algorithm,intermediate
Generates an array, containing the Fibonacci sequence, up until the nth term.
- Use
Array.from()
to create an empty array of the specific length, initializing the first two values (0
and1
). - Use
Array.prototype.reduce()
andArray.prototype.concat()
to add values into the array, using the sum of the last two values, except for the first two.
const fibonacci = n =>
Array.from({ length: n }).reduce(
(acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
[]
);
fibonacci(6); // [0, 1, 1, 2, 3, 5]
filterNonUnique
- title: filterNonUnique
- tags: array,beginner
Creates an array with the non-unique values filtered out.
- Use
new Set()
and the spread operator (...
) to create an array of the unique values inarr
. - Use
Array.prototype.filter()
to create an array containing only the unique values.
const filterNonUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]
filterNonUniqueBy
- title: filterNonUniqueBy
- tags: array,intermediate
Creates an array with the non-unique values filtered out, based on a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.every()
to create an array containing only the unique values, based on the comparator function,fn
. - The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterNonUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));
filterNonUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id === b.id
); // [ { id: 2, value: 'c' } ]
filterUnique
- title: filterUnique
- tags: array,beginner
Creates an array with the unique values filtered out.
- Use
new Set()
and the spread operator (...
) to create an array of the unique values inarr
. - Use
Array.prototype.filter()
to create an array containing only the non-unique values.
const filterUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) !== arr.lastIndexOf(i));
filterUnique([1, 2, 2, 3, 4, 4, 5]); // [2, 4]
filterUniqueBy
- title: filterUniqueBy
- tags: array,intermediate
Creates an array with the unique values filtered out, based on a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.every()
to create an array containing only the non-unique values, based on the comparator function,fn
. - The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.some((x, j) => (i !== j) === fn(v, x, i, j)));
filterUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 3, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 0, value: 'e' } ]
findKey
- title: findKey
- tags: object,intermediate
Finds the first key that satisfies the provided testing function.
Otherwise undefined
is returned.
- Use
Object.keys(obj)
to get all the properties of the object,Array.prototype.find()
to test each key-value pair usingfn
. - The callback receives three arguments - the value, the key and the object.
const findKey = (obj, fn) =>
Object.keys(obj).find(key => fn(obj[key], key, obj));
findKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
x => x['active']
); // 'barney'
findKeys
- title: findKeys
- tags: object,beginner
Finds all the keys in the provided object that match the given value.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.filter()
to test each key-value pair and return all keys that are equal to the given value.
const findKeys = (obj, val) =>
Object.keys(obj).filter(key => obj[key] === val);
const ages = {
Leo: 20,
Zoey: 21,
Jane: 20,
};
findKeys(ages, 20); // [ 'Leo', 'Jane' ]
findLast
- title: findLast
- tags: array,beginner
Finds the last element for which the provided function returns a truthy value.
- Use
Array.prototype.filter()
to remove elements for whichfn
returns falsy values. - Use
Array.prototype.pop()
to get the last element in the filtered array.
const findLast = (arr, fn) => arr.filter(fn).pop();
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3
findLastIndex
- title: findLastIndex
- tags: array,intermediate
Finds the index of the last element for which the provided function returns a truthy value.
- Use
Array.prototype.map()
to map each element to an array with its index and value. - Use
Array.prototype.filter()
to remove elements for whichfn
returns falsy values - Use
Array.prototype.pop()
to get the last element in the filtered array. - Return
-1
if there are no matching elements.
const findLastIndex = (arr, fn) =>
(arr
.map((val, i) => [i, val])
.filter(([i, val]) => fn(val, i, arr))
.pop() || [-1])[0];
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)
findLastKey
- title: findLastKey
- tags: object,intermediate
Finds the last key that satisfies the provided testing function.
Otherwise undefined
is returned.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.reverse()
to reverse the order andArray.prototype.find()
to test the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const findLastKey = (obj, fn) =>
Object.keys(obj)
.reverse()
.find(key => fn(obj[key], key, obj));
findLastKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
x => x['active']
); // 'pebbles'
flatten
- title: flatten
- tags: array,recursion,intermediate
Flattens an array up to the specified depth.
- Use recursion, decrementing
depth
by1
for each level of depth. - Use
Array.prototype.reduce()
andArray.prototype.concat()
to merge elements or arrays. - Base case, for
depth
equal to1
stops recursion. - Omit the second argument,
depth
, to flatten only to a depth of1
(single flatten).
const flatten = (arr, depth = 1) =>
arr.reduce(
(a, v) =>
a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v),
[]
);
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]
flattenObject
- title: flattenObject
- tags: object,recursion,advanced
Flattens an object with the paths for keys.
- Use recursion.
- Use
Object.keys(obj)
combined withArray.prototype.reduce()
to convert every leaf node to a flattened path node. - If the value of a key is an object, the function calls itself with the appropriate
prefix
to create the path usingObject.assign()
. - Otherwise, it adds the appropriate prefixed key-value pair to the accumulator object.
- You should always omit the second argument,
prefix
, unless you want every key to have a prefix.
const flattenObject = (obj, prefix = '') =>
Object.keys(obj).reduce((acc, k) => {
const pre = prefix.length ? `${prefix}.` : '';
if (
typeof obj[k] === 'object' &&
obj[k] !== null &&
Object.keys(obj[k]).length > 0
)
Object.assign(acc, flattenObject(obj[k], pre + k));
else acc[pre + k] = obj[k];
return acc;
}, {});
flattenObject({ a: { b: { c: 1 } }, d: 1 }); // { 'a.b.c': 1, d: 1 }
flip
- title: flip
- tags: function,intermediate
Takes a function as an argument, then makes the first argument the last.
- Use argument destructuring and a closure with variadic arguments.
- Splice the first argument, using the spread operator (
...
), to make it the last before applying the rest.
const flip = fn => (first, ...rest) => fn(...rest, first);
let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b
forEachRight
- title: forEachRight
- tags: array,intermediate
Executes a provided function once for each array element, starting from the array's last element.
- Use
Array.prototype.slice()
to clone the given array andArray.prototype.reverse()
to reverse it. - Use
Array.prototype.forEach()
to iterate over the reversed array.
const forEachRight = (arr, callback) =>
arr
.slice()
.reverse()
.forEach(callback);
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'
forOwn
- title: forOwn
- tags: object,intermediate
Iterates over all own properties of an object, running a callback for each one.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.forEach()
to run the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const forOwn = (obj, fn) =>
Object.keys(obj).forEach(key => fn(obj[key], key, obj));
forOwn({ foo: 'bar', a: 1 }, v => console.log(v)); // 'bar', 1
forOwnRight
- title: forOwnRight
- tags: object,intermediate
Iterates over all own properties of an object in reverse, running a callback for each one.
- Use
Object.keys(obj)
to get all the properties of the object,Array.prototype.reverse()
to reverse their order. - Use
Array.prototype.forEach()
to run the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const forOwnRight = (obj, fn) =>
Object.keys(obj)
.reverse()
.forEach(key => fn(obj[key], key, obj));
forOwnRight({ foo: 'bar', a: 1 }, v => console.log(v)); // 1, 'bar'
formToObject
- title: formToObject
- tags: browser,object,intermediate
Encodes a set of form elements as an object
.
- Use the
Foata
constructor to convert the HTMLform
toFoata
andArray.from()
to convert to an array. - Collect the object from the array using
Array.prototype.reduce()
.
const formToObject = form =>
Array.from(new Foata(form)).reduce(
(acc, [key, value]) => ({
...acc,
[key]: value
}),
{}
);
formToObject(document.querySelector('##form'));
// { email: 'test@email.com', name: 'Test Name' }
formatDuration
- title: formatDuration
- tags: date,math,string,intermediate
Returns the human-readable format of the given number of milliseconds.
- Divide
ms
with the appropriate values to obtain the appropriate values forday
,hour
,minute
,second
andmillisecond
. - Use
Object.entries()
withArray.prototype.filter()
to keep only non-zero values. - Use
Array.prototype.map()
to create the string for each value, pluralizing appropriately. - Use
String.prototype.join(', ')
to combine the values into a string.
const formatDuration = ms => {
if (ms < 0) ms = -ms;
const time = {
day: Math.floor(ms / 86400000),
hour: Math.floor(ms / 3600000) % 24,
minute: Math.floor(ms / 60000) % 60,
second: Math.floor(ms / 1000) % 60,
millisecond: Math.floor(ms) % 1000
};
return Object.entries(time)
.filter(val => val[1] !== 0)
.map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
.join(', ');
};
formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574);
// '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'
formatNumber
- title: formatNumber
- tags: string,math,beginner
Formats a number using the local number format order.
- Use
Number.prototype.toLocaleString()
to convert a number to using the local number format separators.
const formatNumber = num => num.toLocaleString();
formatNumber(123456); // '123,456' in `en-US`
formatNumber(15675436903); // '15.675.436.903' in `de-DE`
frequencies
- title: frequencies
- tags: array,object,intermediate
Creates an object with the unique values of an array as keys and their frequencies as the values.
- Use
Array.prototype.reduce()
to map unique values to an object's keys, adding to existing keys every time the same value is encountered.
const frequencies = arr =>
arr.reduce((a, v) => {
a[v] = a[v] ? a[v] + 1 : 1;
return a;
}, {});
frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // { a: 4, b: 2, c: 1 }
frequencies([...'ball']); // { b: 1, a: 1, l: 2 }
fromCamelCase
- title: fromCamelCase
- tags: string,intermediate
Converts a string from camelcase.
- Use
String.prototype.replace()
to break the string into words and add aseparator
between them. - Omit the second argument to use a default
separator
of_
.
const fromCamelCase = (str, separator = '_') =>
str
.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
.replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
.toLowerCase();
fromCamelCase('someDatabaseFieldName', ' '); // 'some database field name'
fromCamelCase('someLabelThatNeedsToBeDecamelized', '-');
// 'some-label-that-needs-to-be-decamelized'
fromCamelCase('someJavascriptProperty', '_'); // 'some_javascript_property'
fromCamelCase('JSONToCSV', '.'); // 'json.to.csv'
fromTimestamp
- title: fromTimestamp
- tags: date,beginner
Creates a Date
object from a Unix timestamp.
- Convert the timestamp to milliseconds by multiplying with
1000
. - Use
new Date()
to create a newDate
object.
const fromTimestamp = timestamp => new Date(timestamp * 1000);
fromTimestamp(1602162242); // 2020-10-08T13:04:02.000Z
frozenSet
- title: frozenSet
- tags: array,intermediate
Creates a frozen Set
object.
- Use the
new Set()
constructor to create a newSet
object fromiterable
. - Set the
add
,delete
andclear
methods of the newly created object toundefined
, so that they cannot be used, practically freezing the object.
const frozenSet = iterable => {
const s = new Set(iterable);
s.add = undefined;
s.delete = undefined;
s.clear = undefined;
return s;
};
frozenSet([1, 2, 3, 1, 2]);
// Set { 1, 2, 3, add: undefined, delete: undefined, clear: undefined }
fullscreen
- title: fullscreen
- tags: browser,intermediate
Opens or closes an element in fullscreen mode.
- Use
Document.querySelector()
andElement.requestFullscreen()
to open the given element in fullscreen. - Use
Document.exitFullscreen()
to exit fullscreen mode. - Omit the second argument,
el
, to usebody
as the default element. - Omit the first element,
mode
, to open the element in fullscreen mode by default.
const fullscreen = (mode = true, el = 'body') =>
mode
? document.querySelector(el).requestFullscreen()
: document.exitFullscreen();
fullscreen(); // Opens `body` in fullscreen mode
fullscreen(false); // Exits fullscreen mode
functionName
- title: functionName
- tags: function,beginner
Logs the name of a function.
- Use
console.debug()
and thename
property of the passed function to log the function's name to thedebug
channel of the console. - Return the given function
fn
.
const functionName = fn => (console.debug(fn.name), fn);
let m = functionName(Math.max)(5, 6);
// max (logged in debug channel of console)
// m = 6
functions
- title: functions
- tags: object,function,advanced
Gets an array of function property names from own (and optionally inherited) enumerable properties of an object.
- Use
Object.keys(obj)
to iterate over the object's own properties. - If
inherited
istrue
, useObject.getPrototypeOf(obj)
to also get the object's inherited properties. - Use
Array.prototype.filter()
to keep only those properties that are functions. - Omit the second argument,
inherited
, to not include inherited properties by default.
const functions = (obj, inherited = false) =>
(inherited
? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
: Object.keys(obj)
).filter(key => typeof obj[key] === 'function');
function Foo() {
this.a = () => 1;
this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']
gcd
- title: gcd
- tags: math,algorithm,recursion,intermediate
Calculates the greatest common divisor between two or more numbers/arrays.
- The inner
_gcd
function uses recursion. - Base case is when
y
equals0
. In this case, returnx
. - Otherwise, return the GCD of
y
and the remainder of the divisionx/y
.
const gcd = (...arr) => {
const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
return [...arr].reduce((a, b) => _gcd(a, b));
};
gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4
generateItems
- title: generateItems
- tags: array,function,intermediate
Generates an array with the given amount of items, using the given function.
- Use
Array.from()
to create an empty array of the specific length, callingfn
with the index of each newly created element. - The callback takes one argument - the index of each element.
const generateItems = (n, fn) => Array.from({ length: n }, (_, i) => fn(i));
generateItems(10, Math.random);
// [0.21, 0.08, 0.40, 0.96, 0.96, 0.24, 0.19, 0.96, 0.42, 0.70]
generatorToArray
- title: generatorToArray
- tags: function,array,generator,beginner
Converts the output of a generator function to an array.
- Use the spread operator (
...
) to convert the output of the generator function to an array.
const generatorToArray = gen => [...gen];
const s = new Set([1, 2, 1, 3, 1, 4]);
generatorToArray(s.entries()); // [[ 1, 1 ], [ 2, 2 ], [ 3, 3 ], [ 4, 4 ]]
geometricProgression
- title: geometricProgression
- tags: math,algorithm,intermediate
Initializes an array containing the numbers in the specified range where start
and end
are inclusive and the ratio between two terms is step
.
Returns an error if step
equals 1
.
- Use
Array.from()
,Math.log()
andMath.floor()
to create an array of the desired length,Array.prototype.map()
to fill with the desired values in a range. - Omit the second argument,
start
, to use a default value of1
. - Omit the third argument,
step
, to use a default value of2
.
const geometricProgression = (end, start = 1, step = 2) =>
Array.from({
length: Math.floor(Math.log(end / start) / Math.log(step)) + 1,
}).map((_, i) => start * step ** i);
geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]
get
- title: get
- tags: object,regexp,intermediate
Retrieves a set of properties indicated by the given selectors from an object.
- Use
Array.prototype.map()
for each selector,String.prototype.replace()
to replace square brackets with dots. - Use
String.prototype.split('.')
to split each selector. - Use
Array.prototype.filter()
to remove empty values andArray.prototype.reduce()
to get the value indicated by each selector.
const get = (from, ...selectors) =>
[...selectors].map(s =>
s
.replace(/\[([^\[\]]*)\]/g, '.$1.')
.split('.')
.filter(t => t !== '')
.reduce((prev, cur) => prev && prev[cur], from)
);
const obj = {
selector: { to: { val: 'val to select' } },
target: [1, 2, { a: 'test' }],
};
get(obj, 'selector.to.val', 'target[0]', 'target[2].a');
// ['val to select', 1, 'test']
getAncestors
- title: getAncestors
- tags: browser,beginner
Returns all the ancestors of an element from the document root to the given element.
- Use
Node.parentNode
and awhile
loop to move up the ancestor tree of the element. - Use
Array.prototype.unshift()
to add each new ancestor to the start of the array.
const getAncestors = el => {
let ancestors = [];
while (el) {
ancestors.unshift(el);
el = el.parentNode;
}
return ancestors;
};
getAncestors(document.querySelector('nav'));
// [document, html, body, header, nav]
getBaseURL
- title: getBaseURL
- tags: string,browser,regexp,beginner
Gets the current URL without any parameters or fragment identifiers.
- Use
String.prototype.replace()
with an appropriate regular expression to remove everything after either'?'
or'##'
, if found.
const getBaseURL = url => url.replace(/[?##].*$/, '');
getBaseURL('http://url.com/page?name=Adam&surname=Smith');
// 'http://url.com/page'
getColonTimeFrate
- title: getColonTimeFrate
- tags: date,string,beginner
Returns a string of the form HH:MM:SS
from a Date
object.
- Use
Date.prototype.toTimeString()
andString.prototype.slice()
to get theHH:MM:SS
part of a givenDate
object.
const getColonTimeFrate = date => date.toTimeString().slice(0, 8);
getColonTimeFrate(new Date()); // '08:38:00'
getDaysDiffBetweenDates
- title: getDaysDiffBetweenDates
- tags: date,intermediate
Calculates the difference (in days) between two dates.
- Subtract the two
Date
object and divide by the number of milliseconds in a day to get the difference (in days) between them.
const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
(dateFinal - dateInitial) / (1000 * 3600 * 24);
getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9
getElementsBiggerThanViewport
- title: getElementsBiggerThanViewport
- tags: browser,intermediate
Returns an array of HTML elements whose width is larger than that of the viewport's.
- Use
HTMLElement.offsetWidth
to get the width of thedocument
. - Use
Array.prototype.filter()
on the result ofDocument.querySelectorAll()
to check the width of all elements in the document.
const getElementsBiggerThanViewport = () => {
const docWidth = document.documentElement.offsetWidth;
return [...document.querySelectorAll('*')].filter(
el => el.offsetWidth > docWidth
);
};
getElementsBiggerThanViewport(); // <div id="ultra-wide-item" />
getImages
- title: getImages
- tags: browser,intermediate
Fetches all images from within an element and puts them into an array.
- Use
Element.getElementsByTagName()
to get all<img>
elements inside the provided element. - Use
Array.prototype.map()
to map everysrc
attribute of each<img>
element. - If
includeDuplicates
isfalse
, create a newSet
to eliminate duplicates and return it after spreading into an array. - Omit the second argument,
includeDuplicates
, to discard duplicates by default.
const getImages = (el, includeDuplicates = false) => {
const images = [...el.getElementsByTagName('img')].map(img =>
img.getAttribute('src')
);
return includeDuplicates ? images : [...new Set(images)];
};
getImages(document, true); // ['image1.jpg', 'image2.png', 'image1.png', '...']
getImages(document, false); // ['image1.jpg', 'image2.png', '...']
getMeridiemSuffixOfInteger
- title: getMeridiemSuffixOfInteger
- tags: date,beginner
Converts an integer to a suffixed string, adding am
or pm
based on its value.
- Use the modulo operator (
%
) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.
const getMeridiemSuffixOfInteger = num =>
num === 0 || num === 24
? 12 + 'am'
: num === 12
? 12 + 'pm'
: num < 12
? (num % 12) + 'am'
: (num % 12) + 'pm';
getMeridiemSuffixOfInteger(0); // '12am'
getMeridiemSuffixOfInteger(11); // '11am'
getMeridiemSuffixOfInteger(13); // '1pm'
getMeridiemSuffixOfInteger(25); // '1pm'
getMonthsDiffBetweenDates
- title: getMonthsDiffBetweenDates
- tags: date,intermediate
Calculates the difference (in months) between two dates.
- Use
Date.prototype.getFullYear()
andDate.prototype.getMonth()
to calculate the difference (in months) between twoDate
objects.
const getMonthsDiffBetweenDates = (dateInitial, dateFinal) =>
Math.max(
(dateFinal.getFullYear() - dateInitial.getFullYear()) * 12 +
dateFinal.getMonth() -
dateInitial.getMonth(),
0
);
getMonthsDiffBetweenDates(new Date('2017-12-13'), new Date('2018-04-29')); // 4
getParentsUntil
- title: getParentsUntil
- tags: browser,intermediate
Finds all the ancestors of an element up until the element matched by the specified selector.
- Use
Node.parentNode
and awhile
loop to move up the ancestor tree of the element. - Use
Array.prototype.unshift()
to add each new ancestor to the start of the array. - Use
Element.matches()
to check if the current element matches the specifiedselector
.
const getParentsUntil = (el, selector) => {
let parents = [],
_el = el.parentNode;
while (_el && typeof _el.matches === 'function') {
parents.unshift(_el);
if (_el.matches(selector)) return parents;
else _el = _el.parentNode;
}
return [];
};
getParentsUntil(document.querySelector('##home-link'), 'header');
// [header, nav, ul, li]
getProtocol
- title: getProtocol
- tags: browser,beginner
Gets the protocol being used on the current page.
- Use
Window.location.protocol
to get the protocol (http:
orhttps:
) of the current page.
const getProtocol = () => window.location.protocol;
getProtocol(); // 'https:'
getScrollPosition
- title: getScrollPosition
- tags: browser,intermediate
Returns the scroll position of the current page.
- Use
Window.pageXOffset
andWindow.pageYOffset
if they are defined, otherwiseElement.scrollLeft
andElement.scrollTop
. - Omit the single argument,
el
, to use a default value ofwindow
.
const getScrollPosition = (el = window) => ({
x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});
getScrollPosition(); // {x: 0, y: 200}
getSelectedText
- title: getSelectedText
- tags: browser,beginner
Gets the currently selected text.
- Use
Window.getSelection()
andSelection.toString()
to get the currently selected text.
const getSelectedText = () => window.getSelection().toString();
getSelectedText(); // 'Lorem ipsum'
getSiblings
- title: getSiblings
- tags: browser,intermediate
Returns an array containing all the siblings of the given element.
- Use
Node.parentNode
andNode.childNodes
to get aNodeList
of all the elements contained in the element's parent. - Use the spread operator (
...
) andArray.prototype.filter()
to convert to an array and remove the given element from it.
const getSiblings = el =>
[...el.parentNode.childNodes].filter(node => node !== el);
getSiblings(document.querySelector('head')); // ['body']
getStyle
- title: getStyle
- tags: browser,css,beginner
Retrieves the value of a CSS rule for the specified element.
- Use
Window.getComputedStyle()
to get the value of the CSS rule for the specified element.
const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];
getStyle(document.querySelector('p'), 'font-size'); // '16px'
getTimestamp
- title: getTimestamp
- tags: date,beginner
Gets the Unix timestamp from a Date
object.
- Use
Date.prototype.getTime()
to get the timestamp in milliseconds and divide by1000
to get the timestamp in seconds. - Use
Math.floor()
to appropriately round the resulting timestamp to an integer. - Omit the argument,
date
, to use the current date.
const getTimestamp = (date = new Date()) => Math.floor(date.getTime() / 1000);
getTimestamp(); // 1602162242
getType
- title: getType
- tags: type,beginner
Returns the native type of a value.
- Return
'undefined'
or'null'
if the value isundefined
ornull
. - Otherwise, use
Object.prototype.constructor.name
to get the name of the constructor.
const getType = v =>
(v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name);
getType(new Set([1, 2, 3])); // 'Set'
getURLParameters
- title: getURLParameters
- tags: browser,string,regexp,intermediate
Creates an object containing the parameters of the current URL.
- Use
String.prototype.match()
with an appropriate regular expression to get all key-value pairs. - Use
Array.prototype.reduce()
to map and combine them into a single object. - Pass
location.search
as the argument to apply to the currenturl
.
const getURLParameters = url =>
(url.match(/([^?=&]+)(=([^&]*))/g) || []).reduce(
(a, v) => (
(a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a
),
{}
);
getURLParameters('google.com'); // {}
getURLParameters('http://url.com/page?name=Adam&surname=Smith');
// {name: 'Adam', surname: 'Smith'}
getVerticalOffset
- title: getVerticalOffset
- tags: browser,beginner
Finds the distance from a given element to the top of the document.
- Use a
while
loop andHTMLElement.offsetParent
to move up the offset parents of the given element. - Add
HTMLElement.offsetTop
for each element and return the result.
const getVerticalOffset = el => {
let offset = el.offsetTop,
_el = el;
while (_el.offsetParent) {
_el = _el.offsetParent;
offset += _el.offsetTop;
}
return offset;
};
getVerticalOffset('.my-element'); // 120
groupBy
- title: groupBy
- tags: array,object,intermediate
Groups the elements of an array based on the given function.
- Use
Array.prototype.map()
to map the values of the array to a function or property name. - Use
Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const groupBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val, i) => {
acc[val] = (acc[val] || []).concat(arr[i]);
return acc;
}, {});
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}
hammingDistance
- title: hammingDistance
- tags: math,algorithm,intermediate
Calculates the Hamming distance between two values.
- Use the XOR operator (
^
) to find the bit difference between the two numbers. - Convert to a binary string using
Number.prototype.toString(2)
. - Count and return the number of
1
s in the string, usingString.prototype.match(/1/g)
.
const hammingDistance = (num1, num2) =>
((num1 ^ num2).toString(2).match(/1/g) || '').length;
hammingDistance(2, 3); // 1
hasClass
- title: hasClass
- tags: browser,css,beginner
Checks if the given element has the specified class.
- Use
Element.classList
andDOMTokenList.contains()
to check if the element has the specified class.
const hasClass = (el, className) => el.classList.contains(className);
hasClass(document.querySelector('p.special'), 'special'); // true
hasDuplicates
- title: hasDuplicates
- tags: array,beginner
Checks if there are duplicate values in a flat array.
- Use
Set()
to get the unique values in the array. - Use
Set.prototype.size
andArray.prototype.length
to check if the count of the unique values is the same as elements in the original array.
const hasDuplicates = arr => new Set(arr).size !== arr.length;
hasDuplicates([0, 1, 1, 2]); // true
hasDuplicates([0, 1, 2, 3]); // false
hasFlags
- title: hasFlags
- tags: node,intermediate
Checks if the current process's arguments contain the specified flags.
- Use
Array.prototype.every()
andArray.prototype.includes()
to check ifprocess.argv
contains all the specified flags. - Use a regular expression to test if the specified flags are prefixed with
-
or--
and prefix them accordingly.
const hasFlags = (...flags) =>
flags.every(flag =>
process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag)
);
// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // false
hasKey
- title: hasKey
- tags: object,intermediate
Checks if the target value exists in a JSON object.
- Check if
keys
is non-empty and useArray.prototype.every()
to sequentially check its keys to internal depth of the object,obj
. - Use
Object.prototype.hasOwnProperty()
to check ifobj
does not have the current key or is not an object, stop propagation and returnfalse
. - Otherwise assign the key's value to
obj
to use on the next iteration. - Return
false
beforehand if given key list is empty.
const hasKey = (obj, keys) => {
return (
keys.length > 0 &&
keys.every(key => {
if (typeof obj !== 'object' || !obj.hasOwnProperty(key)) return false;
obj = obj[key];
return true;
})
);
};
let obj = {
a: 1,
b: { c: 4 },
'b.d': 5
};
hasKey(obj, ['a']); // true
hasKey(obj, ['b']); // true
hasKey(obj, ['b', 'c']); // true
hasKey(obj, ['b.d']); // true
hasKey(obj, ['d']); // false
hasKey(obj, ['c']); // false
hasKey(obj, ['b', 'f']); // false
hashBrowser
- title: hashBrowser
- tags: browser,promise,advanced
Creates a hash for a value using the SHA-256 algorithm. Returns a promise.
- Use the SubtleCrypto API to create a hash for the given value.
- Create a new
TextEncoder
and use it to encodeval
, passing its value toSubtleCrypto.digest()
to generate a digest of the given data. - Use
DataView.prototype.getUint32()
to read data from the resolvedArrayBuffer
. - Add the data to an array using
Array.prototype.push()
after converting it to its hexadecimal representation usingNumber.prototype.toString(16)
. - Finally, use
Array.prototype.join()
to combine values in the array ofhexes
into a string.
const hashBrowser = val =>
crypto.subtle
.digest('SHA-256', new TextEncoder('utf-8').encode(val))
.then(h => {
let hexes = [],
view = new DataView(h);
for (let i = 0; i < view.byteLength; i += 4)
hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
return hexes.join('');
});
hashBrowser(
JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })
).then(console.log);
// '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'
hashNode
- title: hashNode
- tags: node,promise,advanced
Creates a hash for a value using the SHA-256 algorithm. Returns a promise.
- Use
crypto.createHash()
to create aHash
object with the appropriate algorithm. - Use
hash.update()
to add the data fromval
to theHash
,hash.digest()
to calculate the digest of the data. - Use
setTimeout()
to prevent blocking on a long operation, and return aPromise
to give it a familiar interface.
const crypto = require('crypto');
const hashNode = val =>
new Promise(resolve =>
setTimeout(
() => resolve(crypto.createHash('sha256').update(val).digest('hex')),
0
)
);
hashNode(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(
console.log
);
// '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'
haveSameContents
- title: haveSameContents
- tags: array,intermediate
Checks if two arrays contain the same elements regardless of order.
- Use a
for...of
loop over aSet
created from the values of both arrays. - Use
Array.prototype.filter()
to compare the amount of occurrences of each distinct value in both arrays. - Return
false
if the counts do not match for any element,true
otherwise.
const haveSameContents = (a, b) => {
for (const v of new Set([...a, ...b]))
if (a.filter(e => e === v).length !== b.filter(e => e === v).length)
return false;
return true;
};
haveSameContents([1, 2, 4], [2, 4, 1]); // true
head
- title: head
- tags: array,beginner
Returns the head of an array.
- Check if
arr
is truthy and has alength
property. - Use
arr[0]
if possible to return the first element, otherwise returnundefined
.
const head = arr => (arr && arr.length ? arr[0] : undefined);
head([1, 2, 3]); // 1
head([]); // undefined
head(null); // undefined
head(undefined); // undefined
heapsort
- title: heapsort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the heapsort algorithm.
- Use recursion.
- Use the spread operator (
...
) to clone the original array,arr
. - Use closures to declare a variable,
l
, and a functionheapify
. - Use a
for
loop andMath.floor()
in combination withheapify
to create a max heap from the array. - Use a
for
loop to repeatedly narrow down the considered range, usingheapify
and swapping values as necessary in order to sort the cloned array.
const heapsort = arr => {
const a = [...arr];
let l = a.length;
const heapify = (a, i) => {
const left = 2 * i + 1;
const right = 2 * i + 2;
let max = i;
if (left < l && a[left] > a[max]) max = left;
if (right < l && a[right] > a[max]) max = right;
if (max !== i) {
[a[max], a[i]] = [a[i], a[max]];
heapify(a, max);
}
};
for (let i = Math.floor(l / 2); i >= 0; i -= 1) heapify(a, i);
for (i = a.length - 1; i > 0; i--) {
[a[0], a[i]] = [a[i], a[0]];
l--;
heapify(a, 0);
}
return a;
};
heapsort([6, 3, 4, 1]); // [1, 3, 4, 6]
hexToRGB
- title: hexToRGB
- tags: string,math,advanced
Converts a color code to an rgb()
or rgba()
string if alpha value is provided.
- Use bitwise right-shift operator and mask bits with
&
(and) operator to convert a hexadecimal color code (with or without prefixed with##
) to a string with the RGB values. - If it's 3-digit color code, first convert to 6-digit version.
- If an alpha value is provided alongside 6-digit hex, give
rgba()
string in return.
const hexToRGB = hex => {
let alpha = false,
h = hex.slice(hex.startsWith('##') ? 1 : 0);
if (h.length === 3) h = [...h].map(x => x + x).join('');
else if (h.length === 8) alpha = true;
h = parseInt(h, 16);
return (
'rgb' +
(alpha ? 'a' : '') +
'(' +
(h >>> (alpha ? 24 : 16)) +
', ' +
((h & (alpha ? 0x00ff0000 : 0x00ff00)) >>> (alpha ? 16 : 8)) +
', ' +
((h & (alpha ? 0x0000ff00 : 0x0000ff)) >>> (alpha ? 8 : 0)) +
(alpha ? `, ${h & 0x000000ff}` : '') +
')'
);
};
hexToRGB('##27ae60ff'); // 'rgba(39, 174, 96, 255)'
hexToRGB('27ae60'); // 'rgb(39, 174, 96)'
hexToRGB('##fff'); // 'rgb(255, 255, 255)'
hide
- title: hide
- tags: browser,css,beginner
Hides all the elements specified.
- Use
NodeList.prototype.forEach()
to applydisplay: none
to each element specified.
const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));
hide(document.querySelectorAll('img')); // Hides all <img> elements on the page
httpDelete
- title: httpDelete
- tags: browser,intermediate
Makes a DELETE
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aDELETE
request to the givenurl
. - Handle the
onload
event, by running the providedcallback
function. - Handle the
onerror
event, by running the providederr
function. - Omit the third argument,
err
to log the request to the console's error stream by default.
const httpDelete = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('DELETE', url, true);
request.onload = () => callback(request);
request.onerror = () => err(request);
request.send();
};
httpDelete('https://jsonplaceholder.typicode.com/posts/1', request => {
console.log(request.responseText);
}); // Logs: {}
httpGet
- title: httpGet
- tags: browser,intermediate
Makes a GET
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aGET
request to the givenurl
. - Handle the
onload
event, by calling the givencallback
theresponseText
. - Handle the
onerror
event, by running the providederr
function. - Omit the third argument,
err
, to log errors to the console'serror
stream by default.
const httpGet = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('GET', url, true);
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send();
};
httpGet(
'https://jsonplaceholder.typicode.com/posts/1',
console.log
); /*
Logs: {
"userId": 1,
"id": 1,
"title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
"body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}
*/
httpPost
- title: httpPost
- tags: browser,intermediate
Makes a POST
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aPOST
request to the givenurl
. - Set the value of an
HTTP
request header withsetRequestHeader
method. - Handle the
onload
event, by calling the givencallback
theresponseText
. - Handle the
onerror
event, by running the providederr
function. - Omit the fourth argument,
err
, to log errors to the console'serror
stream by default.
const httpPost = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('POST', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send(data);
};
const newPost = {
userId: 1,
id: 1337,
- title: 'Foo',
body: 'bar bar bar'
};
const data = JSON.stringify(newPost);
httpPost(
'https://jsonplaceholder.typicode.com/posts',
data,
console.log
); /*
Logs: {
"userId": 1,
"id": 1337,
"title": "Foo",
"body": "bar bar bar"
}
*/
httpPost(
'https://jsonplaceholder.typicode.com/posts',
null, // does not send a body
console.log
); /*
Logs: {
"id": 101
}
*/
httpPut
- title: httpPut
- tags: browser,intermediate
Makes a PUT
request to the passed URL.
- Use
XMLHttpRequest
web api to make aPUT
request to the givenurl
. - Set the value of an
HTTP
request header withsetRequestHeader
method. - Handle the
onload
event, by running the providedcallback
function. - Handle the
onerror
event, by running the providederr
function. - Omit the last argument,
err
to log the request to the console's error stream by default.
const httpPut = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('PUT', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request);
request.onerror = () => err(request);
request.send(data);
};
const password = 'fooBaz';
const data = JSON.stringify({
id: 1,
- title: 'foo',
body: 'bar',
userId: 1
});
httpPut('https://jsonplaceholder.typicode.com/posts/1', data, request => {
console.log(request.responseText);
}); /*
Logs: {
id: 1,
- title: 'foo',
body: 'bar',
userId: 1
}
*/
httpsRedirect
- title: httpsRedirect
- tags: browser,intermediate
Redirects the page to HTTPS if it's currently in HTTP.
- Use
location.protocol
to get the protocol currently being used. - If it's not HTTPS, use
location.replace()
to replace the existing page with the HTTPS version of the page. - Use
location.href
to get the full address, split it withString.prototype.split()
and remove the protocol part of the URL. - Note that pressing the back button doesn't take it back to the HTTP page as its replaced in the history.
const httpsRedirect = () => {
if (location.protocol !== 'https:')
location.replace('https://' + location.href.split('//')[1]);
};
httpsRedirect();
// If you are on http://mydomain.com, you are redirected to https://mydomain.com
hz
- title: hz
- tags: function,intermediate unlisted: true
Measures the number of times a function is executed per second (hz
/hertz
).
- Use
performance.now()
to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the functioniterations
times. - Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed.
- Omit the second argument,
iterations
, to use the default of 100 iterations.
const hz = (fn, iterations = 100) => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return (1000 * iterations) / (performance.now() - before);
};
const numbers = Array(10000).fill().map((_, i) => i);
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
let sum = 0;
for (let i = 0; i < numbers.length; i++) sum += numbers[i];
return sum;
};
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784
inRange
- title: inRange
- tags: math,beginner
Checks if the given number falls within the given range.
- Use arithmetic comparison to check if the given number is in the specified range.
- If the second argument,
end
, is not specified, the range is considered to be from0
tostart
.
const inRange = (n, start, end = null) => {
if (end && start > end) [end, start] = [start, end];
return end == null ? n >= 0 && n < start : n >= start && n < end;
};
inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // false
includesAll
- title: includesAll
- tags: array,beginner
Checks if all the elements in values
are included in arr
.
- Use
Array.prototype.every()
andArray.prototype.includes()
to check if all elements ofvalues
are included inarr
.
const includesAll = (arr, values) => values.every(v => arr.includes(v));
includesAll([1, 2, 3, 4], [1, 4]); // true
includesAll([1, 2, 3, 4], [1, 5]); // false
includesAny
- title: includesAny
- tags: array,beginner
Checks if at least one element of values
is included in arr
.
- Use
Array.prototype.some()
andArray.prototype.includes()
to check if at least one element ofvalues
is included inarr
.
const includesAny = (arr, values) => values.some(v => arr.includes(v));
includesAny([1, 2, 3, 4], [2, 9]); // true
includesAny([1, 2, 3, 4], [8, 9]); // false
indentString
- title: indentString
- tags: string,beginner
Indents each line in the provided string.
- Use
String.prototype.replace()
and a regular expression to add the character specified byindent
count
times at the start of each line. - Omit the third argument,
indent
, to use a default indentation character of' '
.
const indentString = (str, count, indent = ' ') =>
str.replace(/^/gm, indent.repeat(count));
indentString('Lorem\nIpsum', 2); // ' Lorem\n Ipsum'
indentString('Lorem\nIpsum', 2, '_'); // '__Lorem\n__Ipsum'
indexOfAll
- title: indexOfAll
- tags: array,intermediate
Finds all indexes of val
in an array.
If val
never occurs, returns an empty array.
- Use
Array.prototype.reduce()
to loop over elements and store indexes for matching elements.
const indexOfAll = (arr, val) =>
arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3]
indexOfAll([1, 2, 3], 4); // []
indexOfSubstrings
- title: indexOfSubstrings
- tags: string,algorithm,generator,intermediate
Finds all the indexes of a substring in a given string.
- Use
Array.prototype.indexOf()
to look forsearchValue
instr
. - Use
yield
to return the index if the value is found and update the index,i
. - Use a
while
loop that will terminate the generator as soon as the value returned fromArray.prototype.indexOf()
is-1
.
const indexOfSubstrings = function* (str, searchValue) {
let i = 0;
while (true) {
const r = str.indexOf(searchValue, i);
if (r !== -1) {
yield r;
i = r + 1;
} else return;
}
};
[...indexOfSubstrings('tiktok tok tok tik tok tik', 'tik')]; // [0, 15, 23]
[...indexOfSubstrings('tutut tut tut', 'tut')]; // [0, 2, 6, 10]
[...indexOfSubstrings('hello', 'hi')]; // []
initial
- title: initial
- tags: array,beginner
Returns all the elements of an array except the last one.
- Use
Array.prototype.slice(0, -1)
to return all but the last element of the array.
const initial = arr => arr.slice(0, -1);
initial([1, 2, 3]); // [1, 2]
initialize2DArray
- title: initialize2DArray
- tags: array,intermediate
Initializes a 2D array of given width and height and value.
- Use
Array.from()
andArray.prototype.map()
to generateh
rows where each is a new array of sizew
. - Use
Array.prototype.fill()
to initialize all items with valueval
. - Omit the last argument,
val
, to use a default value ofnull
.
const initialize2DArray = (w, h, val = null) =>
Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));
initialize2DArray(2, 2, 0); // [[0, 0], [0, 0]]
initializeArrayWithRange
- title: initializeArrayWithRange
- tags: array,intermediate
Initializes an array containing the numbers in the specified range where start
and end
are inclusive with their common difference step
.
- Use
Array.from()
to create an array of the desired length. - Use
(end - start + 1)/step
and a map function to fill the array with the desired values in the given range. - Omit the second argument,
start
, to use a default value of0
. - Omit the last argument,
step
, to use a default value of1
.
const initializeArrayWithRange = (end, start = 0, step = 1) =>
Array.from(
{ length: Math.ceil((end - start + 1) / step) },
(_, i) => i * step + start
);
initializeArrayWithRange(5); // [0, 1, 2, 3, 4, 5]
initializeArrayWithRange(7, 3); // [3, 4, 5, 6, 7]
initializeArrayWithRange(9, 0, 2); // [0, 2, 4, 6, 8]
initializeArrayWithRangeRight
- title: initializeArrayWithRangeRight
- tags: array,intermediate
Initializes an array containing the numbers in the specified range (in reverse) where start
and end
are inclusive with their common difference step
.
- Use
Array.from(Math.ceil((end+1-start)/step))
to create an array of the desired length(the amounts of elements is equal to(end-start)/step
or(end+1-start)/step
for inclusive end),Array.prototype.map()
to fill with the desired values in a range. - Omit the second argument,
start
, to use a default value of0
. - Omit the last argument,
step
, to use a default value of1
.
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
(v, i, arr) => (arr.length - i - 1) * step + start
);
initializeArrayWithRangeRight(5); // [5, 4, 3, 2, 1, 0]
initializeArrayWithRangeRight(7, 3); // [7, 6, 5, 4, 3]
initializeArrayWithRangeRight(9, 0, 2); // [8, 6, 4, 2, 0]
initializeArrayWithValues
- title: initializeArrayWithValues
- tags: array,intermediate
Initializes and fills an array with the specified values.
- Use
Array.from()
to create an array of the desired length,Array.prototype.fill()
to fill it with the desired values. - Omit the last argument,
val
, to use a default value of0
.
const initializeArrayWithValues = (n, val = 0) =>
Array.from({ length: n }).fill(val);
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]
initializeNDArray
- title: initializeNDArray
- tags: array,recursion,intermediate
Create a n-dimensional array with given value.
- Use recursion.
- Use
Array.from()
,Array.prototype.map()
to generate rows where each is a new array initialized usinginitializeNDArray()
.
const initializeNDArray = (val, ...args) =>
args.length === 0
? val
: Array.from({ length: args[0] }).map(() =>
initializeNDArray(val, ...args.slice(1))
);
initializeNDArray(1, 3); // [1, 1, 1]
initializeNDArray(5, 2, 2, 2); // [[[5, 5], [5, 5]], [[5, 5], [5, 5]]]
injectCSS
- title: injectCSS
- tags: browser,css,intermediate
Injects the given CSS code into the current document
- Use
Document.createElement()
to create a newstyle
element and set its type totext/css
. - Use
Element.innerText
to set the value to the given CSS string. - Use
Document.head
andElement.appendChild()
to append the new element to the document head. - Return the newly created
style
element.
const injectCSS = css => {
let el = document.createElement('style');
el.type = 'text/css';
el.innerText = css;
document.head.appendChild(el);
return el;
};
injectCSS('body { background-color: ##000 }');
// '<style type="text/css">body { background-color: ##000 }</style>'
insertAfter
- title: insertAfter
- tags: browser,beginner
Inserts an HTML string after the end of the specified element.
- Use
Element.insertAdjacentHTML()
with a position of'afterend'
to parsehtmlString
and insert it after the end ofel
.
const insertAfter = (el, htmlString) =>
el.insertAdjacentHTML('afterend', htmlString);
insertAfter(document.getElementById('myId'), '<p>after</p>');
// <div id="myId">...</div> <p>after</p>
insertAt
- title: insertAt
- tags: array,intermediate
Mutates the original array to insert the given values after the specified index.
- Use
Array.prototype.splice()
with an appropriate index and a delete count of0
, spreading the given values to be inserted.
const insertAt = (arr, i, ...v) => {
arr.splice(i + 1, 0, ...v);
return arr;
};
let myArray = [1, 2, 3, 4];
insertAt(myArray, 2, 5); // myArray = [1, 2, 3, 5, 4]
let otherArray = [2, 10];
insertAt(otherArray, 0, 4, 6, 8); // otherArray = [2, 4, 6, 8, 10]
insertBefore
- title: insertBefore
- tags: browser,beginner
Inserts an HTML string before the start of the specified element.
- Use
Element.insertAdjacentHTML()
with a position of'beforebegin'
to parsehtmlString
and insert it before the start ofel
.
const insertBefore = (el, htmlString) =>
el.insertAdjacentHTML('beforebegin', htmlString);
insertBefore(document.getElementById('myId'), '<p>before</p>');
// <p>before</p> <div id="myId">...</div>
insertionSort
- title: insertionSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the insertion sort algorithm.
- Use
Array.prototype.reduce()
to iterate over all the elements in the given array. - If the
length
of the accumulator is0
, add the current element to it. - Use
Array.prototype.some()
to iterate over the results in the accumulator until the correct position is found. - Use
Array.prototype.splice()
to insert the current element into the accumulator.
const insertionSort = arr =>
arr.reduce((acc, x) => {
if (!acc.length) return [x];
acc.some((y, j) => {
if (x <= y) {
acc.splice(j, 0, x);
return true;
}
if (x > y && j === acc.length - 1) {
acc.splice(j + 1, 0, x);
return true;
}
return false;
});
return acc;
}, []);
insertionSort([6, 3, 4, 1]); // [1, 3, 4, 6]
intersection
- title: intersection
- tags: array,intermediate
Returns the elements that exist in both arrays, filtering duplicate values.
- Create a
Set
fromb
, then useArray.prototype.filter()
ona
to only keep values contained inb
.
const intersection = (a, b) => {
const s = new Set(b);
return [...new Set(a)].filter(x => s.has(x));
};
intersection([1, 2, 3], [4, 3, 2]); // [2, 3]
intersectionBy
- title: intersectionBy
- tags: array,intermediate
Returns the elements that exist in both arrays, after applying the provided function to each array element of both.
- Create a
Set
by applyingfn
to all elements inb
. - Use
Array.prototype.filter()
ona
to only keep elements, which produce values contained inb
whenfn
is applied to them.
const intersectionBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return [...new Set(a)].filter(x => s.has(fn(x)));
};
intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]
intersectionBy(
[{ - title: 'Apple' }, { - title: 'Orange' }],
[{ - title: 'Orange' }, { - title: 'Melon' }],
x => x.title
); // [{ - title: 'Orange' }]
intersectionWith
- title: intersectionWith
- tags: array,intermediate
Returns the elements that exist in both arrays, using a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
in combination with the provided comparator to determine intersecting values.
const intersectionWith = (a, b, comp) =>
a.filter(x => b.findIndex(y => comp(x, y)) !== -1);
intersectionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1.5, 3, 0]
invertKeyValues
- title: invertKeyValues
- tags: object,advanced
Inverts the key-value pairs of an object, without mutating it.
- Use
Object.keys()
andArray.prototype.reduce()
to invert the key-value pairs of an object and apply the function provided (if any). - Omit the second argument,
fn
, to get the inverted keys without applying a function to them. - The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.
const invertKeyValues = (obj, fn) =>
Object.keys(obj).reduce((acc, key) => {
const val = fn ? fn(obj[key]) : obj[key];
acc[val] = acc[val] || [];
acc[val].push(key);
return acc;
}, {});
invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value);
// { group1: [ 'a', 'c' ], group2: [ 'b' ] }
is
- title: is
- tags: type,array,intermediate
Checks if the provided value is of the specified type.
- Ensure the value is not
undefined
ornull
usingArray.prototype.includes()
. - Compare the
constructor
property on the value withtype
to check if the provided value is of the specifiedtype
.
const is = (type, val) => ![, null].includes(val) && val.constructor === type;
is(Array, [1]); // true
is(ArrayBuffer, new ArrayBuffer()); // true
is(Map, new Map()); // true
is(RegExp, /./g); // true
is(Set, new Set()); // true
is(WeakMap, new WeakMap()); // true
is(WeakSet, new WeakSet()); // true
is(String, ''); // true
is(String, new String('')); // true
is(Number, 1); // true
is(Number, new Number(1)); // true
is(Boolean, true); // true
is(Boolean, new Boolean(true)); // true
isAbsoluteURL
- title: isAbsoluteURL
- tags: string,browser,regexp,intermediate
Checks if the given string is an absolute URL.
- Use
RegExp.prototype.test()
to test if the string is an absolute URL.
const isAbsoluteURL = str => /^[a-z][a-z0-9+.-]*:/.test(str);
isAbsoluteURL('https://google.com'); // true
isAbsoluteURL('ftp://www.myserver.net'); // true
isAbsoluteURL('/foo/bar'); // false
isAfterDate
- title: isAfterDate
- tags: date,beginner
Checks if a date is after another date.
- Use the greater than operator (
>
) to check if the first date comes after the second one.
const isAfterDate = (dateA, dateB) => dateA > dateB;
isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // true
isAlpha
- title: isAlpha
- tags: string,regexp,beginner
Checks if a string contains only alpha characters.
- Use
RegExp.prototype.test()
to check if the given string matches against the alphabetic regexp pattern.
const isAlpha = str => /^[a-zA-Z]*$/.test(str);
isAlpha('sampleInput'); // true
isAlpha('this Will fail'); // false
isAlpha('123'); // false
isAlphaNumeric
- title: isAlphaNumeric
- tags: string,regexp,beginner
Checks if a string contains only alphanumeric characters.
- Use
RegExp.prototype.test()
to check if the input string matches against the alphanumeric regexp pattern.
const isAlphaNumeric = str => /^[a-z0-9]+$/gi.test(str);
isAlphaNumeric('hello123'); // true
isAlphaNumeric('123'); // true
isAlphaNumeric('hello 123'); // false (space character is not alphanumeric)
isAlphaNumeric('##$hello'); // false
isAnagram
- title: isAnagram
- tags: string,regexp,intermediate
Checks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).
- Use
String.prototype.toLowerCase()
andString.prototype.replace()
with an appropriate regular expression to remove unnecessary characters. - Use
String.prototype.split('')
,Array.prototype.sort()
andArray.prototype.join('')
on both strings to normalize them, then check if their normalized forms are equal.
const isAnagram = (str1, str2) => {
const normalize = str =>
str
.toLowerCase()
.replace(/[^a-z0-9]/gi, '')
.split('')
.sort()
.join('');
return normalize(str1) === normalize(str2);
};
isAnagram('iceman', 'cinema'); // true
isArrayLike
- title: isArrayLike
- tags: type,array,intermediate
Checks if the provided argument is array-like (i.e. is iterable).
- Check if the provided argument is not
null
and that itsSymbol.iterator
property is a function.
const isArrayLike = obj =>
obj != null && typeof obj[Symbol.iterator] === 'function';
isArrayLike([1, 2, 3]); // true
isArrayLike(document.querySelectorAll('.className')); // true
isArrayLike('abc'); // true
isArrayLike(null); // false
isAsyncFunction
- title: isAsyncFunction
- tags: type,function,intermediate
Checks if the given argument is an async
function.
- Use
Object.prototype.toString()
andFunction.prototype.call()
and check if the result is'[object AsyncFunction]'
.
const isAsyncFunction = val =>
Object.prototype.toString.call(val) === '[object AsyncFunction]';
isAsyncFunction(function() {}); // false
isAsyncFunction(async function() {}); // true
isBeforeDate
- title: isBeforeDate
- tags: date,beginner
Checks if a date is before another date.
- Use the less than operator (
<
) to check if the first date comes before the second one.
const isBeforeDate = (dateA, dateB) => dateA < dateB;
isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // true
isBetweenDates
- title: isBetweenDates
- tags: date,beginner
Checks if a date is between two other dates.
- Use the greater than (
>
) and less than (<
) operators to check ifdate
is betweendateStart
anddateEnd
.
const isBetweenDates = (dateStart, dateEnd, date) =>
date > dateStart && date < dateEnd;
isBetweenDates(
new Date(2010, 11, 20),
new Date(2010, 11, 30),
new Date(2010, 11, 19)
); // false
isBetweenDates(
new Date(2010, 11, 20),
new Date(2010, 11, 30),
new Date(2010, 11, 25)
); // true
isBoolean
- title: isBoolean
- tags: type,beginner
Checks if the given argument is a native boolean element.
- Use
typeof
to check if a value is classified as a boolean primitive.
const isBoolean = val => typeof val === 'boolean';
isBoolean(null); // false
isBoolean(false); // true
isBrowser
- title: isBrowser
- tags: browser,node,intermediate
Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.
- Use
Array.prototype.includes()
on thetypeof
values of bothwindow
anddocument
(globals usually only available in a browser environment unless they were explicitly defined), which will returntrue
if one of them isundefined
. typeof
allows globals to be checked for existence without throwing aReferenceError
.- If both of them are not
undefined
, then the current environment is assumed to be a browser.
const isBrowser = () => ![typeof window, typeof document].includes('undefined');
isBrowser(); // true (browser)
isBrowser(); // false (Node)
isBrowserTabFocused
- title: isBrowserTabFocused
- tags: browser,beginner
Checks if the browser tab of the page is focused.
- Use the
Document.hidden
property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.
const isBrowserTabFocused = () => !document.hidden;
isBrowserTabFocused(); // true
isContainedIn
- title: isContainedIn
- tags: array,intermediate
Checks if the elements of the first array are contained in the second one regardless of order.
- Use a
for...of
loop over aSet
created from the first array. - Use
Array.prototype.some()
to check if all distinct values are contained in the second array. - Use
Array.prototype.filter()
to compare the number of occurrences of each distinct value in both arrays. - Return
false
if the count of any element is greater in the first array than the second one,true
otherwise.
const isContainedIn = (a, b) => {
for (const v of new Set(a)) {
if (
!b.some(e => e === v) ||
a.filter(e => e === v).length > b.filter(e => e === v).length
)
return false;
}
return true;
};
isContainedIn([1, 4], [2, 4, 1]); // true
isDateValid
- title: isDateValid
- tags: date,intermediate
Checks if a valid date object can be created from the given values.
- Use the spread operator (
...
) to pass the array of arguments to theDate
constructor. - Use
Date.prototype.valueOf()
andNumber.isNaN()
to check if a validDate
object can be created from the given values.
const isDateValid = (...val) => !Number.isNaN(new Date(...val).valueOf());
isDateValid('December 17, 1995 03:24:00'); // true
isDateValid('1995-12-17T03:24:00'); // true
isDateValid('1995-12-17 T03:24:00'); // false
isDateValid('Duck'); // false
isDateValid(1995, 11, 17); // true
isDateValid(1995, 11, 17, 'Duck'); // false
isDateValid({}); // false
isDeepFrozen
- title: isDeepFrozen
- tags: object,recursion,intermediate
Checks if an object is deeply frozen.
- Use recursion.
- Use
Object.isFrozen()
on the given object. - Use
Object.keys()
,Array.prototype.every()
to check that all keys are either deeply frozen objects or non-object values.
const isDeepFrozen = obj =>
Object.isFrozen(obj) &&
Object.keys(obj).every(
prop => typeof obj[prop] !== 'object' || isDeepFrozen(obj[prop])
);
const x = Object.freeze({ a: 1 });
const y = Object.freeze({ b: { c: 2 } });
isDeepFrozen(x); // true
isDeepFrozen(y); // false
isDisjoint
- title: isDisjoint
- tags: array,intermediate
Checks if the two iterables are disjointed (have no common values).
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that the two iterables have no common values.
const isDisjoint = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sA].every(v => !sB.has(v));
};
isDisjoint(new Set([1, 2]), new Set([3, 4])); // true
isDisjoint(new Set([1, 2]), new Set([1, 3])); // false
isDivisible
- title: isDivisible
- tags: math,beginner
Checks if the first numeric argument is divisible by the second one.
- Use the modulo operator (
%
) to check if the remainder is equal to0
.
const isDivisible = (dividend, divisor) => dividend % divisor === 0;
isDivisible(6, 3); // true
isDuplexStream
- title: isDuplexStream
- tags: node,type,intermediate
Checks if the given argument is a duplex (readable and writable) stream.
- Check if the value is different from
null
. - Use
typeof
to check if a value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_read
,_write
and_readableState
,_writableState
properties arefunction
andobject
respectively.
const isDuplexStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';
const Stream = require('stream');
isDuplexStream(new Stream.Duplex()); // true
isEmpty
- title: isEmpty
- tags: type,array,object,string,beginner
Checks if the a value is an empty object/collection, has no enumerable properties or is any type that is not considered a collection.
- Check if the provided value is
null
or if itslength
is equal to0
.
const isEmpty = val => val == null || !(Object.keys(val) || val).length;
isEmpty([]); // true
isEmpty({}); // true
isEmpty(''); // true
isEmpty([1, 2]); // false
isEmpty({ a: 1, b: 2 }); // false
isEmpty('text'); // false
isEmpty(123); // true - type is not considered a collection
isEmpty(true); // true - type is not considered a collection
isEven
- title: isEven
- tags: math,beginner
Checks if the given number is even.
- Checks whether a number is odd or even using the modulo (
%
) operator. - Returns
true
if the number is even,false
if the number is odd.
const isEven = num => num % 2 === 0;
isEven(3); // false
isFunction
- title: isFunction
- tags: type,function,beginner
Checks if the given argument is a function.
- Use
typeof
to check if a value is classified as a function primitive.
const isFunction = val => typeof val === 'function';
isFunction('x'); // false
isFunction(x => x); // true
isGeneratorFunction
- title: isGeneratorFunction
- tags: type,function,intermediate
Checks if the given argument is a generator function.
- Use
Object.prototype.toString()
andFunction.prototype.call()
and check if the result is'[object GeneratorFunction]'
.
const isGeneratorFunction = val =>
Object.prototype.toString.call(val) === '[object GeneratorFunction]';
isGeneratorFunction(function() {}); // false
isGeneratorFunction(function*() {}); // true
isISOString
- title: isISOString
- tags: date,intermediate
Checks if the given string is valid in the simplified extended ISO format (ISO 8601).
- Use
new Date()
to create a date object from the given string. - Use
Date.prototype.valueOf()
andNumber.isNaN()
to check if the produced date object is valid. - Use
Date.prototype.toISOString()
to compare the ISO formatted string representation of the date with the original string.
const isISOString = val => {
const d = new Date(val);
return !Number.isNaN(d.valueOf()) && d.toISOString() === val;
};
isISOString('2020-10-12T10:10:10.000Z'); // true
isISOString('2020-10-12'); // false
isLeapYear
- title: isLeapYear
- tags: date,beginner
Checks if the given year
is a leap year.
- Use
new Date()
, setting the date to February 29th of the givenyear
. - Use
Date.prototype.getMonth()
to check if the month is equal to1
.
const isLeapYear = year => new Date(year, 1, 29).getMonth() === 1;
isLeapYear(2019); // false
isLeapYear(2020); // true
isLocalStorageEnabled
- title: isLocalStorageEnabled
- tags: browser,intermediate
Checks if localStorage
is enabled.
- Use a
try...catch
block to returntrue
if all operations complete successfully,false
otherwise. - Use
Storage.setItem()
andStorage.removeItem()
to test storing and deleting a value inwindow.localStorage
.
const isLocalStorageEnabled = () => {
try {
const key = `__storage__test`;
window.localStorage.setItem(key, null);
window.localStorage.removeItem(key);
return true;
} catch (e) {
return false;
}
};
isLocalStorageEnabled(); // true, if localStorage is accessible
isLowerCase
- title: isLowerCase
- tags: string,beginner
Checks if a string is lower case.
- Convert the given string to lower case, using
String.prototype.toLowerCase()
and compare it to the original.
const isLowerCase = str => str === str.toLowerCase();
isLowerCase('abc'); // true
isLowerCase('a3@$'); // true
isLowerCase('Ab4'); // false
isNegativeZero
- title: isNegativeZero
- tags: math,intermediate
Checks if the given value is equal to negative zero (-0
).
- Check whether a passed value is equal to
0
and if1
divided by the value equals-Infinity
.
const isNegativeZero = val => val === 0 && 1 / val === -Infinity;
isNegativeZero(-0); // true
isNegativeZero(0); // false
isNil
- title: isNil
- tags: type,beginner
Checks if the specified value is null
or undefined
.
- Use the strict equality operator to check if the value of
val
is equal tonull
orundefined
.
const isNil = val => val === undefined || val === null;
isNil(null); // true
isNil(undefined); // true
isNil(''); // false
isNode
- title: isNode
- tags: node,browser,intermediate
Determines if the current runtime environment is Node.js.
- Use the
process
global object that provides information about the current Node.js process. - Check if
process
is defined andprocess.versions
,process.versions.node
are notnull
.
const isNode = () =>
typeof process !== 'undefined' &&
process.versions !== null &&
process.versions.node !== null;
isNode(); // true (Node)
isNode(); // false (browser)
isNull
- title: isNull
- tags: type,beginner
Checks if the specified value is null
.
- Use the strict equality operator to check if the value of
val
is equal tonull
.
const isNull = val => val === null;
isNull(null); // true
isNumber
- title: isNumber
- tags: type,math,beginner
Checks if the given argument is a number.
- Use
typeof
to check if a value is classified as a number primitive. - To safeguard against
NaN
, check ifval === val
(asNaN
has atypeof
equal tonumber
and is the only value not equal to itself).
const isNumber = val => typeof val === 'number' && val === val;
isNumber(1); // true
isNumber('1'); // false
isNumber(NaN); // false
isObject
- title: isObject
- tags: type,object,beginner
Checks if the passed value is an object or not.
- Uses the
Object
constructor to create an object wrapper for the given value. - If the value is
null
orundefined
, create and return an empty object. - Otherwise, return an object of a type that corresponds to the given value.
const isObject = obj => obj === Object(obj);
isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // false
isObjectLike
- title: isObjectLike
- tags: type,object,beginner
Checks if a value is object-like.
- Check if the provided value is not
null
and itstypeof
is equal to'object'
.
const isObjectLike = val => val !== null && typeof val === 'object';
isObjectLike({}); // true
isObjectLike([1, 2, 3]); // true
isObjectLike(x => x); // false
isObjectLike(null); // false
isOdd
- title: isOdd
- tags: math,beginner
Checks if the given number is odd.
- Check whether a number is odd or even using the modulo (
%
) operator. - Return
true
if the number is odd,false
if the number is even.
const isOdd = num => num % 2 === 1;
isOdd(3); // true
isPlainObject
- title: isPlainObject
- tags: type,object,intermediate
Checks if the provided value is an object created by the Object constructor.
- Check if the provided value is truthy.
- Use
typeof
to check if it is an object andObject.prototype.constructor
to make sure the constructor is equal toObject
.
const isPlainObject = val =>
!!val && typeof val === 'object' && val.constructor === Object;
isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // false
isPowerOfTen
- title: isPowerOfTen
- tags: math,beginner
Checks if the given number is a power of 10
.
- Use
Math.log10()
and the modulo operator (%
) to determine ifn
is a power of10
.
const isPowerOfTen = n => Math.log10(n) % 1 === 0;
isPowerOfTen(1); // true
isPowerOfTen(10); // true
isPowerOfTen(20); // false
isPowerOfTwo
- title: isPowerOfTwo
- tags: math,beginner
Checks if the given number is a power of 2
.
- Use the bitwise binary AND operator (
&
) to determine ifn
is a power of2
. - Additionally, check that
n
is not falsy.
const isPowerOfTwo = n => !!n && (n & (n - 1)) == 0;
isPowerOfTwo(0); // false
isPowerOfTwo(1); // true
isPowerOfTwo(8); // true
isPrime
- title: isPrime
- tags: math,algorithm,beginner
Checks if the provided integer is a prime number.
- Check numbers from
2
to the square root of the given number. - Return
false
if any of them divides the given number, else returntrue
, unless the number is less than2
.
const isPrime = num => {
const boundary = Math.floor(Math.sqrt(num));
for (let i = 2; i <= boundary; i++) if (num % i === 0) return false;
return num >= 2;
};
isPrime(11); // true
isPrimitive
- title: isPrimitive
- tags: type,intermediate
Checks if the passed value is primitive or not.
- Create an object from
val
and compare it withval
to determine if the passed value is primitive (i.e. not equal to the created object).
const isPrimitive = val => Object(val) !== val;
isPrimitive(null); // true
isPrimitive(undefined); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // false
isPrimitive({}); // false
isPromiseLike
- title: isPromiseLike
- tags: type,function,promise,intermediate
Checks if an object looks like a Promise
.
- Check if the object is not
null
, itstypeof
matches eitherobject
orfunction
and if it has a.then
property, which is also afunction
.
const isPromiseLike = obj =>
obj !== null &&
(typeof obj === 'object' || typeof obj === 'function') &&
typeof obj.then === 'function';
isPromiseLike({
then: function() {
return '';
}
}); // true
isPromiseLike(null); // false
isPromiseLike({}); // false
isReadableStream
- title: isReadableStream
- tags: node,type,intermediate
Checks if the given argument is a readable stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_read
and_readableState
properties arefunction
andobject
respectively.
const isReadableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object';
const fs = require('fs');
isReadableStream(fs.createReadStream('test.txt')); // true
isSameDate
- title: isSameDate
- tags: date,beginner
Checks if a date is the same as another date.
- Use
Date.prototype.toISOString()
and strict equality checking (===
) to check if the first date is the same as the second one.
const isSameDate = (dateA, dateB) =>
dateA.toISOString() === dateB.toISOString();
isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // true
isSessionStorageEnabled
- title: isSessionStorageEnabled
- tags: browser,intermediate
Checks if sessionStorage
is enabled.
- Use a
try...catch
block to returntrue
if all operations complete successfully,false
otherwise. - Use
Storage.setItem()
andStorage.removeItem()
to test storing and deleting a value inwindow.sessionStorage
.
const isSessionStorageEnabled = () => {
try {
const key = `__storage__test`;
window.sessionStorage.setItem(key, null);
window.sessionStorage.removeItem(key);
return true;
} catch (e) {
return false;
}
};
isSessionStorageEnabled(); // true, if sessionStorage is accessible
isSorted
- title: isSorted
- tags: array,intermediate
Checks if a numeric array is sorted.
- Calculate the ordering
direction
for the first pair of adjacent array elements. - Return
0
if the given array is empty, only has one element or thedirection
changes for any pair of adjacent array elements. - Use
Math.sign()
to covert the final value ofdirection
to-1
(descending order) or1
(ascending order).
const isSorted = arr => {
if (arr.length <= 1) return 0;
const direction = arr[1] - arr[0];
for (let i = 2; i < arr.length; i++) {
if ((arr[i] - arr[i - 1]) * direction < 0) return 0;
}
return Math.sign(direction);
};
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0
isSorted([4]); // 0
isStream
- title: isStream
- tags: node,type,intermediate
Checks if the given argument is a stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
.
const isStream = val =>
val !== null && typeof val === 'object' && typeof val.pipe === 'function';
const fs = require('fs');
isStream(fs.createReadStream('test.txt')); // true
isString
- title: isString
- tags: type,string,beginner
Checks if the given argument is a string. Only works for string primitives.
- Use
typeof
to check if a value is classified as a string primitive.
const isString = val => typeof val === 'string';
isString('10'); // true
isSymbol
- title: isSymbol
- tags: type,beginner
Checks if the given argument is a symbol.
- Use
typeof
to check if a value is classified as a symbol primitive.
const isSymbol = val => typeof val === 'symbol';
isSymbol(Symbol('x')); // true
isTravisCI
- title: isTravisCI
- tags: node,intermediate
Checks if the current environment is Travis CI.
- Check if the current environment has the
TRAVIS
andCI
environment variables (reference).
const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;
isTravisCI(); // true (if code is running on Travis CI)
isUndefined
- title: isUndefined
- tags: type,beginner
Checks if the specified value is undefined
.
- Use the strict equality operator to check if
val
is equal toundefined
.
const isUndefined = val => val === undefined;
isUndefined(undefined); // true
isUpperCase
- title: isUpperCase
- tags: string,beginner
Checks if a string is upper case.
- Convert the given string to upper case, using
String.prototype.toUpperCase()
and compare it to the original.
const isUpperCase = str => str === str.toUpperCase();
isUpperCase('ABC'); // true
isUpperCase('A3@$'); // true
isUpperCase('aB4'); // false
isValidJSON
- title: isValidJSON
- tags: type,intermediate
Checks if the provided string is a valid JSON.
- Use
JSON.parse()
and atry... catch
block to check if the provided string is a valid JSON.
const isValidJSON = str => {
try {
JSON.parse(str);
return true;
} catch (e) {
return false;
}
};
isValidJSON('{"name":"Adam","age":20}'); // true
isValidJSON('{"name":"Adam",age:"20"}'); // false
isValidJSON(null); // true
isWeekday
- title: isWeekday
- tags: date,beginner
Checks if the given date is a weekday.
- Use
Date.prototype.getDay()
to check weekday by using a modulo operator (%
). - Omit the argument,
d
, to use the current date as default.
const isWeekday = (d = new Date()) => d.getDay() % 6 !== 0;
isWeekday(); // true (if current date is 2019-07-19)
isWeekend
- title: isWeekend
- tags: date,beginner
Checks if the given date is a weekend.
- Use
Date.prototype.getDay()
to check weekend by using a modulo operator (%
). - Omit the argument,
d
, to use the current date as default.
const isWeekend = (d = new Date()) => d.getDay() % 6 === 0;
isWeekend(); // 2018-10-19 (if current date is 2018-10-18)
isWritableStream
- title: isWritableStream
- tags: node,type,intermediate
Checks if the given argument is a writable stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_write
and_writableState
properties arefunction
andobject
respectively.
const isWritableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';
const fs = require('fs');
isWritableStream(fs.createWriteStream('test.txt')); // true
join
- title: join
- tags: array,intermediate
Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.
- Use
Array.prototype.reduce()
to combine elements into a string. - Omit the second argument,
separator
, to use a default separator of','
. - Omit the third argument,
end
, to use the same value asseparator
by default.
const join = (arr, separator = ',', end = separator) =>
arr.reduce(
(acc, val, i) =>
i === arr.length - 2
? acc + val + end
: i === arr.length - 1
? acc + val
: acc + val + separator,
''
);
join(['pen', 'pineapple', 'apple', 'pen'],',','&'); // 'pen,pineapple,apple&pen'
join(['pen', 'pineapple', 'apple', 'pen'], ','); // 'pen,pineapple,apple,pen'
join(['pen', 'pineapple', 'apple', 'pen']); // 'pen,pineapple,apple,pen'
juxt
- title: juxt
- tags: function,advanced
Takes several functions as argument and returns a function that is the juxtaposition of those functions.
- Use
Array.prototype.map()
to return afn
that can take a variable number ofargs
. - When
fn
is called, return an array containing the result of applying eachfn
to theargs
.
const juxt = (...fns) => (...args) => [...fns].map(fn => [...args].map(fn));
juxt(
x => x + 1,
x => x - 1,
x => x * 10
)(1, 2, 3); // [[2, 3, 4], [0, 1, 2], [10, 20, 30]]
juxt(
s => s.length,
s => s.split(' ').join('-')
)('30 seconds of code'); // [[18], ['30-seconds-of-code']]
kMeans
- title: kMeans
- tags: algorithm,array,advanced
Groups the given data into k
clusters, using the k-means clustering algorithm.
- Use
Array.from()
andArray.prototype.slice()
to initialize appropriate variables for the clustercentroids
,distances
andclasses
. - Use a
while
loop to repeat the assignment and update steps as long as there are changes in the previous iteration, as indicated byitr
. - Calculate the euclidean distance between each data point and centroid using
Math.hypot()
,Object.keys()
andArray.prototype.map()
. - Use
Array.prototype.indexOf()
andMath.min()
to find the closest centroid. - Use
Array.from()
andArray.prototype.reduce()
, as well asparseFloat()
andNumber.prototype.toFixed()
to calculate the new centroids.
const kMeans = (data, k = 1) => {
const centroids = data.slice(0, k);
const distances = Array.from({ length: data.length }, () =>
Array.from({ length: k }, () => 0)
);
const classes = Array.from({ length: data.length }, () => -1);
let itr = true;
while (itr) {
itr = false;
for (let d in data) {
for (let c = 0; c < k; c++) {
distances[d][c] = Math.hypot(
...Object.keys(data[0]).map(key => data[d][key] - centroids[c][key])
);
}
const m = distances[d].indexOf(Math.min(...distances[d]));
if (classes[d] !== m) itr = true;
classes[d] = m;
}
for (let c = 0; c < k; c++) {
centroids[c] = Array.from({ length: data[0].length }, () => 0);
const size = data.reduce((acc, _, d) => {
if (classes[d] === c) {
acc++;
for (let i in data[0]) centroids[c][i] += data[d][i];
}
return acc;
}, 0);
for (let i in data[0]) {
centroids[c][i] = parseFloat(Number(centroids[c][i] / size).toFixed(2));
}
}
}
return classes;
};
kMeans([[0, 0], [0, 1], [1, 3], [2, 0]], 2); // [0, 1, 1, 0]
kNearestNeighbors
- title: kNearestNeighbors
- tags: algorithm,array,advanced
Classifies a data point relative to a labelled data set, using the k-nearest neighbors algorithm.
- Use
Array.prototype.map()
to map thedata
to objects containing the euclidean distance of each element frompoint
, calculated usingMath.hypot()
,Object.keys()
and itslabel
. - Use
Array.prototype.sort()
andArray.prototype.slice()
to get thek
nearest neighbors ofpoint
. - Use
Array.prototype.reduce()
in combination withObject.keys()
andArray.prototype.indexOf()
to find the most frequentlabel
among them.
const kNearestNeighbors = (data, labels, point, k = 3) => {
const kNearest = data
.map((el, i) => ({
dist: Math.hypot(...Object.keys(el).map(key => point[key] - el[key])),
label: labels[i]
}))
.sort((a, b) => a.dist - b.dist)
.slice(0, k);
return kNearest.reduce(
(acc, { label }, i) => {
acc.classCounts[label] =
Object.keys(acc.classCounts).indexOf(label) !== -1
? acc.classCounts[label] + 1
: 1;
if (acc.classCounts[label] > acc.topClassCount) {
acc.topClassCount = acc.classCounts[label];
acc.topClass = label;
}
return acc;
},
{
classCounts: {},
topClass: kNearest[0].label,
topClassCount: 0
}
).topClass;
};
const data = [[0, 0], [0, 1], [1, 3], [2, 0]];
const labels = [0, 1, 1, 0];
kNearestNeighbors(data, labels, [1, 2], 2); // 1
kNearestNeighbors(data, labels, [1, 0], 2); // 0
kmToMiles
- title: kmToMiles
- tags: math,beginner unlisted: true
Converts kilometers to miles.
- Follow the conversion formula
mi = km * 0.621371
.
const kmToMiles = km => km * 0.621371;
kmToMiles(8.1) // 5.0331051
last
- title: last
- tags: array,beginner
Returns the last element in an array.
- Check if
arr
is truthy and has alength
property. - Use
Array.prototype.length - 1
to compute the index of the last element of the given array and return it, otherwise returnundefined
.
const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
last([1, 2, 3]); // 3
last([]); // undefined
last(null); // undefined
last(undefined); // undefined
lastDateOfMonth
- title: lastDateOfMonth
- tags: date,intermediate
Returns the string representation of the last date in the given date's month.
- Use
Date.prototype.getFullYear()
,Date.prototype.getMonth()
to get the current year and month from the given date. - Use the
new Date()
constructor to create a new date with the given year and month incremented by1
, and the day set to0
(last day of previous month). - Omit the argument,
date
, to use the current date by default.
const lastDateOfMonth = (date = new Date()) => {
let d = new Date(date.getFullYear(), date.getMonth() + 1, 0);
return d.toISOString().split('T')[0];
};
lastDateOfMonth(new Date('2015-08-11')); // '2015-08-30'
lcm
- title: lcm
- tags: math,algorithm,recursion,intermediate
Calculates the least common multiple of two or more numbers.
- Use the greatest common divisor (GCD) formula and the fact that
lcm(x, y) = x * y / gcd(x, y)
to determine the least common multiple. - The GCD formula uses recursion.
const lcm = (...arr) => {
const gcd = (x, y) => (!y ? x : gcd(y, x % y));
const _lcm = (x, y) => (x * y) / gcd(x, y);
return [...arr].reduce((a, b) => _lcm(a, b));
};
lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60
levenshteinDistance
- title: levenshteinDistance
- tags: string,algorithm,intermediate
Calculates the difference between two strings, using the Levenshtein distance algorithm.
- If either of the two strings has a
length
of zero, return thelength
of the other one. - Use a
for
loop to iterate over the letters of the target string and a nestedfor
loop to iterate over the letters of the source string. - Calculate the cost of substituting the letters corresponding to
i - 1
andj - 1
in the target and source respectively (0
if they are the same,1
otherwise). - Use
Math.min()
to populate each element in the 2D array with the minimum of the cell above incremented by one, the cell to the left incremented by one or the cell to the top left incremented by the previously calculated cost. - Return the last element of the last row of the produced array.
const levenshteinDistance = (s, t) => {
if (!s.length) return t.length;
if (!t.length) return s.length;
const arr = [];
for (let i = 0; i <= t.length; i++) {
arr[i] = [i];
for (let j = 1; j <= s.length; j++) {
arr[i][j] =
i === 0
? j
: Math.min(
arr[i - 1][j] + 1,
arr[i][j - 1] + 1,
arr[i - 1][j - 1] + (s[j - 1] === t[i - 1] ? 0 : 1)
);
}
}
return arr[t.length][s.length];
};
levenshteinDistance('duck', 'dark'); // 2
linearSearch
- title: linearSearch
- tags: algorithm,array,beginner
Finds the first index of a given element in an array using the linear search algorithm.
- Use a
for...in
loop to iterate over the indexes of the given array. - Check if the element in the corresponding index is equal to
item
. - If the element is found, return the index, using the unary
+
operator to convert it from a string to a number. - If the element is not found after iterating over the whole array, return
-1
.
const linearSearch = (arr, item) => {
for (const i in arr) {
if (arr[i] === item) return +i;
}
return -1;
};
linearSearch([2, 9, 9], 9); // 1
linearSearch([2, 9, 9], 7); // -1
listenOnce
- title: listenOnce
- tags: browser,event,beginner
Adds an event listener to an element that will only run the callback the first time the event is triggered.
- Use
EventTarget.addEventListener()
to add an event listener to an element. - Use
{ once: true }
as options to only run the given callback once.
const listenOnce = (el, evt, fn) =>
el.addEventListener(evt, fn, { once: true });
listenOnce(
document.getElementById('my-id'),
'click',
() => console.log('Hello world')
); // 'Hello world' will only be logged on the first click
logBase
- title: logBase
- tags: math,beginner
Calculates the logarithm of the given number in the given base.
- Use
Math.log()
to get the logarithm from the value and the base and divide them.
const logBase = (n, base) => Math.log(n) / Math.log(base);
logBase(10, 10); // 1
logBase(100, 10); // 2
longestItem
- title: longestItem
- tags: array,intermediate
Takes any number of iterable objects or objects with a length
property and returns the longest one.
- Use
Array.prototype.reduce()
, comparing the length of objects to find the longest one. - If multiple objects have the same length, the first one will be returned.
- Returns
undefined
if no arguments are provided.
const longestItem = (...vals) =>
vals.reduce((a, x) => (x.length > a.length ? x : a));
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'
lowercaseKeys
- title: lowercaseKeys
- tags: object,intermediate
Creates a new object from the specified object, where all the keys are in lowercase.
- Use
Object.keys()
andArray.prototype.reduce()
to create a new object from the specified object. - Convert each key in the original object to lowercase, using
String.prototype.toLowerCase()
.
const lowercaseKeys = obj =>
Object.keys(obj).reduce((acc, key) => {
acc[key.toLowerCase()] = obj[key];
return acc;
}, {});
const myObj = { Name: 'Adam', sUrnAME: 'Smith' };
const myObjLower = lowercaseKeys(myObj); // {name: 'Adam', surname: 'Smith'};
luhnCheck
- title: luhnCheck
- tags: math,algorithm,advanced
Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
- Use
String.prototype.split('')
,Array.prototype.reverse()
andArray.prototype.map()
in combination withparseInt()
to obtain an array of digits. - Use
Array.prototype.splice(0, 1)
to obtain the last digit. - Use
Array.prototype.reduce()
to implement the Luhn Algorithm. - Return
true
ifsum
is divisible by10
,false
otherwise.
const luhnCheck = num => {
let arr = (num + '')
.split('')
.reverse()
.map(x => parseInt(x));
let lastDigit = arr.splice(0, 1)[0];
let sum = arr.reduce(
(acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9),
0
);
sum += lastDigit;
return sum % 10 === 0;
};
luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); // false
luhnCheck(123456789); // false
mapKeys
- title: mapKeys
- tags: object,intermediate
Maps the keys of an object using the provided function, generating a new object.
- Use
Object.keys()
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same values and mapped keys usingfn
.
const mapKeys = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[fn(obj[k], k, obj)] = obj[k];
return acc;
}, {});
mapKeys({ a: 1, b: 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }
mapNumRange
- title: mapNumRange
- tags: math,beginner
Maps a number from one range to another range.
- Return
num
mapped betweenoutMin
-outMax
frominMin
-inMax
.
const mapNumRange = (num, inMin, inMax, outMin, outMax) =>
((num - inMin) * (outMax - outMin)) / (inMax - inMin) + outMin;
mapNumRange(5, 0, 10, 0, 100); // 50
mapObject
- title: mapObject
- tags: array,object,intermediate
Maps the values of an array to an object using a function.
- Use
Array.prototype.reduce()
to applyfn
to each element inarr
and combine the results into an object. - Use
el
as the key for each property and the result offn
as the value.
const mapObject = (arr, fn) =>
arr.reduce((acc, el, i) => {
acc[el] = fn(el, i, arr);
return acc;
}, {});
mapObject([1, 2, 3], a => a * a); // { 1: 1, 2: 4, 3: 9 }
mapString
- title: mapString
- tags: string,intermediate
Creates a new string with the results of calling a provided function on every character in the given string.
- Use
String.prototype.split('')
andArray.prototype.map()
to call the provided function,fn
, for each character instr
. - Use
Array.prototype.join('')
to recombine the array of characters into a string. - The callback function,
fn
, takes three arguments (the current character, the index of the current character and the stringmapString
was called upon).
const mapString = (str, fn) =>
str
.split('')
.map((c, i) => fn(c, i, str))
.join('');
mapString('lorem ipsum', c => c.toUpperCase()); // 'LOREM IPSUM'
mapValues
- title: mapValues
- tags: object,intermediate
Maps the values of an object using the provided function, generating a new object with the same keys.
- Use
Object.keys()
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same keys and mapped values usingfn
.
const mapValues = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[k] = fn(obj[k], k, obj);
return acc;
}, {});
const users = {
fred: { user: 'fred', age: 40 },
pebbles: { user: 'pebbles', age: 1 }
};
mapValues(users, u => u.age); // { fred: 40, pebbles: 1 }
mask
- title: mask
- tags: string,intermediate
Replaces all but the last num
of characters with the specified mask character.
- Use
String.prototype.slice()
to grab the portion of the characters that will remain unmasked. - Use
String.padStart()
to fill the beginning of the string with themask
character up to the original length. - If
num
is negative, the unmasked characters will be at the start of the string. - Omit the second argument,
num
, to keep a default of4
characters unmasked. - Omit the third argument,
mask
, to use a default character of'*'
for the mask.
const mask = (cc, num = 4, mask = '*') =>
`${cc}`.slice(-num).padStart(`${cc}`.length, mask);
mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'
matches
- title: matches
- tags: object,intermediate
Compares two objects to determine if the first one contains equivalent property values to the second one.
- Use
Object.keys()
to get all the keys of the second object. - Use
Array.prototype.every()
,Object.prototype.hasOwnProperty()
and strict comparison to determine if all keys exist in the first object and have the same values.
const matches = (obj, source) =>
Object.keys(source).every(
key => obj.hasOwnProperty(key) && obj[key] === source[key]
);
matches({ age: 25, hair: 'long', beard: true }, { hair: 'long', beard: true });
// true
matches({ hair: 'long', beard: true }, { age: 25, hair: 'long', beard: true });
// false
matchesWith
- title: matchesWith
- tags: object,intermediate
Compares two objects to determine if the first one contains equivalent property values to the second one, based on a provided function.
- Use
Object.keys()
to get all the keys of the second object. - Use
Array.prototype.every()
,Object.prototype.hasOwnProperty()
and the provided function to determine if all keys exist in the first object and have equivalent values. - If no function is provided, the values will be compared using the equality operator.
const matchesWith = (obj, source, fn) =>
Object.keys(source).every(key =>
obj.hasOwnProperty(key) && fn
? fn(obj[key], source[key], key, obj, source)
: obj[key] == source[key]
);
const isGreeting = val => /^h(?:i|ello)$/.test(val);
matchesWith(
{ greeting: 'hello' },
{ greeting: 'hi' },
(oV, sV) => isGreeting(oV) && isGreeting(sV)
); // true
maxBy
- title: maxBy
- tags: math,array,beginner
Returns the maximum value of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Math.max()
to get the maximum value.
const maxBy = (arr, fn) =>
Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8
maxDate
- title: maxDate
- tags: date,intermediate
Returns the maximum of the given dates.
- Use the ES6 spread syntax with
Math.max()
to find the maximum date value. - Use
new Date()
to convert it to aDate
object.
const maxDate = (...dates) => new Date(Math.max(...dates));
const dates = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
maxDate(...dates); // 2018-03-11T22:00:00.000Z
maxN
- title: maxN
- tags: array,math,intermediate
Returns the n
maximum elements from the provided array.
- Use
Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in descending order. - Use
Array.prototype.slice()
to get the specified number of elements. - Omit the second argument,
n
, to get a one-element array. - If
n
is greater than or equal to the provided array's length, then return the original array (sorted in descending order).
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3, 2]
median
- title: median
- tags: math,array,intermediate
Calculates the median of an array of numbers.
- Find the middle of the array, use
Array.prototype.sort()
to sort the values. - Return the number at the midpoint if
Array.prototype.length
is odd, otherwise the average of the two middle numbers.
const median = arr => {
const mid = Math.floor(arr.length / 2),
nums = [...arr].sort((a, b) => a - b);
return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};
median([5, 6, 50, 1, -5]); // 5
memoize
- title: memoize
- tags: function,advanced
Returns the memoized (cached) function.
- Create an empty cache by instantiating a new
Map
object. - Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not.
- The
function
keyword must be used in order to allow the memoized function to have itsthis
context changed if necessary. - Allow access to the
cache
by setting it as a property on the returned function.
const memoize = fn => {
const cache = new Map();
const cached = function (val) {
return cache.has(val)
? cache.get(val)
: cache.set(val, fn.call(this, val)) && cache.get(val);
};
cached.cache = cache;
return cached;
};
// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's cached
console.log(anagramsCached.cache); // The cached anagrams map
merge
- title: merge
- tags: object,array,intermediate
Creates a new object from the combination of two or more objects.
- Use
Array.prototype.reduce()
combined withObject.keys()
to iterate over all objects and keys. - Use
Object.prototype.hasOwnProperty()
andArray.prototype.concat()
to append values for keys existing in multiple objects.
const merge = (...objs) =>
[...objs].reduce(
(acc, obj) =>
Object.keys(obj).reduce((a, k) => {
acc[k] = acc.hasOwnProperty(k)
? [].concat(acc[k]).concat(obj[k])
: obj[k];
return acc;
}, {}),
{}
);
const object = {
a: [{ x: 2 }, { y: 4 }],
b: 1
};
const other = {
a: { z: 3 },
b: [2, 3],
c: 'foo'
};
merge(object, other);
// { a: [ { x: 2 }, { y: 4 }, { z: 3 } ], b: [ 1, 2, 3 ], c: 'foo' }
mergeSort
- title: mergeSort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the merge sort algorithm.
- Use recursion.
- If the
length
of the array is less than2
, return the array. - Use
Math.floor()
to calculate the middle point of the array. - Use
Array.prototype.slice()
to slice the array in two and recursively callmergeSort()
on the created subarrays. - Finally, use
Array.from()
andArray.prototype.shift()
to combine the two sorted subarrays into one.
const mergeSort = arr => {
if (arr.length < 2) return arr;
const mid = Math.floor(arr.length / 2);
const l = mergeSort(arr.slice(0, mid));
const r = mergeSort(arr.slice(mid, arr.length));
return Array.from({ length: l.length + r.length }, () => {
if (!l.length) return r.shift();
else if (!r.length) return l.shift();
else return l[0] > r[0] ? r.shift() : l.shift();
});
};
mergeSort([5, 1, 4, 2, 3]); // [1, 2, 3, 4, 5]
mergeSortedArrays
- title: mergeSortedArrays
- tags: array,intermediate
Merges two sorted arrays into one.
- Use the spread operator (
...
) to clone both of the given arrays. - Use
Array.from()
to create an array of the appropriate length based on the given arrays. - Use
Array.prototype.shift()
to populate the newly created array from the removed elements of the cloned arrays.
const mergeSortedArrays = (a, b) => {
const _a = [...a],
_b = [...b];
return Array.from({ length: _a.length + _b.length }, () => {
if (!_a.length) return _b.shift();
else if (!_b.length) return _a.shift();
else return _a[0] > _b[0] ? _b.shift() : _a.shift();
});
};
mergeSortedArrays([1, 4, 5], [2, 3, 6]); // [1, 2, 3, 4, 5, 6]
midpoint
- title: midpoint
- tags: math,beginner
Calculates the midpoint between two pairs of (x,y) points.
- Destructure the array to get
x1
,y1
,x2
andy2
. - Calculate the midpoint for each dimension by dividing the sum of the two endpoints by
2
.
const midpoint = ([x1, y1], [x2, y2]) => [(x1 + x2) / 2, (y1 + y2) / 2];
midpoint([2, 2], [4, 4]); // [3, 3]
midpoint([4, 4], [6, 6]); // [5, 5]
midpoint([1, 3], [2, 4]); // [1.5, 3.5]
milesToKm
- title: milesToKm
- tags: math,beginner unlisted: true
Converts miles to kilometers.
- Follow the conversion formula
km = mi * 1.609344
.
const milesToKm = miles => miles * 1.609344;
milesToKm(5); // ~8.04672
minBy
- title: minBy
- tags: math,array,beginner
Returns the minimum value of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Math.min()
to get the minimum value.
const minBy = (arr, fn) =>
Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2
minDate
- title: minDate
- tags: date,intermediate
Returns the minimum of the given dates.
- Use the ES6 spread syntax with
Math.min()
to find the minimum date value. - Use
new Date()
to convert it to aDate
object.
const minDate = (...dates) => new Date(Math.min(...dates));
const dates = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
minDate(...dates); // 2016-01-08T22:00:00.000Z
minN
- title: minN
- tags: array,math,intermediate
Returns the n
minimum elements from the provided array.
- Use
Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in ascending order. - Use
Array.prototype.slice()
to get the specified number of elements. - Omit the second argument,
n
, to get a one-element array. - If
n
is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).
const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1, 2]
mostFrequent
- title: mostFrequent
- tags: array,intermediate
Returns the most frequent element in an array.
- Use
Array.prototype.reduce()
to map unique values to an object's keys, adding to existing keys every time the same value is encountered. - Use
Object.entries()
on the result in combination withArray.prototype.reduce()
to get the most frequent value in the array.
const mostFrequent = arr =>
Object.entries(
arr.reduce((a, v) => {
a[v] = a[v] ? a[v] + 1 : 1;
return a;
}, {})
).reduce((a, v) => (v[1] >= a[1] ? v : a), [null, 0])[0];
mostFrequent(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // 'a'
mostPerformant
- title: mostPerformant
- tags: function,advanced
Returns the index of the function in an array of functions which executed the fastest.
- Use
Array.prototype.map()
to generate an array where each value is the total time taken to execute the function afteriterations
times. - Use the difference in
performance.now()
values before and after to get the total time in milliseconds to a high degree of accuracy. - Use
Math.min()
to find the minimum execution time, and return the index of that shortest time which corresponds to the index of the most performant function. - Omit the second argument,
iterations
, to use a default of10000
iterations. - The more iterations, the more reliable the result but the longer it will take.
const mostPerformant = (fns, iterations = 10000) => {
const times = fns.map(fn => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return performance.now() - before;
});
return times.indexOf(Math.min(...times));
};
mostPerformant([
() => {
// Loops through the entire array before returning `false`
[1, 2, 3, 4, 5, 6, 7, 8, 9, '10'].every(el => typeof el === 'number');
},
() => {
// Only needs to reach index `1` before returning `false`
[1, '2', 3, 4, 5, 6, 7, 8, 9, 10].every(el => typeof el === 'number');
}
]); // 1
negate
- title: negate
- tags: function,beginner
Negates a predicate function.
- Take a predicate function and apply the not operator (
!
) to it with its arguments.
const negate = func => (...args) => !func(...args);
[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 === 0)); // [ 1, 3, 5 ]
nest
- title: nest
- tags: object,recursion,intermediate
Nests recursively objects linked to one another in a flat array.
- Use recursion.
- Use
Array.prototype.filter()
to filter the items where theid
matches thelink
. - Use
Array.prototype.map()
to map each item to a new object that has achildren
property which recursively nests the items based on which ones are children of the current item. - Omit the second argument,
id
, to default tonull
which indicates the object is not linked to another one (i.e. it is a top level object). - Omit the third argument,
link
, to use'parent_id'
as the default property which links the object to another one by itsid
.
const nest = (items, id = null, link = 'parent_id') =>
items
.filter(item => item[link] === id)
.map(item => ({ ...item, children: nest(items, item.id, link) }));
const comments = [
{ id: 1, parent_id: null },
{ id: 2, parent_id: 1 },
{ id: 3, parent_id: 1 },
{ id: 4, parent_id: 2 },
{ id: 5, parent_id: 4 }
];
const nestedComments = nest(comments);
// [{ id: 1, parent_id: null, children: [...] }]
nodeListToArray
- title: nodeListToArray
- tags: browser,array,beginner
Converts a NodeList
to an array.
- Use spread operator (
...
) inside new array to convert aNodeList
to an array.
const nodeListToArray = nodeList => [...nodeList];
nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]
none
- title: none
- tags: array,beginner
Checks if the provided predicate function returns false
for all elements in a collection.
- Use
Array.prototype.some()
to test if any elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const none = (arr, fn = Boolean) => !arr.some(fn);
none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // true
normalizeLineEndings
- title: normalizeLineEndings
- tags: string,regexp,intermediate
Normalizes line endings in a string.
- Use
String.prototype.replace()
and a regular expression to match and replace line endings with thenormalized
version. - Omit the second argument,
normalized
, to use the default value of'\r\n'
.
const normalizeLineEndings = (str, normalized = '\r\n') =>
str.replace(/\r?\n/g, normalized);
normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n');
// 'This\r\nis a\r\nmultiline\r\nstring.\r\n'
normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n', '\n');
// 'This\nis a\nmultiline\nstring.\n'
not
- title: not
- tags: math,logic,beginner unlisted: true
Returns the logical inverse of the given value.
- Use the logical not (
!
) operator to return the inverse of the given value.
const not = a => !a;
not(true); // false
not(false); // true
nthArg
- title: nthArg
- tags: function,beginner
Creates a function that gets the argument at index n
.
- Use
Array.prototype.slice()
to get the desired argument at indexn
. - If
n
is negative, the nth argument from the end is returned.
const nthArg = n => (...args) => args.slice(n)[0];
const third = nthArg(2);
third(1, 2, 3); // 3
third(1, 2); // undefined
const last = nthArg(-1);
last(1, 2, 3, 4, 5); // 5
nthElement
- title: nthElement
- tags: array,beginner
Returns the nth element of an array.
- Use
Array.prototype.slice()
to get an array containing the nth element at the first place. - If the index is out of bounds, return
undefined
. - Omit the second argument,
n
, to get the first element of the array.
const nthElement = (arr, n = 0) =>
(n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'
nthRoot
- title: nthRoot
- tags: math,beginner
Calculates the nth root of a given number.
- Use
Math.pow()
to calculatex
to the power of1/n
which is equal to the nth root ofx
.
const nthRoot = (x, n) => Math.pow(x, 1 / n);
nthRoot(32, 5); // 2
objectFromPairs
- title: objectFromPairs
- tags: object,array,beginner
Creates an object from the given key-value pairs.
- Use
Array.prototype.reduce()
to create and combine key-value pairs.
const objectFromPairs = arr =>
arr.reduce((a, [key, val]) => ((a[key] = val), a), {});
objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}
objectToEntries
- title: objectToEntries
- tags: object,array,beginner
Creates an array of key-value pair arrays from an object.
- Use
Object.keys()
andArray.prototype.map()
to iterate over the object's keys and produce an array with key-value pairs.
const objectToEntries = obj => Object.keys(obj).map(k => [k, obj[k]]);
objectToEntries({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]
objectToPairs
- title: objectToPairs
- tags: object,array,beginner
Creates an array of key-value pair arrays from an object.
- Use
Object.entries()
to get an array of key-value pair arrays from the given object.
const objectToPairs = obj => Object.entries(obj);
objectToPairs({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]
objectToQueryString
- title: objectToQueryString
- tags: object,advanced
Generates a query string from the key-value pairs of the given object.
- Use
Array.prototype.reduce()
onObject.entries(queryParameters)
to create the query string. - Determine the
symbol
to be either?
or&
based on the length ofqueryString
. - Concatenate
val
toqueryString
only if it's a string. - Return the
queryString
or an empty string when thequeryParameters
are falsy.
const objectToQueryString = queryParameters => {
return queryParameters
? Object.entries(queryParameters).reduce(
(queryString, [key, val], index) => {
const symbol = queryString.length === 0 ? '?' : '&';
queryString +=
typeof val === 'string' ? `${symbol}${key}=${val}` : '';
return queryString;
},
''
)
: '';
};
objectToQueryString({ page: '1', size: '2kg', key: undefined });
// '?page=1&size=2kg'
observeMutations
- title: observeMutations
- tags: browser,event,advanced
Creates a new MutationObserver
and runs the provided callback for each mutation on the specified element.
- Use a
MutationObserver
to observe mutations on the given element. - Use
Array.prototype.forEach()
to run the callback for each mutation that is observed. - Omit the third argument,
options
, to use the default options (alltrue
).
const observeMutations = (element, callback, options) => {
const observer = new MutationObserver(mutations =>
mutations.forEach(m => callback(m))
);
observer.observe(
element,
Object.assign(
{
childList: true,
attributes: true,
attributeOldValue: true,
characterData: true,
characterDataOldValue: true,
subtree: true,
},
options
)
);
return observer;
};
const obs = observeMutations(document, console.log);
// Logs all mutations that happen on the page
obs.disconnect();
// Disconnects the observer and stops logging mutations on the page
off
- title: off
- tags: browser,event,intermediate
Removes an event listener from an element.
- Use
EventTarget.removeEventListener()
to remove an event listener from an element. - Omit the fourth argument
opts
to usefalse
or specify it based on the options used when the event listener was added.
const off = (el, evt, fn, opts = false) =>
el.removeEventListener(evt, fn, opts);
const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the page
offset
- title: offset
- tags: array,beginner
Moves the specified amount of elements to the end of the array.
- Use
Array.prototype.slice()
twice to get the elements after the specified index and the elements before that. - Use the spread operator (
...
) to combine the two into one array. - If
offset
is negative, the elements will be moved from end to start.
const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];
offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]
omit
- title: omit
- tags: object,intermediate
Omits the key-value pairs corresponding to the given keys from an object.
- Use
Object.keys()
,Array.prototype.filter()
andArray.prototype.includes()
to remove the provided keys. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs.
const omit = (obj, arr) =>
Object.keys(obj)
.filter(k => !arr.includes(k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omit({ a: 1, b: '2', c: 3 }, ['b']); // { 'a': 1, 'c': 3 }
omitBy
- title: omitBy
- tags: object,intermediate
Omits the key-value pairs corresponding to the keys of the object for which the given function returns falsy.
- Use
Object.keys()
andArray.prototype.filter()
to remove the keys for whichfn
returns a truthy value. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs. - The callback function is invoked with two arguments: (value, key).
const omitBy = (obj, fn) =>
Object.keys(obj)
.filter(k => !fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }
on
- title: on
- tags: browser,event,intermediate
Adds an event listener to an element with the ability to use event delegation.
- Use
EventTarget.addEventListener()
to add an event listener to an element. - If there is a
target
property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correctthis
context. - Omit
opts
to default to non-delegation behavior and event bubbling. - Returns a reference to the custom delegator function, in order to be possible to use with
off
.
const on = (el, evt, fn, opts = {}) => {
const delegatorFn = e =>
e.target.matches(opts.target) && fn.call(e.target, e);
el.addEventListener(
evt,
opts.target ? delegatorFn : fn,
opts.options || false
);
if (opts.target) return delegatorFn;
};
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' });
// logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true });
// use capturing instead of bubbling
onClickOutside
- title: onClickOutside
- tags: browser,event,intermediate
Runs the callback whenever the user clicks outside of the specified element.
- Use
EventTarget.addEventListener()
to listen for'click'
events. - Use
Node.contains()
to check ifEvent.target
is a descendant ofelement
and runcallback
if not.
const onClickOutside = (element, callback) => {
document.addEventListener('click', e => {
if (!element.contains(e.target)) callback();
});
};
onClickOutside('##my-element', () => console.log('Hello'));
// Will log 'Hello' whenever the user clicks outside of ##my-element
onScrollStop
- title: onScrollStop
- tags: browser,event,intermediate
Runs the callback whenever the user has stopped scrolling.
- Use
EventTarget.addEventListener()
to listen for the'scroll'
event. - Use
setTimeout()
to wait150
ms until calling the givencallback
. - Use
clearTimeout()
to clear the timeout if a new'scroll'
event is fired in under150
ms.
const onScrollStop = callback => {
let isScrolling;
window.addEventListener(
'scroll',
e => {
clearTimeout(isScrolling);
isScrolling = setTimeout(() => {
callback();
}, 150);
},
false
);
};
onScrollStop(() => {
console.log('The user has stopped scrolling');
});
onUserInputChange
- title: onUserInputChange
- tags: browser,event,advanced
Runs the callback whenever the user input type changes (mouse
or touch
).
- Use two event listeners.
- Assume
mouse
input initially and bind a'touchstart'
event listener to the document. - On
'touchstart'
, add a'mousemove'
event listener to listen for two consecutive'mousemove'
events firing within 20ms, usingperformance.now()
. - Run the callback with the input type as an argument in either of these situations.
const onUserInputChange = callback => {
let type = 'mouse',
lastTime = 0;
const mousemoveHandler = () => {
const now = performance.now();
if (now - lastTime < 20)
(type = 'mouse'),
callback(type),
document.removeEventListener('mousemove', mousemoveHandler);
lastTime = now;
};
document.addEventListener('touchstart', () => {
if (type === 'touch') return;
(type = 'touch'),
callback(type),
document.addEventListener('mousemove', mousemoveHandler);
});
};
onUserInputChange(type => {
console.log('The user is now using', type, 'as an input method.');
});
once
- title: once
- tags: function,intermediate
Ensures a function is called only once.
- Utilizing a closure, use a flag,
called
, and set it totrue
once the function is called for the first time, preventing it from being called again. - In order to allow the function to have its
this
context changed (such as in an event listener), thefunction
keyword must be used, and the supplied function must have the context applied. - Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (
...
) operator.
const once = fn => {
let called = false;
return function(...args) {
if (called) return;
called = true;
return fn.apply(this, args);
};
};
const startApp = function(event) {
console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp));
// only runs `startApp` once upon click
or
- title: or
- tags: math,logic,beginner unlisted: true
Checks if at least one of the arguments is true
.
- Use the logical or (
||
) operator on the two given values.
const or = (a, b) => a || b;
or(true, true); // true
or(true, false); // true
or(false, false); // false
orderBy
- title: orderBy
- tags: object,array,advanced
Sorts an array of objects, ordered by properties and orders.
- Uses
Array.prototype.sort()
,Array.prototype.reduce()
on theprops
array with a default value of0
. - Use array destructuring to swap the properties position depending on the order supplied.
- If no
orders
array is supplied, sort by'asc'
by default.
const orderBy = (arr, props, orders) =>
[...arr].sort((a, b) =>
props.reduce((acc, prop, i) => {
if (acc === 0) {
const [p1, p2] =
orders && orders[i] === 'desc'
? [b[prop], a[prop]]
: [a[prop], b[prop]];
acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;
}
return acc;
}, 0)
);
const users = [
{ name: 'fred', age: 48 },
{ name: 'barney', age: 36 },
{ name: 'fred', age: 40 },
];
orderBy(users, ['name', 'age'], ['asc', 'desc']);
// [{name: 'barney', age: 36}, {name: 'fred', age: 48}, {name: 'fred', age: 40}]
orderBy(users, ['name', 'age']);
// [{name: 'barney', age: 36}, {name: 'fred', age: 40}, {name: 'fred', age: 48}]
orderWith
- title: orderWith
- tags: array,object,intermediate
Sorts an array of objects, ordered by a property, based on the array of orders provided.
- Use
Array.prototype.reduce()
to create an object from theorder
array with the values as keys and their original index as the value. - Use
Array.prototype.sort()
to sort the given array, skipping elements for whichprop
is empty or not in theorder
array.
const orderWith = (arr, prop, order) => {
const orderValues = order.reduce((acc, v, i) => {
acc[v] = i;
return acc;
}, {});
return [...arr].sort((a, b) => {
if (orderValues[a[prop]] === undefined) return 1;
if (orderValues[b[prop]] === undefined) return -1;
return orderValues[a[prop]] - orderValues[b[prop]];
});
};
const users = [
{ name: 'fred', language: 'Javascript' },
{ name: 'barney', language: 'TypeScript' },
{ name: 'frannie', language: 'Javascript' },
{ name: 'anna', language: 'Java' },
{ name: 'jimmy' },
{ name: 'nicky', language: 'Python' },
];
orderWith(users, 'language', ['Javascript', 'TypeScript', 'Java']);
/*
[
{ name: 'fred', language: 'Javascript' },
{ name: 'frannie', language: 'Javascript' },
{ name: 'barney', language: 'TypeScript' },
{ name: 'anna', language: 'Java' },
{ name: 'jimmy' },
{ name: 'nicky', language: 'Python' }
]
*/
over
- title: over
- tags: function,intermediate
Creates a function that invokes each provided function with the arguments it receives and returns the results.
- Use
Array.prototype.map()
andFunction.prototype.apply()
to apply each function to the given arguments.
const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));
const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1, 5]
overArgs
- title: overArgs
- tags: function,intermediate
Creates a function that invokes the provided function with its arguments transformed.
- Use
Array.prototype.map()
to applytransforms
toargs
in combination with the spread operator (...
) to pass the transformed arguments tofn
.
const overArgs = (fn, transforms) =>
(...args) => fn(...args.map((val, i) => transforms[i](val)));
const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]
pad
- title: pad
- tags: string,beginner
Pads a string on both sides with the specified character, if it's shorter than the specified length
.
- Use
String.prototype.padStart()
andString.prototype.padEnd()
to pad both sides of the given string. - Omit the third argument,
char
, to use the whitespace character as the default padding character.
const pad = (str, length, char = ' ') =>
str.padStart((str.length + length) / 2, char).padEnd(length, char);
pad('cat', 8); // ' cat '
pad(String(42), 6, '0'); // '004200'
pad('foobar', 3); // 'foobar'
padNumber
- title: padNumber
- tags: string,math,beginner
Pads a given number to the specified length.
- Use
String.prototype.padStart()
to pad the number to specified length, after converting it to a string.
const padNumber = (n, l) => `${n}`.padStart(l, '0');
padNumber(1234, 6); // '001234'
palindrome
- title: palindrome
- tags: string,intermediate
Checks if the given string is a palindrome.
- Normalize the string to
String.prototype.toLowerCase()
and useString.prototype.replace()
to remove non-alphanumeric characters from it. - Use the spread operator (
...
) to split the normalized string into individual characters. - Use
Array.prototype.reverse()
,String.prototype.join('')
and compare the result to the normalized string.
const palindrome = str => {
const s = str.toLowerCase().replace(/[\W_]/g, '');
return s === [...s].reverse().join('');
};
palindrome('taco cat'); // true
parseCookie
- title: parseCookie
- tags: browser,string,intermediate
Parses an HTTP Cookie header string, returning an object of all cookie name-value pairs.
- Use
String.prototype.split(';')
to separate key-value pairs from each other. - Use
Array.prototype.map()
andString.prototype.split('=')
to separate keys from values in each pair. - Use
Array.prototype.reduce()
anddecodeURIComponent()
to create an object with all key-value pairs.
const parseCookie = str =>
str
.split(';')
.map(v => v.split('='))
.reduce((acc, v) => {
acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
return acc;
}, {});
parseCookie('foo=bar; equation=E%3Dmc%5E2');
// { foo: 'bar', equation: 'E=mc^2' }
partial
- title: partial
- tags: function,intermediate
Creates a function that invokes fn
with partials
prepended to the arguments it receives.
- Use the spread operator (
...
) to prependpartials
to the list of arguments offn
.
const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'
partialRight
- title: partialRight
- tags: function,intermediate
Creates a function that invokes fn
with partials
appended to the arguments it receives.
- Use the spread operator (
...
) to appendpartials
to the list of arguments offn
.
const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'
partition
- title: partition
- tags: array,object,intermediate
Groups the elements into two arrays, depending on the provided function's truthiness for each element.
- Use
Array.prototype.reduce()
to create an array of two arrays. - Use
Array.prototype.push()
to add elements for whichfn
returnstrue
to the first array and elements for whichfn
returnsfalse
to the second one.
const partition = (arr, fn) =>
arr.reduce(
(acc, val, i, arr) => {
acc[fn(val, i, arr) ? 0 : 1].push(val);
return acc;
},
[[], []]
);
const users = [
{ user: 'barney', age: 36, active: false },
{ user: 'fred', age: 40, active: true },
];
partition(users, o => o.active);
// [
// [{ user: 'fred', age: 40, active: true }],
// [{ user: 'barney', age: 36, active: false }]
// ]
partitionBy
- title: partitionBy
- tags: array,object,advanced
Applies fn
to each value in arr
, splitting it each time the provided function returns a new value.
- Use
Array.prototype.reduce()
with an accumulator object that will hold the resulting array and the last value returned fromfn
. - Use
Array.prototype.push()
to add each value inarr
to the appropriate partition in the accumulator array.
const partitionBy = (arr, fn) =>
arr.reduce(
({ res, last }, v, i, a) => {
const next = fn(v, i, a);
if (next !== last) res.push([v]);
else res[res.length - 1].push(v);
return { res, last: next };
},
{ res: [] }
).res;
const numbers = [1, 1, 3, 3, 4, 5, 5, 5];
partitionBy(numbers, n => n % 2 === 0); // [[1, 1, 3, 3], [4], [5, 5, 5]]
partitionBy(numbers, n => n); // [[1, 1], [3, 3], [4], [5, 5, 5]]
percentile
- title: percentile
- tags: math,intermediate
Calculates the percentage of numbers in the given array that are less or equal to the given value.
- Use
Array.prototype.reduce()
to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.
const percentile = (arr, val) =>
(100 *
arr.reduce(
(acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0),
0
)) /
arr.length;
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55
permutations
- title: permutations
- tags: array,algorithm,recursion,advanced
Generates all permutations of an array's elements (contains duplicates).
- Use recursion.
- For each element in the given array, create all the partial permutations for the rest of its elements.
- Use
Array.prototype.map()
to combine the element with each partial permutation, thenArray.prototype.reduce()
to combine all permutations in one array. - Base cases are for
Array.prototype.length
equal to2
or1
. - ⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries may cause your browser to hang as it tries to solve all the different combinations.
const permutations = arr => {
if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
return arr.reduce(
(acc, item, i) =>
acc.concat(
permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
item,
...val,
])
),
[]
);
};
permutations([1, 33, 5]);
// [ [1, 33, 5], [1, 5, 33], [33, 1, 5], [33, 5, 1], [5, 1, 33], [5, 33, 1] ]
pick
- title: pick
- tags: object,intermediate
Picks the key-value pairs corresponding to the given keys from an object.
- Use
Array.prototype.reduce()
to convert the filtered/picked keys back to an object with the corresponding key-value pairs if the key exists in the object.
const pick = (obj, arr) =>
arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
pick({ a: 1, b: '2', c: 3 }, ['a', 'c']); // { 'a': 1, 'c': 3 }
pickBy
- title: pickBy
- tags: object,intermediate
Creates an object composed of the properties the given function returns truthy for.
- Use
Object.keys(obj)
andArray.prototype.filter()
to remove the keys for whichfn
returns a falsy value. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs. - The callback function is invoked with two arguments: (value, key).
const pickBy = (obj, fn) =>
Object.keys(obj)
.filter(k => fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number');
// { 'a': 1, 'c': 3 }
pipeAsyncFunctions
- title: pipeAsyncFunctions
- tags: function,promise,intermediate
Performs left-to-right function composition for asynchronous functions.
- Use
Array.prototype.reduce()
and the spread operator (...
) to perform function composition usingPromise.prototype.then()
. - The functions can return a combination of normal values,
Promise
s or beasync
, returning throughawait
. - All functions must accept a single argument.
const pipeAsyncFunctions = (...fns) =>
arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));
const sum = pipeAsyncFunctions(
x => x + 1,
x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
x => x + 3,
async x => (await x) + 4
);
(async() => {
console.log(await sum(5)); // 15 (after one second)
})();
pipeFunctions
- title: pipeFunctions
- tags: function,intermediate
Performs left-to-right function composition.
- Use
Array.prototype.reduce()
with the spread operator (...
) to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const pipeFunctions = (...fns) =>
fns.reduce((f, g) => (...args) => g(f(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15
pluck
- title: pluck
- tags: array,object,beginner
Converts an array of objects into an array of values corresponding to the specified key
.
- Use
Array.prototype.map()
to map the array of objects to the value ofkey
for each one.
const pluck = (arr, key) => arr.map(i => i[key]);
const simpsons = [
{ name: 'lisa', age: 8 },
{ name: 'homer', age: 36 },
{ name: 'marge', age: 34 },
{ name: 'bart', age: 10 }
];
pluck(simpsons, 'age'); // [8, 36, 34, 10]
pluralize
- title: pluralize
- tags: string,advanced
Returns the singular or plural form of the word based on the input number, using an optional dictionary if supplied.
- Use a closure to define a function that pluralizes the given
word
based on the value ofnum
. - If
num
is either-1
or1
, return the singular form of the word. - If
num
is any other number, return theplural
form. - Omit the third argument,
plural
, to use the default of the singular word +s
, or supply a custom pluralizedword
when necessary. - If the first argument is an
object
, return a function which can use the supplied dictionary to resolve the correct plural form of the word.
const pluralize = (val, word, plural = word + 's') => {
const _pluralize = (num, word, plural = word + 's') =>
[1, -1].includes(Number(num)) ? word : plural;
if (typeof val === 'object')
return (num, word) => _pluralize(num, word, val[word]);
return _pluralize(val, word, plural);
};
pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'
const PLURALS = {
person: 'people',
radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'
powerset
- title: powerset
- tags: math,algorithm,beginner
Returns the powerset of a given array of numbers.
- Use
Array.prototype.reduce()
combined withArray.prototype.map()
to iterate over elements and combine into an array containing all combinations.
const powerset = arr =>
arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
powerset([1, 2]); // [[], [1], [2], [2, 1]]
prefersDarkColorScheme
- title: prefersDarkColorScheme
- tags: browser,intermediate
Checks if the user color scheme preference is dark
.
- Use
Window.matchMedia()
with the appropriate media query to check the user color scheme preference.
const prefersDarkColorScheme = () =>
window &&
window.matchMedia &&
window.matchMedia('(prefers-color-scheme: dark)').matches;
prefersDarkColorScheme(); // true
prefersLightColorScheme
- title: prefersLightColorScheme
- tags: browser,intermediate
Checks if the user color scheme preference is light
.
- Use
Window.matchMedia()
with the appropriate media query to check the user color scheme preference.
const prefersLightColorScheme = () =>
window &&
window.matchMedia &&
window.matchMedia('(prefers-color-scheme: light)').matches;
prefersLightColorScheme(); // true
prefix
- title: prefix
- tags: browser,intermediate
Prefixes a CSS property based on the current browser.
- Use
Array.prototype.findIndex()
on an array of vendor prefix strings to test ifDocument.body
has one of them defined in itsCSSStyleDeclaration
object, otherwise returnnull
. - Use
String.prototype.charAt()
andString.prototype.toUpperCase()
to capitalize the property, which will be appended to the vendor prefix string.
const prefix = prop => {
const capitalizedProp = prop.charAt(0).toUpperCase() + prop.slice(1);
const prefixes = ['', 'webkit', 'moz', 'ms', 'o'];
const i = prefixes.findIndex(
prefix =>
typeof document.body.style[prefix ? prefix + capitalizedProp : prop] !==
'undefined'
);
return i !== -1 ? (i === 0 ? prop : prefixes[i] + capitalizedProp) : null;
};
prefix('appearance');
// 'appearance' on a supported browser, otherwise 'webkitAppearance', 'mozAppearance', 'msAppearance' or 'oAppearance'
prettyBytes
- title: prettyBytes
- tags: string,math,advanced
Converts a number in bytes to a human-readable string.
- Use an array dictionary of units to be accessed based on the exponent.
- Use
Number.prototype.toPrecision()
to truncate the number to a certain number of digits. - Return the prettified string by building it up, taking into account the supplied options and whether it is negative or not.
- Omit the second argument,
precision
, to use a default precision of3
digits. - Omit the third argument,
addSpace
, to add space between the number and unit by default.
const prettyBytes = (num, precision = 3, addSpace = true) => {
const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
const exponent = Math.min(
Math.floor(Math.log10(num < 0 ? -num : num) / 3),
UNITS.length - 1
);
const n = Number(
((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision)
);
return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};
prettyBytes(1000); // '1 KB'
prettyBytes(-27145424323.5821, 5); // '-27.145 GB'
prettyBytes(123456789, 3, false); // '123MB'
primeFactors
- title: primeFactors
- tags: math,algorithm,beginner
Finds the prime factors of a given number using the trial division algorithm.
- Use a
while
loop to iterate over all possible prime factors, starting with2
. - If the current factor,
f
, exactly dividesn
, addf
to the factors array and dividen
byf
. Otherwise, incrementf
by one.
const primeFactors = n => {
let a = [],
f = 2;
while (n > 1) {
if (n % f === 0) {
a.push(f);
n /= f;
} else {
f++;
}
}
return a;
};
primeFactors(147); // [3, 7, 7]
primes
- title: primes
- tags: math,algorithm,intermediate
Generates primes up to a given number, using the Sieve of Eratosthenes.
- Generate an array from
2
to the given number. - Use
Array.prototype.filter()
to filter out the values divisible by any number from2
to the square root of the provided number.
const primes = num => {
let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
sqroot = Math.floor(Math.sqrt(num)),
numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
return arr;
};
primes(10); // [2, 3, 5, 7]
prod
- title: prod
- tags: math,array,intermediate
Calculates the product of two or more numbers/arrays.
- Use
Array.prototype.reduce()
to multiply each value with an accumulator, initialized with a value of1
.
const prod = (...arr) => [...arr].reduce((acc, val) => acc * val, 1);
prod(1, 2, 3, 4); // 24
prod(...[1, 2, 3, 4]); // 24
promisify
- title: promisify
- tags: function,promise,intermediate
Converts an asynchronous function to return a promise.
- Use currying to return a function returning a
Promise
that calls the original function. - Use the rest operator (
...
) to pass in all the parameters. - Note: In Node 8+, you can use
util.promisify
.
const promisify = func => (...args) =>
new Promise((resolve, reject) =>
func(...args, (err, result) => (err ? reject(err) : resolve(result)))
);
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // Promise resolves after 2s
pull
- title: pull
- tags: array,intermediate
Mutates the original array to filter out the values specified.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values.
const pull = (arr, ...args) => {
let argState = Array.isArray(args[0]) ? args[0] : args;
let pulled = arr.filter(v => !argState.includes(v));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]
pullAtIndex
- title: pullAtIndex
- tags: array,advanced
Mutates the original array to filter out the values at the specified indexes. Returns the removed elements.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values. - Use
Array.prototype.push()
to keep track of pulled values.
const pullAtIndex = (arr, pullArr) => {
let removed = [];
let pulled = arr
.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
.filter((v, i) => !pullArr.includes(i));
arr.length = 0;
pulled.forEach(v => arr.push(v));
return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]);
// myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
pullAtValue
- title: pullAtValue
- tags: array,advanced
Mutates the original array to filter out the values specified. Returns the removed elements.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values. - Use
Array.prototype.push()
to keep track of pulled values.
const pullAtValue = (arr, pullArr) => {
let removed = [],
pushToRemove = arr.forEach((v, i) =>
pullArr.includes(v) ? removed.push(v) : v
),
mutateTo = arr.filter((v, i) => !pullArr.includes(v));
arr.length = 0;
mutateTo.forEach(v => arr.push(v));
return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']);
// myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
pullBy
- title: pullBy
- tags: array,advanced
Mutates the original array to filter out the values specified, based on a given iterator function.
- Check if the last argument provided is a function.
- Use
Array.prototype.map()
to apply the iterator functionfn
to all array elements. - Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values.
const pullBy = (arr, ...args) => {
const length = args.length;
let fn = length > 1 ? args[length - 1] : undefined;
fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]
quarterOfYear
- title: quarterOfYear
- tags: date,beginner
Returns the quarter and year to which the supplied date belongs to.
- Use
Date.prototype.getMonth()
to get the current month in the range (0, 11), add1
to map it to the range (1, 12). - Use
Math.ceil()
and divide the month by3
to get the current quarter. - Use
Date.prototype.getFullYear()
to get the year from the givendate
. - Omit the argument,
date
, to use the current date by default.
const quarterOfYear = (date = new Date()) => [
Math.ceil((date.getMonth() + 1) / 3),
date.getFullYear()
];
quarterOfYear(new Date('07/10/2018')); // [ 3, 2018 ]
quarterOfYear(); // [ 4, 2020 ]
queryStringToObject
- title: queryStringToObject
- tags: object,intermediate
Generates an object from the given query string or URL.
- Use
String.prototype.split()
to get the params from the givenurl
. - Use
new URLSearchParams()
to create an appropriate object and convert it to an array of key-value pairs using the spread operator (...
). - Use
Array.prototype.reduce()
to convert the array of key-value pairs into an object.
const queryStringToObject = url =>
[...new URLSearchParams(url.split('?')[1])].reduce(
(a, [k, v]) => ((a[k] = v), a),
{}
);
queryStringToObject('https://google.com?page=1&count=10');
// {page: '1', count: '10'}
quickSort
- title: quickSort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the quicksort algorithm.
- Use recursion.
- Use the spread operator (
...
) to clone the original array,arr
. - If the
length
of the array is less than2
, return the cloned array. - Use
Math.floor()
to calculate the index of the pivot element. - Use
Array.prototype.reduce()
andArray.prototype.push()
to split the array into two subarrays (elements smaller or equal to thepivot
and elements greater than it), destructuring the result into two arrays. - Recursively call
quickSort()
on the created subarrays.
const quickSort = arr => {
const a = [...arr];
if (a.length < 2) return a;
const pivotIndex = Math.floor(arr.length / 2);
const pivot = a[pivotIndex];
const [lo, hi] = a.reduce(
(acc, val, i) => {
if (val < pivot || (val === pivot && i != pivotIndex)) {
acc[0].push(val);
} else if (val > pivot) {
acc[1].push(val);
}
return acc;
},
[[], []]
);
return [...quickSort(lo), pivot, ...quickSort(hi)];
};
quickSort([1, 6, 1, 5, 3, 2, 1, 4]); // [1, 1, 1, 2, 3, 4, 5, 6]
radsToDegrees
- title: radsToDegrees
- tags: math,beginner
Converts an angle from radians to degrees.
- Use
Math.PI
and the radian to degree formula to convert the angle from radians to degrees.
const radsToDegrees = rad => (rad * 180.0) / Math.PI;
radsToDegrees(Math.PI / 2); // 90
randomAlphaNumeric
- title: randomAlphaNumeric
- tags: string,random,advanced
Generates a random string with the specified length.
- Use
Array.from()
to create a new array with the specifiedlength
. - Use
Math.random()
generate a random floating-point number,Number.prototype.toString(36)
to convert it to an alphanumeric string. - Use
String.prototype.slice(2)
to remove the integral part and decimal point from each generated number. - Use
Array.prototype.some()
to repeat this process as many times as required, up tolength
, as it produces a variable-length string each time. - Finally, use
String.prototype.slice()
to trim down the generated string if it's longer than the givenlength
.
const randomAlphaNumeric = length => {
let s = '';
Array.from({ length }).some(() => {
s += Math.random().toString(36).slice(2);
return s.length >= length;
});
return s.slice(0, length);
};
randomAlphaNumeric(5); // '0afad'
randomBoolean
- title: randomBoolean
- tags: math,random,beginner
Generates a random boolean value.
- Use
Math.random()
to generate a random number and check if it is greater than or equal to0.5
.
const randomBoolean = () => Math.random() >= 0.5;
randomBoolean(); // true
randomHexColorCode
- title: randomHexColorCode
- tags: math,random,beginner
Generates a random hexadecimal color code.
- Use
Math.random()
to generate a random 24-bit (6 * 4bits) hexadecimal number. - Use bit shifting and then convert it to an hexadecimal string using
Number.prototype.toString(16)
.
const randomHexColorCode = () => {
let n = (Math.random() * 0xfffff * 1000000).toString(16);
return '##' + n.slice(0, 6);
};
randomHexColorCode(); // '##e34155'
randomIntArrayInRange
- title: randomIntArrayInRange
- tags: math,random,intermediate
Generates an array of n
random integers in the specified range.
- Use
Array.from()
to create an empty array of the specific length. - Use
Math.random()
to generate random numbers and map them to the desired range, usingMath.floor()
to make them integers.
const randomIntArrayInRange = (min, max, n = 1) =>
Array.from(
{ length: n },
() => Math.floor(Math.random() * (max - min + 1)) + min
);
randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]
randomIntegerInRange
- title: randomIntegerInRange
- tags: math,random,beginner
Generates a random integer in the specified range.
- Use
Math.random()
to generate a random number and map it to the desired range. - Use
Math.floor()
to make it an integer.
const randomIntegerInRange = (min, max) =>
Math.floor(Math.random() * (max - min + 1)) + min;
randomIntegerInRange(0, 5); // 2
randomNumberInRange
- title: randomNumberInRange
- tags: math,random,beginner
Generates a random number in the specified range.
- Use
Math.random()
to generate a random value, map it to the desired range using multiplication.
const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;
randomNumberInRange(2, 10); // 6.0211363285087005
rangeGenerator
- title: rangeGenerator
- tags: function,generator,advanced
Creates a generator, that generates all values in the given range using the given step.
- Use a
while
loop to iterate fromstart
toend
, usingyield
to return each value and then incrementing bystep
. - Omit the third argument,
step
, to use a default value of1
.
const rangeGenerator = function* (start, end, step = 1) {
let i = start;
while (i < end) {
yield i;
i += step;
}
};
for (let i of rangeGenerator(6, 10)) console.log(i);
// Logs 6, 7, 8, 9
readFileLines
- title: readFileLines
- tags: node,array,beginner
Returns an array of lines from the specified file.
- Use
fs.readFileSync()
to create aBuffer
from a file. - Convert buffer to string using
buf.toString(encoding)
function. - Use
String.prototype.split(\n)
to create an array of lines from the contents of the file.
const fs = require('fs');
const readFileLines = filename =>
fs
.readFileSync(filename)
.toString('UTF8')
.split('\n');
/*
contents of test.txt :
line1
line2
line3
___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']
rearg
- title: rearg
- tags: function,intermediate
Creates a function that invokes the provided function with its arguments arranged according to the specified indexes.
- Use
Array.prototype.map()
to reorder arguments based onindexes
. - Use the spread operator (
...
) to pass the transformed arguments tofn
.
const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));
var rearged = rearg(
function(a, b, c) {
return [a, b, c];
},
[2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']
recordAnimationFrames
- title: recordAnimationFrames
- tags: browser,recursion,intermediate
Invokes the provided callback on each animation frame.
- Use recursion.
- Provided that
running
istrue
, continue invokingWindow.requestAnimationFrame()
which invokes the provided callback. - Return an object with two methods
start
andstop
to allow manual control of the recording. - Omit the second argument,
autoStart
, to implicitly callstart
when the function is invoked.
const recordAnimationFrames = (callback, autoStart = true) => {
let running = false,
raf;
const stop = () => {
if (!running) return;
running = false;
cancelAnimationFrame(raf);
};
const start = () => {
if (running) return;
running = true;
run();
};
const run = () => {
raf = requestAnimationFrame(() => {
callback();
if (running) run();
});
};
if (autoStart) start();
return { start, stop };
};
const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb);
// logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false);
// `start` needs to be explicitly called to begin recording frames
redirect
- title: redirect
- tags: browser,beginner
Redirects to a specified URL.
- Use
Window.location.href
orWindow.location.replace()
to redirect tourl
. - Pass a second argument to simulate a link click (
true
- default) or an HTTP redirect (false
).
const redirect = (url, asLink = true) =>
asLink ? (window.location.href = url) : window.location.replace(url);
redirect('https://google.com');
reduceSuccessive
- title: reduceSuccessive
- tags: array,intermediate
Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.
- Use
Array.prototype.reduce()
to apply the given function to the given array, storing each new result.
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce(
(res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res),
[acc]
);
reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0);
// [0, 1, 3, 6, 10, 15, 21]
reduceWhich
- title: reduceWhich
- tags: array,intermediate
Returns the minimum/maximum value of an array, after applying the provided function to set the comparing rule.
- Use
Array.prototype.reduce()
in combination with thecomparator
function to get the appropriate element in the array. - Omit the second argument,
comparator
, to use the default one that returns the minimum element in the array.
const reduceWhich = (arr, comparator = (a, b) => a - b) =>
arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));
reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
[
{ name: 'Tom', age: 12 },
{ name: 'Jack', age: 18 },
{ name: 'Lucy', age: 9 }
],
(a, b) => a.age - b.age
); // {name: 'Lucy', age: 9}
reducedFilter
- title: reducedFilter
- tags: array,intermediate
Filters an array of objects based on a condition while also filtering out unspecified keys.
- Use
Array.prototype.filter()
to filter the array based on the predicatefn
so that it returns the objects for which the condition returned a truthy value. - On the filtered array, use
Array.prototype.map()
to return the new object. - Use
Array.prototype.reduce()
to filter out the keys which were not supplied as thekeys
argument.
const reducedFilter = (data, keys, fn) =>
data.filter(fn).map(el =>
keys.reduce((acc, key) => {
acc[key] = el[key];
return acc;
}, {})
);
const data = [
{
id: 1,
name: 'john',
age: 24
},
{
id: 2,
name: 'mike',
age: 50
}
];
reducedFilter(data, ['id', 'name'], item => item.age > 24);
// [{ id: 2, name: 'mike'}]
reject
- title: reject
- tags: array,beginner
Filters an array's values based on a predicate function, returning only values for which the predicate function returns false
.
- Use
Array.prototype.filter()
in combination with the predicate function,pred
, to return only the values for which it returnsfalse
.
const reject = (pred, array) => array.filter((...args) => !pred(...args));
reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']);
// ['Pear', 'Kiwi']
remove
- title: remove
- tags: array,intermediate
Mutates an array by removing elements for which the given function returns false
.
- Use
Array.prototype.filter()
to find array elements that return truthy values. - Use
Array.prototype.reduce()
to remove elements usingArray.prototype.splice()
. - The callback function is invoked with three arguments (value, index, array).
const remove = (arr, func) =>
Array.isArray(arr)
? arr.filter(func).reduce((acc, val) => {
arr.splice(arr.indexOf(val), 1);
return acc.concat(val);
}, [])
: [];
remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]
removeAccents
- title: removeAccents
- tags: string,beginner
Removes accents from strings.
- Use
String.prototype.normalize()
to convert the string to a normalized Unicode format. - Use
String.prototype.replace()
to replace diacritical marks in the given Unicode range by empty strings.
const removeAccents = str =>
str.normalize('NFD').replace(/[\u0300-\u036f]/g, '');
removeAccents('Antoine de Saint-Exupéry'); // 'Antoine de Saint-Exupery'
removeClass
- title: removeClass
- tags: browser,beginner
Removes a class from an HTML element.
- Use
Element.classList
andDOMTokenList.remove()
to remove the specified class from the element.
const removeClass = (el, className) => el.classList.remove(className);
removeClass(document.querySelector('p.special'), 'special');
// The paragraph will not have the 'special' class anymore
removeElement
- title: removeElement
- tags: browser,beginner
Removes an element from the DOM.
- Use
Element.parentNode
to get the given element's parent node. - Use
Element.removeChild()
to remove the given element from its parent node.
const removeElement = el => el.parentNode.removeChild(el);
removeElement(document.querySelector('##my-element'));
// Removes ##my-element from the DOM
removeNonASCII
- title: removeNonASCII
- tags: string,regexp,intermediate
Removes non-printable ASCII characters.
- Use
String.prototype.replace()
with a regular expression to remove non-printable ASCII characters.
const removeNonASCII = str => str.replace(/[^\x20-\x7E]/g, '');
removeNonASCII('äÄçÇéÉêlorem-ipsumöÖÐþúÚ'); // 'lorem-ipsum'
removeWhitespace
- title: removeWhitespace
- tags: string,regexp,beginner
Returns a string with whitespaces removed.
- Use
String.prototype.replace()
with a regular expression to replace all occurrences of whitespace characters with an empty string.
const removeWhitespace = str => str.replace(/\s+/g, '');
removeWhitespace('Lorem ipsum.\n Dolor sit amet. ');
// 'Loremipsum.Dolorsitamet.'
renameKeys
- title: renameKeys
- tags: object,intermediate
Replaces the names of multiple object keys with the values provided.
- Use
Object.keys()
in combination withArray.prototype.reduce()
and the spread operator (...
) to get the object's keys and rename them according tokeysMap
.
const renameKeys = (keysMap, obj) =>
Object.keys(obj).reduce(
(acc, key) => ({
...acc,
...{ [keysMap[key] || key]: obj[key] }
}),
{}
);
const obj = { name: 'Bobo', job: 'Front-End Master', shoeSize: 100 };
renameKeys({ name: 'firstName', job: 'passion' }, obj);
// { firstName: 'Bobo', passion: 'Front-End Master', shoeSize: 100 }
renderElement
- title: renderElement
- tags: browser,recursion,advanced
Renders the given DOM tree in the specified DOM element.
- Destructure the first argument into
type
andprops
, usingtype
to determine if the given element is a text element. - Based on the element's
type
, use eitherDocument.createTextNode()
orDocument.createElement()
to create the DOM element. - Use
Object.keys()
to add attributes to the DOM element and setting event listeners, as necessary. - Use recursion to render
props.children
, if any. - Finally, use
Node.appendChild()
to append the DOM element to the specifiedcontainer
.
const renderElement = ({ type, props = {} }, container) => {
const isTextElement = !type;
const element = isTextElement
? document.createTextNode('')
: document.createElement(type);
const isListener = p => p.startsWith('on');
const isAttribute = p => !isListener(p) && p !== 'children';
Object.keys(props).forEach(p => {
if (isAttribute(p)) element[p] = props[p];
if (!isTextElement && isListener(p))
element.addEventListener(p.toLowerCase().slice(2), props[p]);
});
if (!isTextElement && props.children && props.children.length)
props.children.forEach(childElement =>
renderElement(childElement, element)
);
container.appendChild(element);
};
const myElement = {
type: 'button',
props: {
type: 'button',
className: 'btn',
onClick: () => alert('Clicked'),
children: [{ props: { nodeValue: 'Click me' } }]
}
};
renderElement(myElement, document.body);
repeatGenerator
- title: repeatGenerator
- tags: function,generator,advanced
Creates a generator, repeating the given value indefinitely.
- Use a non-terminating
while
loop, that willyield
a value every timeGenerator.prototype.next()
is called. - Use the return value of the
yield
statement to update the returned value if the passed value is notundefined
.
const repeatGenerator = function* (val) {
let v = val;
while (true) {
let newV = yield v;
if (newV !== undefined) v = newV;
}
};
const repeater = repeatGenerator(5);
repeater.next(); // { value: 5, done: false }
repeater.next(); // { value: 5, done: false }
repeater.next(4); // { value: 4, done: false }
repeater.next(); // { value: 4, done: false }
requireUncached
- title: requireUncached
- tags: node,advanced
Loads a module after removing it from the cache (if exists).
- Use
delete
to remove the module from the cache (if exists). - Use
require()
to load the module again.
const requireUncached = module => {
delete require.cache[require.resolve(module)];
return require(module);
};
const fs = requireUncached('fs'); // 'fs' will be loaded fresh every time
reverseNumber
- title: reverseNumber
- tags: math,string,beginner
Reverses a number.
- Use
Object.prototype.toString()
to convertn
to a string. - Use
String.prototype.split('')
,Array.prototype.reverse()
andString.prototype.join('')
to get the reversed value ofn
as a string. - Use
parseFloat()
to convert the string to a number andMath.sign()
to preserve its sign.
const reverseNumber = n =>
parseFloat(`${n}`.split('').reverse().join('')) * Math.sign(n);
reverseNumber(981); // 189
reverseNumber(-500); // -5
reverseNumber(73.6); // 6.37
reverseNumber(-5.23); // -32.5
reverseString
- title: reverseString
- tags: string,beginner
Reverses a string.
- Use the spread operator (
...
) andArray.prototype.reverse()
to reverse the order of the characters in the string. - Combine characters to get a string using
String.prototype.join('')
.
const reverseString = str => [...str].reverse().join('');
reverseString('foobar'); // 'raboof'
round
- title: round
- tags: math,intermediate
Rounds a number to a specified amount of digits.
- Use
Math.round()
and template literals to round the number to the specified number of digits. - Omit the second argument,
decimals
, to round to an integer.
const round = (n, decimals = 0) =>
Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
round(1.005, 2); // 1.01
runAsync
- title: runAsync
- tags: browser,function,promise,advanced
Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.
- Create a
new Worker()
using aBlob
object URL, the contents of which should be the stringified version of the supplied function. - Immediately post the return value of calling the function back.
- Return a
new Promise()
, listening foronmessage
andonerror
events and resolving the data posted back from the worker, or throwing an error.
const runAsync = fn => {
const worker = new Worker(
URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
type: 'application/javascript; charset=utf-8'
})
);
return new Promise((res, rej) => {
worker.onmessage = ({ data }) => {
res(data), worker.terminate();
};
worker.onerror = err => {
rej(err), worker.terminate();
};
});
};
const longRunningFunction = () => {
let result = 0;
for (let i = 0; i < 1000; i++)
for (let j = 0; j < 700; j++)
for (let k = 0; k < 300; k++) result = result + i + j + k;
return result;
};
/*
NOTE: Since the function is running in a different context, closures are not supported.
The function supplied to `runAsync` gets stringified, so everything becomes literal.
All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'
runPromisesInSeries
- title: runPromisesInSeries
- tags: function,promise,intermediate
Runs an array of promises in series.
- Use
Array.prototype.reduce()
to create a promise chain, where each promise returns the next promise when resolved.
const runPromisesInSeries = ps =>
ps.reduce((p, next) => p.then(next), Promise.resolve());
const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]);
// Executes each promise sequentially, taking a total of 3 seconds to complete
sample
- title: sample
- tags: array,string,random,beginner
Gets a random element from an array.
- Use
Math.random()
to generate a random number. - Multiply it by
Array.prototype.length
and round it off to the nearest whole number usingMath.floor()
. - This method also works with strings.
const sample = arr => arr[Math.floor(Math.random() * arr.length)];
sample([3, 7, 9, 11]); // 9
sampleSize
- title: sampleSize
- tags: array,random,intermediate
Gets n
random elements at unique keys from an array up to the size of the array.
- Shuffle the array using the Fisher-Yates algorithm.
- Use
Array.prototype.slice()
to get the firstn
elements. - Omit the second argument,
n
, to get only one element at random from the array.
const sampleSize = ([...arr], n = 1) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr.slice(0, n);
};
sampleSize([1, 2, 3], 2); // [3, 1]
sampleSize([1, 2, 3], 4); // [2, 3, 1]
scrollToTop
- title: scrollToTop
- tags: browser,intermediate
Smooth-scrolls to the top of the page.
- Get distance from top using
Document.documentElement
orDocument.body
andElement.scrollTop
. - Scroll by a fraction of the distance from the top.
- Use
Window.requestAnimationFrame()
to animate the scrolling.
const scrollToTop = () => {
const c = document.documentElement.scrollTop || document.body.scrollTop;
if (c > 0) {
window.requestAnimationFrame(scrollToTop);
window.scrollTo(0, c - c / 8);
}
};
scrollToTop(); // Smooth-scrolls to the top of the page
sdbm
- title: sdbm
- tags: math,intermediate
Hashes the input string into a whole number.
- Use
String.prototype.split('')
andArray.prototype.reduce()
to create a hash of the input string, utilizing bit shifting.
const sdbm = str => {
let arr = str.split('');
return arr.reduce(
(hashCode, currentVal) =>
(hashCode =
currentVal.charCodeAt(0) +
(hashCode << 6) +
(hashCode << 16) -
hashCode),
0
);
};
sdbm('name'); // -3521204949
selectionSort
- title: selectionSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the selection sort algorithm.
- Use the spread operator (
...
) to clone the original array,arr
. - Use a
for
loop to iterate over elements in the array. - Use
Array.prototype.slice()
andArray.prototype.reduce()
to find the index of the minimum element in the subarray to the right of the current index and perform a swap, if necessary.
const selectionSort = arr => {
const a = [...arr];
for (let i = 0; i < a.length; i++) {
const min = a
.slice(i + 1)
.reduce((acc, val, j) => (val < a[acc] ? j + i + 1 : acc), i);
if (min !== i) [a[i], a[min]] = [a[min], a[i]];
}
return a;
};
selectionSort([5, 1, 4, 2, 3]); // [1, 2, 3, 4, 5]
serializeCookie
- title: serializeCookie
- tags: browser,string,intermediate
Serializes a cookie name-value pair into a Set-Cookie header string.
- Use template literals and
encodeURIComponent()
to create the appropriate string.
const serializeCookie = (name, val) =>
`${encodeURIComponent(name)}=${encodeURIComponent(val)}`;
serializeCookie('foo', 'bar'); // 'foo=bar'
serializeForm
- title: serializeForm
- tags: browser,string,intermediate
Encodes a set of form elements as a query string.
- Use the
Foata
constructor to convert the HTMLform
toFoata
. - Use
Array.from()
to convert to an array, passing a map function as the second argument. - Use
Array.prototype.map()
andencodeURIComponent()
to encode each field's value. - Use
Array.prototype.join()
with appropriate arguments to produce an appropriate query string.
const serializeForm = form =>
Array.from(new Foata(form), field =>
field.map(encodeURIComponent).join('=')
).join('&');
serializeForm(document.querySelector('##form'));
// email=test%40email.com&name=Test%20Name
setStyle
- title: setStyle
- tags: browser,beginner
Sets the value of a CSS rule for the specified HTML element.
- Use
ElementCSSInlineStyle.style
to set the value of the CSSrule
for the specified element toval
.
const setStyle = (el, rule, val) => (el.style[rule] = val);
setStyle(document.querySelector('p'), 'font-size', '20px');
// The first <p> element on the page will have a font-size of 20px
shallowClone
- title: shallowClone
- tags: object,beginner
Creates a shallow clone of an object.
- Use
Object.assign()
and an empty object ({}
) to create a shallow clone of the original.
const shallowClone = obj => Object.assign({}, obj);
const a = { x: true, y: 1 };
const b = shallowClone(a); // a !== b
shank
- title: shank
- tags: array,intermediate
Has the same functionality as Array.prototype.splice()
, but returning a new array instead of mutating the original array.
- Use
Array.prototype.slice()
andArray.prototype.concat()
to get an array with the new contents after removing existing elements and/or adding new elements. - Omit the second argument,
index
, to start at0
. - Omit the third argument,
delCount
, to remove0
elements. - Omit the fourth argument,
elements
, in order to not add any new elements.
const shank = (arr, index = 0, delCount = 0, ...elements) =>
arr
.slice(0, index)
.concat(elements)
.concat(arr.slice(index + delCount));
const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta');
// [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']
show
- title: show
- tags: browser,css,beginner
Shows all the elements specified.
- Use the spread operator (
...
) andArray.prototype.forEach()
to clear thedisplay
property for each element specified.
const show = (...el) => [...el].forEach(e => (e.style.display = ''));
show(...document.querySelectorAll('img'));
// Shows all <img> elements on the page
shuffle
- title: shuffle
- tags: array,random,intermediate
Randomizes the order of the values of an array, returning a new array.
- Use the Fisher-Yates algorithm to reorder the elements of the array.
const shuffle = ([...arr]) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr;
};
const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]
similarity
- title: similarity
- tags: array,math,beginner
Returns an array of elements that appear in both arrays.
- Use
Array.prototype.includes()
to determine values that are not part ofvalues
. - Use
Array.prototype.filter()
to remove them.
const similarity = (arr, values) => arr.filter(v => values.includes(v));
similarity([1, 2, 3], [1, 2, 4]); // [1, 2]
size
- title: size
- tags: object,array,string,intermediate
Gets the size of an array, object or string.
- Get type of
val
(array
,object
orstring
). - Use
Array.prototype.length
property for arrays. - Use
length
orsize
value if available or number of keys for objects. - Use
size
of aBlob
object created fromval
for strings. - Split strings into array of characters with
split('')
and return its length.
const size = val =>
Array.isArray(val)
? val.length
: val && typeof val === 'object'
? val.size || val.length || Object.keys(val).length
: typeof val === 'string'
? new Blob([val]).size
: 0;
size([1, 2, 3, 4, 5]); // 5
size('size'); // 4
size({ one: 1, two: 2, three: 3 }); // 3
sleep
- title: sleep
- tags: function,promise,intermediate
Delays the execution of an asynchronous function.
- Delay executing part of an
async
function, by putting it to sleep, returning anew Promise()
.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
async function sleepyWork() {
console.log("I'm going to sleep for 1 second.");
await sleep(1000);
console.log('I woke up after 1 second.');
}
slugify
- title: slugify
- tags: string,regexp,intermediate
Converts a string to a URL-friendly slug.
- Use
String.prototype.toLowerCase()
andString.prototype.trim()
to normalize the string. - Use
String.prototype.replace()
to replace spaces, dashes and underscores with-
and remove special characters.
const slugify = str =>
str
.toLowerCase()
.trim()
.replace(/[^\w\s-]/g, '')
.replace(/[\s_-]+/g, '-')
.replace(/^-+|-+$/g, '');
slugify('Hello World!'); // 'hello-world'
smoothScroll
- title: smoothScroll
- tags: browser,css,intermediate
Smoothly scrolls the element on which it's called into the visible area of the browser window.
- Use
Element.scrollIntoView()
to scroll the element. - Use
{ behavior: 'smooth' }
to scroll smoothly.
const smoothScroll = element =>
document.querySelector(element).scrollIntoView({
behavior: 'smooth'
});
smoothScroll('##fooBar'); // scrolls smoothly to the element with the id fooBar
smoothScroll('.fooBar');
// scrolls smoothly to the first element with a class of fooBar
sortCharactersInString
- title: sortCharactersInString
- tags: string,beginner
Alphabetically sorts the characters in a string.
- Use the spread operator (
...
),Array.prototype.sort()
andString.prototype.localeCompare()
to sort the characters instr
. - Recombine using
String.prototype.join('')
.
const sortCharactersInString = str =>
[...str].sort((a, b) => a.localeCompare(b)).join('');
sortCharactersInString('cabbage'); // 'aabbceg'
sortedIndex
- title: sortedIndex
- tags: array,math,intermediate
Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.findIndex()
to find the appropriate index where the element should be inserted.
const sortedIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
return index === -1 ? arr.length : index;
};
sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1
sortedIndexBy
- title: sortedIndexBy
- tags: array,math,intermediate
Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order, based on the provided iterator function.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.findIndex()
to find the appropriate index where the element should be inserted, based on the iterator functionfn
.
const sortedIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr.findIndex(el =>
isDescending ? val >= fn(el) : val <= fn(el)
);
return index === -1 ? arr.length : index;
};
sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0
sortedLastIndex
- title: sortedLastIndex
- tags: array,intermediate
Finds the highest index at which a value should be inserted into an array in order to maintain its sort order.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.reverse()
andArray.prototype.findIndex()
to find the appropriate last index where the element should be inserted.
const sortedLastIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr
.reverse()
.findIndex(el => (isDescending ? n <= el : n >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndex([10, 20, 30, 30, 40], 30); // 4
sortedLastIndexBy
- title: sortedLastIndexBy
- tags: array,intermediate
Finds the highest index at which a value should be inserted into an array in order to maintain its sort order, based on a provided iterator function.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.map()
to apply the iterator function to all elements of the array. - Use
Array.prototype.reverse()
andArray.prototype.findIndex()
to find the appropriate last index where the element should be inserted, based on the provided iterator function.
const sortedLastIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr
.map(fn)
.reverse()
.findIndex(el => (isDescending ? val <= el : val >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1
splitLines
- title: splitLines
- tags: string,regexp,beginner
Splits a multiline string into an array of lines.
- Use
String.prototype.split()
and a regular expression to match line breaks and create an array.
const splitLines = str => str.split(/\r?\n/);
splitLines('This\nis a\nmultiline\nstring.\n');
// ['This', 'is a', 'multiline', 'string.' , '']
spreadOver
- title: spreadOver
- tags: function,intermediate
Takes a variadic function and returns a function that accepts an array of arguments.
- Use a closure and the spread operator (
...
) to map the array of arguments to the inputs of the function.
const spreadOver = fn => argsArr => fn(...argsArr);
const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3
stableSort
- title: stableSort
- tags: array,advanced
Performs stable sorting of an array, preserving the initial indexes of items when their values are the same.
- Use
Array.prototype.map()
to pair each element of the input array with its corresponding index. - Use
Array.prototype.sort()
and acompare
function to sort the list, preserving their initial order if the items compared are equal. - Use
Array.prototype.map()
to convert back to the initial array items. - Does not mutate the original array, but returns a new array instead.
const stableSort = (arr, compare) =>
arr
.map((item, index) => ({ item, index }))
.sort((a, b) => compare(a.item, b.item) || a.index - b.index)
.map(({ item }) => item);
const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
standardDeviation
- title: standardDeviation
- tags: math,array,intermediate
Calculates the standard deviation of an array of numbers.
- Use
Array.prototype.reduce()
to calculate the mean, variance and the sum of the variance of the values and determine the standard deviation. - Omit the second argument,
usePopulation
, to get the sample standard deviation or set it totrue
to get the population standard deviation.
const standardDeviation = (arr, usePopulation = false) => {
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
return Math.sqrt(
arr
.reduce((acc, val) => acc.concat((val - mean) ** 2), [])
.reduce((acc, val) => acc + val, 0) /
(arr.length - (usePopulation ? 0 : 1))
);
};
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true);
// 12.29899614287479 (population)
stringPermutations
- title: stringPermutations
- tags: string,recursion,advanced
Generates all permutations of a string (contains duplicates).
- Use recursion.
- For each letter in the given string, create all the partial permutations for the rest of its letters.
- Use
Array.prototype.map()
to combine the letter with each partial permutation. - Use
Array.prototype.reduce()
to combine all permutations in one array. - Base cases are for
String.prototype.length
equal to2
or1
. - ⚠️ WARNING: The execution time increases exponentially with each character. Anything more than 8 to 10 characters will cause your environment to hang as it tries to solve all the different combinations.
const stringPermutations = str => {
if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
return str
.split('')
.reduce(
(acc, letter, i) =>
acc.concat(
stringPermutations(str.slice(0, i) + str.slice(i + 1)).map(
val => letter + val
)
),
[]
);
};
stringPermutations('abc'); // ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
stringifyCircularJSON
- title: stringifyCircularJSON
- tags: object,advanced
Serializes a JSON object containing circular references into a JSON format.
- Create a
new WeakSet()
to store and check seen values, usingWeakSet.prototype.add()
andWeakSet.prototype.has()
. - Use
JSON.stringify()
with a custom replacer function that omits values already inseen
, adding new values as necessary. - ⚠️ NOTICE: This function finds and removes circular references, which causes circular data loss in the serialized JSON.
const stringifyCircularJSON = obj => {
const seen = new WeakSet();
return JSON.stringify(obj, (k, v) => {
if (v !== null && typeof v === 'object') {
if (seen.has(v)) return;
seen.add(v);
}
return v;
});
};
const obj = { n: 42 };
obj.obj = obj;
stringifyCircularJSON(obj); // '{"n": 42}'
stripHTMLTags
- title: stripHTMLTags
- tags: string,regexp,beginner
Removes HTML/XML tags from string.
- Use a regular expression to remove HTML/XML tags from a string.
const stripHTMLTags = str => str.replace(/<[^>]*>/g, '');
stripHTMLTags('<p><em>lorem</em> <strong>ipsum</strong></p>'); // 'lorem ipsum'
subSet
- title: subSet
- tags: array,intermediate
Checks if the first iterable is a subset of the second one, excluding duplicate values.
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that each value in the first iterable is contained in the second one.
const subSet = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sA].every(v => sB.has(v));
};
subSet(new Set([1, 2]), new Set([1, 2, 3, 4])); // true
subSet(new Set([1, 5]), new Set([1, 2, 3, 4])); // false
sum
- title: sum
- tags: math,array,beginner
Calculates the sum of two or more numbers/arrays.
- Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
.
const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);
sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10
sumBy
- title: sumBy
- tags: math,array,intermediate
Calculates the sum of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
.
const sumBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val) => acc + val, 0);
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
sumN
- title: sumN
- tags: math,beginner
Sums all the numbers between 1
and n
.
- Use the formula
(n * (n + 1)) / 2
to get the sum of all the numbers between 1 andn
.
const sumN = n => (n * (n + 1)) / 2;
sumN(100); // 5050
sumPower
- title: sumPower
- tags: math,intermediate
Calculates the sum of the powers of all the numbers from start
to end
(both inclusive).
- Use
Array.prototype.fill()
to create an array of all the numbers in the target range. - Use
Array.prototype.map()
and the exponent operator (**
) to raise them topower
andArray.prototype.reduce()
to add them together. - Omit the second argument,
power
, to use a default power of2
. - Omit the third argument,
start
, to use a default starting value of1
.
const sumPower = (end, power = 2, start = 1) =>
Array(end + 1 - start)
.fill(0)
.map((x, i) => (i + start) ** power)
.reduce((a, b) => a + b, 0);
sumPower(10); // 385
sumPower(10, 3); // 3025
sumPower(10, 3, 5); // 2925
superSet
- title: superSet
- tags: array,intermediate
Checks if the first iterable is a superset of the second one, excluding duplicate values.
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that each value in the second iterable is contained in the first one.
const superSet = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sB].every(v => sA.has(v));
};
superSet(new Set([1, 2, 3, 4]), new Set([1, 2])); // true
superSet(new Set([1, 2, 3, 4]), new Set([1, 5])); // false
supportsTouchEvents
- title: supportsTouchEvents
- tags: browser,beginner
Checks if touch events are supported.
- Check if
'ontouchstart'
exists inwindow
.
const supportsTouchEvents = () =>
window && 'ontouchstart' in window;
supportsTouchEvents(); // true
swapCase
- title: swapCase
- tags: string,beginner
Creates a string with uppercase characters converted to lowercase and vice versa.
- Use the spread operator (
...
) to convertstr
into an array of characters. - Use
String.prototype.toLowerCase()
andString.prototype.toUpperCase()
to convert lowercase characters to uppercase and vice versa. - Use
Array.prototype.map()
to apply the transformation to each character,Array.prototype.join()
to combine back into a string. - Note that it is not necessarily true that
swapCase(swapCase(str)) === str
.
const swapCase = str =>
[...str]
.map(c => (c === c.toLowerCase() ? c.toUpperCase() : c.toLowerCase()))
.join('');
swapCase('Hello world!'); // 'hELLO WORLD!'
symmetricDifference
- title: symmetricDifference
- tags: array,math,intermediate
Returns the symmetric difference between two arrays, without filtering out duplicate values.
- Create a
new Set()
from each array to get the unique values of each one. - Use
Array.prototype.filter()
on each of them to only keep values not contained in the other.
const symmetricDifference = (a, b) => {
const sA = new Set(a),
sB = new Set(b);
return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};
symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]
symmetricDifferenceBy
- title: symmetricDifferenceBy
- tags: array,intermediate
Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.
- Create a
new Set()
from each array to get the unique values of each one after applyingfn
to them. - Use
Array.prototype.filter()
on each of them to only keep values not contained in the other.
const symmetricDifferenceBy = (a, b, fn) => {
const sA = new Set(a.map(v => fn(v))),
sB = new Set(b.map(v => fn(v)));
return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};
symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]
symmetricDifferenceBy(
[{ id: 1 }, { id: 2 }, { id: 3 }],
[{ id: 1 }, { id: 2 }, { id: 4 }],
i => i.id
);
// [{ id: 3 }, { id: 4 }]
symmetricDifferenceWith
- title: symmetricDifferenceWith
- tags: array,intermediate
Returns the symmetric difference between two arrays, using a provided function as a comparator.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
to find the appropriate values.
const symmetricDifferenceWith = (arr, val, comp) => [
...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
];
symmetricDifferenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 3.9]
tail
- title: tail
- tags: array,beginner
Returns all elements in an array except for the first one.
- Return
Array.prototype.slice(1)
ifArray.prototype.length
is more than1
, otherwise, return the whole array.
const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);
tail([1, 2, 3]); // [2, 3]
tail([1]); // [1]
take
- title: take
- tags: array,beginner
Creates an array with n
elements removed from the beginning.
- Use
Array.prototype.slice()
to create a slice of the array withn
elements taken from the beginning.
const take = (arr, n = 1) => arr.slice(0, n);
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []
takeRight
- title: takeRight
- tags: array,intermediate
Creates an array with n
elements removed from the end.
- Use
Array.prototype.slice()
to create a slice of the array withn
elements taken from the end.
const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);
takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]
takeRightUntil
- title: takeRightUntil
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns true
.
Returns the removed elements.
- Create a reversed copy of the array, using the spread operator (
...
) andArray.prototype.reverse()
. - Loop through the reversed copy, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is truthy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeRightUntil = (arr, fn) => {
for (const [i, val] of [...arr].reverse().entries())
if (fn(val)) return i === 0 ? [] : arr.slice(-i);
return arr;
};
takeRightUntil([1, 2, 3, 4], n => n < 3); // [3, 4]
takeRightWhile
- title: takeRightWhile
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns false
.
Returns the removed elements.
- Create a reversed copy of the array, using the spread operator (
...
) andArray.prototype.reverse()
. - Loop through the reversed copy, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is falsy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeRightWhile = (arr, fn) => {
for (const [i, val] of [...arr].reverse().entries())
if (!fn(val)) return i === 0 ? [] : arr.slice(-i);
return arr;
};
takeRightWhile([1, 2, 3, 4], n => n >= 3); // [3, 4]
takeUntil
- title: takeUntil
- tags: array,intermediate
Removes elements in an array until the passed function returns true
.
Returns the removed elements.
- Loop through the array, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is truthy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeUntil = (arr, fn) => {
for (const [i, val] of arr.entries()) if (fn(val)) return arr.slice(0, i);
return arr;
};
takeUntil([1, 2, 3, 4], n => n >= 3); // [1, 2]
takeWhile
- title: takeWhile
- tags: array,intermediate
Removes elements in an array until the passed function returns false
.
Returns the removed elements.
- Loop through the array, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is falsy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeWhile = (arr, fn) => {
for (const [i, val] of arr.entries()) if (!fn(val)) return arr.slice(0, i);
return arr;
};
takeWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
throttle
- title: throttle
- tags: function,advanced
Creates a throttled function that only invokes the provided function at most once per every wait
milliseconds
- Use
setTimeout()
andclearTimeout()
to throttle the given method,fn
. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessaryarguments
. - Use
Date.now()
to keep track of the last time the throttled function was invoked. - Use a variable,
inThrottle
, to prevent a race condition between the first execution offn
and the next loop. - Omit the second argument,
wait
, to set the timeout at a default of 0 ms.
const throttle = (fn, wait) => {
let inThrottle, lastFn, lastTime;
return function() {
const context = this,
args = arguments;
if (!inThrottle) {
fn.apply(context, args);
lastTime = Date.now();
inThrottle = true;
} else {
clearTimeout(lastFn);
lastFn = setTimeout(function() {
if (Date.now() - lastTime >= wait) {
fn.apply(context, args);
lastTime = Date.now();
}
}, Math.max(wait - (Date.now() - lastTime), 0));
}
};
};
window.addEventListener(
'resize',
throttle(function(evt) {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250ms
timeTaken
- title: timeTaken
- tags: function,beginner
Measures the time it takes for a function to execute.
- Use
Console.time()
andConsole.timeEnd()
to measure the difference between the start and end times to determine how long the callback took to execute.
const timeTaken = callback => {
console.time('timeTaken');
const r = callback();
console.timeEnd('timeTaken');
return r;
};
timeTaken(() => Math.pow(2, 10)); // 1024, (logged): timeTaken: 0.02099609375ms
times
- title: times
- tags: function,intermediate
Iterates over a callback n
times
- Use
Function.prototype.call()
to callfn
n
times or until it returnsfalse
. - Omit the last argument,
context
, to use anundefined
object (or the global object in non-strict mode).
const times = (n, fn, context = undefined) => {
let i = 0;
while (fn.call(context, i) !== false && ++i < n) {}
};
var output = '';
times(5, i => (output += i));
console.log(output); // 01234
toCamelCase
- title: toCamelCase
- tags: string,regexp,intermediate
Converts a string to camelcase.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
,String.prototype.toLowerCase()
andString.prototype.toUpperCase()
to combine them, capitalizing the first letter of each one.
const toCamelCase = str => {
let s =
str &&
str
.match(
/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g
)
.map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
.join('');
return s.slice(0, 1).toLowerCase() + s.slice(1);
};
toCamelCase('some_database_field_name'); // 'someDatabaseFieldName'
toCamelCase('Some label that needs to be camelized');
// 'someLabelThatNeedsToBeCamelized'
toCamelCase('some-javascript-property'); // 'someJavascriptProperty'
toCamelCase('some-mixed_string with spaces_underscores-and-hyphens');
// 'someMixedStringWithSpacesUnderscoresAndHyphens'
toCharArray
- title: toCharArray
- tags: string,beginner
Converts a string to an array of characters.
- Use the spread operator (
...
) to convert the string into an array of characters.
const toCharArray = s => [...s];
toCharArray('hello'); // ['h', 'e', 'l', 'l', 'o']
toCurrency
- title: toCurrency
- tags: math,string,intermediate
Takes a number and returns it in the specified currency formatting.
- Use
Intl.NumberFormat
to enable country / currency sensitive formatting.
const toCurrency = (n, curr, LanguageFormat = undefined) =>
Intl.NumberFormat(LanguageFormat, {
style: 'currency',
currency: curr,
}).format(n);
toCurrency(123456.789, 'EUR');
// €123,456.79 | currency: Euro | currencyLangFormat: Local
toCurrency(123456.789, 'USD', 'en-us');
// $123,456.79 | currency: US Dollar | currencyLangFormat: English (United States)
toCurrency(123456.789, 'USD', 'fa');
// ۱۲۳٬۴۵۶٫۷۹ $ | currency: US Dollar | currencyLangFormat: Farsi
toCurrency(322342436423.2435, 'JPY');
// ¥322,342,436,423 | currency: Japanese Yen | currencyLangFormat: Local
toCurrency(322342436423.2435, 'JPY', 'fi');
// 322 342 436 423 ¥ | currency: Japanese Yen | currencyLangFormat: Finnish
toDecimalMark
- title: toDecimalMark
- tags: math,beginner
Converts a number to a decimal mark formatted string.
- Use
Number.prototype.toLocaleString()
to convert the number to decimal mark format.
const toDecimalMark = num => num.toLocaleString('en-US');
toDecimalMark(12305030388.9087); // '12,305,030,388.909'
toHSLArray
- title: toHSLArray
- tags: string,browser,regexp,beginner
Converts an hsl()
color string to an array of values.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values.
const toHSLArray = hslStr => hslStr.match(/\d+/g).map(Number);
toHSLArray('hsl(50, 10%, 10%)'); // [50, 10, 10]
toHSLObject
- title: toHSLObject
- tags: string,browser,regexp,intermediate
Converts an hsl()
color string to an object with the values of each color.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Use array destructuring to store the values into named variables and create an appropriate object from them.
const toHSLObject = hslStr => {
const [hue, saturation, lightness] = hslStr.match(/\d+/g).map(Number);
return { hue, saturation, lightness };
};
toHSLObject('hsl(50, 10%, 10%)'); // { hue: 50, saturation: 10, lightness: 10 }
toHash
- title: toHash
- tags: array,intermediate
Reduces a given array-like into a value hash (keyed data store).
- Given an iterable object or array-like structure, call
Array.prototype.reduce.call()
on the provided object to step over it and return anObject
, keyed by the reference value.
const toHash = (object, key) =>
Array.prototype.reduce.call(
object,
(acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
{}
);
toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 3: 1 }
toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
// A more in depth example:
let users = [
{ id: 1, first: 'Jon' },
{ id: 2, first: 'Joe' },
{ id: 3, first: 'Moe' },
];
let managers = [{ manager: 1, employees: [2, 3] }];
// We use function here because we want a bindable reference,
// but a closure referencing the hash would work, too.
managers.forEach(
manager =>
(manager.employees = manager.employees.map(function(id) {
return this[id];
}, toHash(users, 'id')))
);
managers;
// [ {manager:1, employees: [ {id: 2, first: 'Joe'}, {id: 3, first: 'Moe'} ] } ]
toISOStringWithTimezone
- title: toISOStringWithTimezone
- tags: date,intermediate
Converts a date to extended ISO format (ISO 8601), including timezone offset.
- Use
Date.prototype.getTimezoneOffset()
to get the timezone offset and reverse it, storing its sign indiff
. - Define a helper function,
pad
, that normalizes any passed number to an integer usingMath.floor()
andMath.abs()
and pads it to2
digits, usingString.prototype.padStart()
. - Use
pad()
and the built-in methods in theDate
prototype to build the ISO 8601 string with timezone offset.
const toISOStringWithTimezone = date => {
const tzOffset = -date.getTimezoneOffset();
const diff = tzOffset >= 0 ? '+' : '-';
const pad = n => `${Math.floor(Math.abs(n))}`.padStart(2, '0');
return date.getFullYear() +
'-' + pad(date.getMonth() + 1) +
'-' + pad(date.getDate()) +
'T' + pad(date.getHours()) +
':' + pad(date.getMinutes()) +
':' + pad(date.getSeconds()) +
diff + pad(tzOffset / 60) +
':' + pad(tzOffset % 60);
};
toISOStringWithTimezone(new Date()); // '2020-10-06T20:43:33-04:00'
toKebabCase
- title: toKebabCase
- tags: string,regexp,intermediate
Converts a string to kebab case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.join()
andString.prototype.toLowerCase()
to combine them, adding-
as a separator.
const toKebabCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('-');
toKebabCase('camelCase'); // 'camel-case'
toKebabCase('some text'); // 'some-text'
toKebabCase('some-mixed_string With spaces_underscores-and-hyphens');
// 'some-mixed-string-with-spaces-underscores-and-hyphens'
toKebabCase('AllThe-small Things'); // 'all-the-small-things'
toKebabCase('IAmEditingSomeXMLAndHTML');
// 'i-am-editing-some-xml-and-html'
toOrdinalSuffix
- title: toOrdinalSuffix
- tags: math,intermediate
Takes a number and returns it as a string with the correct ordinal indicator suffix.
- Use the modulo operator (
%
) to find values of single and tens digits. - Find which ordinal pattern digits match.
- If digit is found in teens pattern, use teens ordinal.
const toOrdinalSuffix = num => {
const int = parseInt(num),
digits = [int % 10, int % 100],
ordinals = ['st', 'nd', 'rd', 'th'],
oPattern = [1, 2, 3, 4],
tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
? int + ordinals[digits[0] - 1]
: int + ordinals[3];
};
toOrdinalSuffix('123'); // '123rd'
toPairs
- title: toPairs
- tags: object,array,intermediate
Creates an array of key-value pair arrays from an object or other iterable.
- Check if
Symbol.iterator
is defined and, if so, useArray.prototype.entries()
to get an iterator for the given iterable. - Use
Array.from()
to convert the result to an array of key-value pair arrays. - If
Symbol.iterator
is not defined forobj
, useObject.entries()
instead.
const toPairs = obj =>
obj[Symbol.iterator] instanceof Function && obj.entries instanceof Function
? Array.from(obj.entries())
: Object.entries(obj);
toPairs({ a: 1, b: 2 }); // [['a', 1], ['b', 2]]
toPairs([2, 4, 8]); // [[0, 2], [1, 4], [2, 8]]
toPairs('shy'); // [['0', 's'], ['1', 'h'], ['2', 'y']]
toPairs(new Set(['a', 'b', 'c', 'a'])); // [['a', 'a'], ['b', 'b'], ['c', 'c']]
toRGBArray
- title: toRGBArray
- tags: string,browser,regexp,beginner
Converts an rgb()
color string to an array of values.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values.
const toRGBArray = rgbStr => rgbStr.match(/\d+/g).map(Number);
toRGBArray('rgb(255, 12, 0)'); // [255, 12, 0]
toRGBObject
- title: toRGBObject
- tags: string,browser,regexp,intermediate
Converts an rgb()
color string to an object with the values of each color.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Use array destructuring to store the values into named variables and create an appropriate object from them.
const toRGBObject = rgbStr => {
const [red, green, blue] = rgbStr.match(/\d+/g).map(Number);
return { red, green, blue };
};
toRGBObject('rgb(255, 12, 0)'); // {red: 255, green: 12, blue: 0}
toRomanNumeral
- title: toRomanNumeral
- tags: math,string,intermediate
Converts an integer to its roman numeral representation.
Accepts value between 1
and 3999
(both inclusive).
- Create a lookup table containing 2-value arrays in the form of (roman value, integer).
- Use
Array.prototype.reduce()
to loop over the values inlookup
and repeatedly dividenum
by the value. - Use
String.prototype.repeat()
to add the roman numeral representation to the accumulator.
const toRomanNumeral = num => {
const lookup = [
['M', 1000],
['CM', 900],
['D', 500],
['CD', 400],
['C', 100],
['XC', 90],
['L', 50],
['XL', 40],
['X', 10],
['IX', 9],
['V', 5],
['IV', 4],
['I', 1],
];
return lookup.reduce((acc, [k, v]) => {
acc += k.repeat(Math.floor(num / v));
num = num % v;
return acc;
}, '');
};
toRomanNumeral(3); // 'III'
toRomanNumeral(11); // 'XI'
toRomanNumeral(1998); // 'MCMXCVIII'
toSafeInteger
- title: toSafeInteger
- tags: math,beginner
Converts a value to a safe integer.
- Use
Math.max()
andMath.min()
to find the closest safe value. - Use
Math.round()
to convert to an integer.
const toSafeInteger = num =>
Math.round(
Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER)
);
toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991
toSnakeCase
- title: toSnakeCase
- tags: string,regexp,intermediate
Converts a string to snake case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
andString.prototype.toLowerCase()
to combine them, adding_
as a separator.
const toSnakeCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('_');
toSnakeCase('camelCase'); // 'camel_case'
toSnakeCase('some text'); // 'some_text'
toSnakeCase('some-mixed_string With spaces_underscores-and-hyphens');
// 'some_mixed_string_with_spaces_underscores_and_hyphens'
toSnakeCase('AllThe-small Things'); // 'all_the_small_things'
toKebabCase('IAmEditingSomeXMLAndHTML');
// 'i_am_editing_some_xml_and_html'
toTitleCase
- title: toTitleCase
- tags: string,regexp,intermediate
Converts a string to title case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
andString.prototype.toUpperCase()
to combine them, capitalizing the first letter of each word and adding a whitespace between them.
const toTitleCase = str =>
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.charAt(0).toUpperCase() + x.slice(1))
.join(' ');
toTitleCase('some_database_field_name'); // 'Some Database Field Name'
toTitleCase('Some label that needs to be title-cased');
// 'Some Label That Needs To Be Title Cased'
toTitleCase('some-package-name'); // 'Some Package Name'
toTitleCase('some-mixed_string with spaces_underscores-and-hyphens');
// 'Some Mixed String With Spaces Underscores And Hyphens'
toggleClass
- title: toggleClass
- tags: browser,beginner
Toggles a class for an HTML element.
- Use
Element.classList
andDOMTokenList.toggle()
to toggle the specified class for the element.
const toggleClass = (el, className) => el.classList.toggle(className);
toggleClass(document.querySelector('p.special'), 'special');
// The paragraph will not have the 'special' class anymore
tomorrow
- title: tomorrow
- tags: date,intermediate
Results in a string representation of tomorrow's date.
- Use
new Date()
to get the current date. - Increment it by one using
Date.prototype.getDate()
and set the value to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const tomorrow = () => {
let d = new Date();
d.setDate(d.getDate() + 1);
return d.toISOString().split('T')[0];
};
tomorrow(); // 2018-10-19 (if current date is 2018-10-18)
transform
- title: transform
- tags: object,intermediate
Applies a function against an accumulator and each key in the object (from left to right).
- Use
Object.keys()
to iterate over each key in the object. - Use
Array.prototype.reduce()
to apply the specified function against the given accumulator.
const transform = (obj, fn, acc) =>
Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);
transform(
{ a: 1, b: 2, c: 1 },
(r, v, k) => {
(r[v] || (r[v] = [])).push(k);
return r;
},
{}
); // { '1': ['a', 'c'], '2': ['b'] }
triggerEvent
- title: triggerEvent
- tags: browser,event,intermediate
Triggers a specific event on a given element, optionally passing custom data.
- Use
new CustomEvent()
to create an event from the specifiedeventType
and details. - Use
EventTarget.dispatchEvent()
to trigger the newly created event on the given element. - Omit the third argument,
detail
, if you do not want to pass custom data to the triggered event.
const triggerEvent = (el, eventType, detail) =>
el.dispatchEvent(new CustomEvent(eventType, { detail }));
triggerEvent(document.getElementById('myId'), 'click');
triggerEvent(document.getElementById('myId'), 'click', { username: 'bob' });
truncateString
- title: truncateString
- tags: string,beginner
Truncates a string up to a specified length.
- Determine if
String.prototype.length
is greater thannum
. - Return the string truncated to the desired length, with
'...'
appended to the end or the original string.
const truncateString = (str, num) =>
str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
truncateString('boomerang', 7); // 'boom...'
truncateStringAtWhitespace
- title: truncateStringAtWhitespace
- tags: string,intermediate
Truncates a string up to specified length, respecting whitespace when possible.
- Determine if
String.prototype.length
is greater or equal tolim
. If not, return it as-is. - Use
String.prototype.slice()
andString.prototype.lastIndexOf()
to find the index of the last space below the desiredlim
. - Use
String.prototype.slice()
to appropriately truncatestr
based onlastSpace
, respecting whitespace if possible and appendingending
at the end. - Omit the third argument,
ending
, to use the default ending of'...'
.
const truncateStringAtWhitespace = (str, lim, ending = '...') => {
if (str.length <= lim) return str;
const lastSpace = str.slice(0, lim - ending.length + 1).lastIndexOf(' ');
return str.slice(0, lastSpace > 0 ? lastSpace : lim - ending.length) + ending;
};
truncateStringAtWhitespace('short', 10); // 'short'
truncateStringAtWhitespace('not so short', 10); // 'not so...'
truncateStringAtWhitespace('trying a thing', 10); // 'trying...'
truncateStringAtWhitespace('javascripting', 10); // 'javascr...'
truthCheckCollection
- title: truthCheckCollection
- tags: object,logic,array,intermediate
Checks if the predicate function is truthy for all elements of a collection.
- Use
Array.prototype.every()
to check if each passed object has the specified property and if it returns a truthy value.
const truthCheckCollection = (collection, pre) =>
collection.every(obj => obj[pre]);
truthCheckCollection(
[
{ user: 'Tinky-Winky', sex: 'male' },
{ user: 'Dipsy', sex: 'male' },
],
'sex'
); // true
unary
- title: unary
- tags: function,beginner
Creates a function that accepts up to one argument, ignoring any additional arguments.
- Call the provided function,
fn
, with just the first argument supplied.
const unary = fn => val => fn(val);
['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]
uncurry
- title: uncurry
- tags: function,advanced
Uncurries a function up to depth n
.
- Return a variadic function.
- Use
Array.prototype.reduce()
on the provided arguments to call each subsequent curry level of the function. - If the
length
of the provided arguments is less thann
throw an error. - Otherwise, call
fn
with the proper amount of arguments, usingArray.prototype.slice(0, n)
. - Omit the second argument,
n
, to uncurry up to depth1
.
const uncurry = (fn, n = 1) => (...args) => {
const next = acc => args => args.reduce((x, y) => x(y), acc);
if (n > args.length) throw new RangeError('Arguments too few!');
return next(fn)(args.slice(0, n));
};
const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6
unescapeHTML
- title: unescapeHTML
- tags: string,browser,regexp,beginner
Unescapes escaped HTML characters.
- Use
String.prototype.replace()
with a regexp that matches the characters that need to be unescaped. - Use the function's callback to replace each escaped character instance with its associated unescaped character using a dictionary (object).
const unescapeHTML = str =>
str.replace(
/&|<|>|&##39;|"/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
'&##39;': "'",
'"': '"'
}[tag] || tag)
);
unescapeHTML('<a href="##">Me & you</a>');
// '<a href="##">Me & you</a>'
unflattenObject
- title: unflattenObject
- tags: object,advanced
Unflatten an object with the paths for keys.
- Use nested
Array.prototype.reduce()
to convert the flat path to a leaf node. - Use
String.prototype.split('.')
to split each key with a dot delimiter andArray.prototype.reduce()
to add objects against the keys. - If the current accumulator already contains a value against a particular key, return its value as the next accumulator.
- Otherwise, add the appropriate key-value pair to the accumulator object and return the value as the accumulator.
const unflattenObject = obj =>
Object.keys(obj).reduce((res, k) => {
k.split('.').reduce(
(acc, e, i, keys) =>
acc[e] ||
(acc[e] = isNaN(Number(keys[i + 1]))
? keys.length - 1 === i
? obj[k]
: {}
: []),
res
);
return res;
}, {});
unflattenObject({ 'a.b.c': 1, d: 1 }); // { a: { b: { c: 1 } }, d: 1 }
unflattenObject({ 'a.b': 1, 'a.c': 2, d: 3 }); // { a: { b: 1, c: 2 }, d: 3 }
unflattenObject({ 'a.b.0': 8, d: 3 }); // { a: { b: [ 8 ] }, d: 3 }
unfold
- title: unfold
- tags: function,array,intermediate
Builds an array, using an iterator function and an initial seed value.
- Use a
while
loop andArray.prototype.push()
to call the function repeatedly until it returnsfalse
. - The iterator function accepts one argument (
seed
) and must always return an array with two elements ([value
,nextSeed
]) orfalse
to terminate.
const unfold = (fn, seed) => {
let result = [],
val = [null, seed];
while ((val = fn(val[1]))) result.push(val[0]);
return result;
};
var f = n => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]
union
- title: union
- tags: array,beginner
Returns every element that exists in any of the two arrays at least once.
- Create a
new Set()
with all values ofa
andb
and convert it to an array.
const union = (a, b) => Array.from(new Set([...a, ...b]));
union([1, 2, 3], [4, 3, 2]); // [1, 2, 3, 4]
unionBy
- title: unionBy
- tags: array,intermediate
Returns every element that exists in any of the two arrays at least once, after applying the provided function to each array element of both.
- Create a
new Set()
by applying allfn
to all values ofa
. - Create a
new Set()
froma
and all elements inb
whose value, after applyingfn
does not match a value in the previously created set. - Return the last set converted to an array.
const unionBy = (a, b, fn) => {
const s = new Set(a.map(fn));
return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
};
unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]
unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], x => x.id)
// [{ id: 1 }, { id: 2 }, { id: 3 }]
unionWith
- title: unionWith
- tags: array,intermediate
Returns every element that exists in any of the two arrays at least once, using a provided comparator function.
- Create a
new Set()
with all values ofa
and values inb
for which the comparator finds no matches ina
, usingArray.prototype.findIndex()
.
const unionWith = (a, b, comp) =>
Array.from(
new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])
);
unionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
);
// [1, 1.2, 1.5, 3, 0, 3.9]
uniqueElements
- title: uniqueElements
- tags: array,beginner
Finds all unique values in an array.
- Create a
new Set()
from the given array to discard duplicated values. - Use the spread operator (
...
) to convert it back to an array.
const uniqueElements = arr => [...new Set(arr)];
uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]
uniqueElementsBy
- title: uniqueElementsBy
- tags: array,intermediate
Finds all unique values of an array, based on a provided comparator function.
- Use
Array.prototype.reduce()
andArray.prototype.some()
for an array containing only the first unique occurrence of each value, based on the comparator function,fn
. - The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsBy = (arr, fn) =>
arr.reduce((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);
uniqueElementsBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]
uniqueElementsByRight
- title: uniqueElementsByRight
- tags: array,intermediate
Finds all unique values of an array, based on a provided comparator function, starting from the right.
- Use
Array.prototype.reduceRight()
andArray.prototype.some()
for an array containing only the last unique occurrence of each value, based on the comparator function,fn
. - The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsByRight = (arr, fn) =>
arr.reduceRight((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);
uniqueElementsByRight(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]
uniqueSymmetricDifference
- title: uniqueSymmetricDifference
- tags: array,math,intermediate
Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.
- Use
Array.prototype.filter()
andArray.prototype.includes()
on each array to remove values contained in the other. - Create a
new Set()
from the results, removing duplicate values.
const uniqueSymmetricDifference = (a, b) => [
...new Set([
...a.filter(v => !b.includes(v)),
...b.filter(v => !a.includes(v)),
]),
];
uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]
untildify
- title: untildify
- tags: node,string,beginner
Converts a tilde path to an absolute path.
- Use
String.prototype.replace()
with a regular expression andos.homedir()
to replace the~
in the start of the path with the home directory.
const untildify = str =>
str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);
untildify('~/node'); // '/Users/aUser/node'
unzip
- title: unzip
- tags: array,intermediate
Creates an array of arrays, ungrouping the elements in an array produced by zip.
- Use
Math.max()
,Function.prototype.apply()
to get the longest subarray in the array,Array.prototype.map()
to make each element an array. - Use
Array.prototype.reduce()
andArray.prototype.forEach()
to map grouped values to individual arrays.
const unzip = arr =>
arr.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
);
unzip([['a', 1, true], ['b', 2, false]]); // [['a', 'b'], [1, 2], [true, false]]
unzip([['a', 1, true], ['b', 2]]); // [['a', 'b'], [1, 2], [true]]
unzipWith
- title: unzipWith
- tags: array,advanced
Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.
- Use
Math.max()
,Function.prototype.apply()
to get the longest subarray in the array,Array.prototype.map()
to make each element an array. - Use
Array.prototype.reduce()
andArray.prototype.forEach()
to map grouped values to individual arrays. - Use
Array.prototype.map()
and the spread operator (...
) to applyfn
to each individual group of elements.
const unzipWith = (arr, fn) =>
arr
.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
)
.map(val => fn(...val));
unzipWith(
[
[1, 10, 100],
[2, 20, 200],
],
(...args) => args.reduce((acc, v) => acc + v, 0)
);
// [3, 30, 300]
validateNumber
- title: validateNumber
- tags: math,intermediate
Checks if the given value is a number.
- Use
parseFloat()
to try to convertn
to a number. - Use
!Number.isNaN()
to check ifnum
is a number. - Use
Number.isFinite()
to check ifnum
is finite. - Use
Number()
and the loose equality operator (==
) to check if the coercion holds.
const validateNumber = n => {
const num = parseFloat(n);
return !Number.isNaN(num) && Number.isFinite(num) && Number(n) == n;
}
validateNumber('10'); // true
validateNumber('a'); // false
vectorAngle
- title: vectorAngle
- tags: math,beginner
Calculates the angle (theta) between two vectors.
- Use
Array.prototype.reduce()
,Math.pow()
andMath.sqrt()
to calculate the magnitude of each vector and the scalar product of the two vectors. - Use
Math.acos()
to calculate the arccosine and get the theta value.
const vectorAngle = (x, y) => {
let mX = Math.sqrt(x.reduce((acc, n) => acc + Math.pow(n, 2), 0));
let mY = Math.sqrt(y.reduce((acc, n) => acc + Math.pow(n, 2), 0));
return Math.acos(x.reduce((acc, n, i) => acc + n * y[i], 0) / (mX * mY));
};
vectorAngle([3, 4], [4, 3]); // 0.283794109208328
vectorDistance
- title: vectorDistance
- tags: math,algorithm,beginner
Calculates the distance between two vectors.
- Use
Array.prototype.reduce()
,Math.pow()
andMath.sqrt()
to calculate the Euclidean distance between two vectors.
const vectorDistance = (x, y) =>
Math.sqrt(x.reduce((acc, val, i) => acc + Math.pow(val - y[i], 2), 0));
vectorDistance([10, 0, 5], [20, 0, 10]); // 11.180339887498949
walkThrough
- title: walkThrough
- tags: object,recursion,generator,advanced
Creates a generator, that walks through all the keys of a given object.
- Use recursion.
- Define a generator function,
walk
, that takes an object and an array of keys. - Use a
for...of
loop andObject.keys()
to iterate over the keys of the object. - Use
typeof
to check if each value in the given object is itself an object. - If so, use the
yield*
expression to recursively delegate to the same generator function,walk
, appending the currentkey
to the array of keys. Otherwise,yield
the an array of keys representing the current path and the value of the givenkey
. - Use the
yield*
expression to delegate to thewalk
generator function.
const walkThrough = function* (obj) {
const walk = function* (x, previous = []) {
for (let key of Object.keys(x)) {
if (typeof x[key] === 'object') yield* walk(x[key], [...previous, key]);
else yield [[...previous, key], x[key]];
}
};
yield* walk(obj);
};
const obj = {
a: 10,
b: 20,
c: {
d: 10,
e: 20,
f: [30, 40]
},
g: [
{
h: 10,
i: 20
},
{
j: 30
},
40
]
};
[...walkThrough(obj)];
/*
[
[['a'], 10],
[['b'], 20],
[['c', 'd'], 10],
[['c', 'e'], 20],
[['c', 'f', '0'], 30],
[['c', 'f', '1'], 40],
[['g', '0', 'h'], 10],
[['g', '0', 'i'], 20],
[['g', '1', 'j'], 30],
[['g', '2'], 40]
]
*/
weightedAverage
- title: weightedAverage
- tags: math,intermediate
Calculates the weighted average of two or more numbers.
- Use
Array.prototype.reduce()
to create the weighted sum of the values and the sum of the weights. - Divide them with each other to get the weighted average.
const weightedAverage = (nums, weights) => {
const [sum, weightSum] = weights.reduce(
(acc, w, i) => {
acc[0] = acc[0] + nums[i] * w;
acc[1] = acc[1] + w;
return acc;
},
[0, 0]
);
return sum / weightSum;
};
weightedAverage([1, 2, 3], [0.6, 0.2, 0.3]); // 1.72727
weightedSample
- title: weightedSample
- tags: array,random,advanced
Gets a random element from an array, using the provided weights
as the probabilities for each element.
- Use
Array.prototype.reduce()
to create an array of partial sums for each value inweights
. - Use
Math.random()
to generate a random number andArray.prototype.findIndex()
to find the correct index based on the array previously produced. - Finally, return the element of
arr
with the produced index.
const weightedSample = (arr, weights) => {
let roll = Math.random();
return arr[
weights
.reduce(
(acc, w, i) => (i === 0 ? [w] : [...acc, acc[acc.length - 1] + w]),
[]
)
.findIndex((v, i, s) => roll >= (i === 0 ? 0 : s[i - 1]) && roll < v)
];
};
weightedSample([3, 7, 9, 11], [0.1, 0.2, 0.6, 0.1]); // 9
when
- title: when
- tags: function,logic,beginner
Returns a function that takes one argument and runs a callback if it's truthy or returns it if falsy.
- Return a function expecting a single value,
x
, that returns the appropriate value based onpred
.
const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);
const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1
without
- title: without
- tags: array,beginner
Filters out the elements of an array that have one of the specified values.
- Use
Array.prototype.includes()
to find values to exclude. - Use
Array.prototype.filter()
to create an array excluding them.
const without = (arr, ...args) => arr.filter(v => !args.includes(v));
without([2, 1, 2, 3], 1, 2); // [3]
wordWrap
- title: wordWrap
- tags: string,regexp,intermediate
Wraps a string to a given number of characters using a string break character.
- Use
String.prototype.replace()
and a regular expression to insert a given break character at the nearest whitespace ofmax
characters. - Omit the third argument,
br
, to use the default value of'\n'
.
const wordWrap = (str, max, br = '\n') => str.replace(
new RegExp(`(?![^\\n]{1,${max}}$)([^\\n]{1,${max}})\\s`, 'g'), '$1' + br
);
wordWrap(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce tempus.',
32
);
// 'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit.\nFusce tempus.'
wordWrap(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce tempus.',
32,
'\r\n'
);
// 'Lorem ipsum dolor sit amet,\r\nconsectetur adipiscing elit.\r\nFusce tempus.'
words
- title: words
- tags: string,regexp,intermediate
Converts a given string into an array of words.
- Use
String.prototype.split()
with a suppliedpattern
(defaults to non-alpha as a regexp) to convert to an array of strings. - Use
Array.prototype.filter()
to remove any empty strings. - Omit the second argument,
pattern
, to use the default regexp.
const words = (str, pattern = /[^a-zA-Z-]+/) =>
str.split(pattern).filter(Boolean);
words('I love javaScript!!'); // ['I', 'love', 'javaScript']
words('python, javaScript & coffee'); // ['python', 'javaScript', 'coffee']
xProd
- title: xProd
- tags: array,math,intermediate
Creates a new array out of the two supplied by creating each possible pair from the arrays.
- Use
Array.prototype.reduce()
,Array.prototype.map()
andArray.prototype.concat()
to produce every possible pair from the elements of the two arrays.
const xProd = (a, b) =>
a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);
xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]
xor
- title: xor
- tags: math,logic,beginner unlisted: true
Checks if only one of the arguments is true
.
- Use the logical or (
||
), and (&&
) and not (!
) operators on the two given values to create the logical xor.
const xor = (a, b) => (( a || b ) && !( a && b ));
xor(true, true); // false
xor(true, false); // true
xor(false, true); // true
xor(false, false); // false
yesNo
- title: yesNo
- tags: string,regexp,intermediate unlisted: true
Returns true
if the string is y
/yes
or false
if the string is n
/no
.
- Use
RegExp.prototype.test()
to check if the string evaluates toy/yes
orn/no
. - Omit the second argument,
def
to set the default answer asno
.
const yesNo = (val, def = false) =>
/^(y|yes)$/i.test(val) ? true : /^(n|no)$/i.test(val) ? false : def;
yesNo('Y'); // true
yesNo('yes'); // true
yesNo('No'); // false
yesNo('Foo', true); // true
yesterday
- title: yesterday
- tags: date,intermediate
Results in a string representation of yesterday's date.
- Use
new Date()
to get the current date. - Decrement it by one using
Date.prototype.getDate()
and set the value to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const yesterday = () => {
let d = new Date();
d.setDate(d.getDate() - 1);
return d.toISOString().split('T')[0];
};
yesterday(); // 2018-10-17 (if current date is 2018-10-18)
zip
- title: zip
- tags: array,intermediate
Creates an array of elements, grouped based on their position in the original arrays.
- Use
Math.max()
,Function.prototype.apply()
to get the longest array in the arguments. - Create an array with that length as return value and use
Array.from()
with a mapping function to create an array of grouped elements. - If lengths of the argument arrays vary,
undefined
is used where no value could be found.
const zip = (...arrays) => {
const maxLength = Math.max(...arrays.map(x => x.length));
return Array.from({ length: maxLength }).map((_, i) => {
return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
});
};
zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]
zipObject
- title: zipObject
- tags: array,object,intermediate
Associates properties to values, given array of valid property identifiers and an array of values.
- Use
Array.prototype.reduce()
to build an object from the two arrays. - If the length of
props
is longer thanvalues
, remaining keys will beundefined
. - If the length of
values
is longer thanprops
, remaining values will be ignored.
const zipObject = (props, values) =>
props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});
zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}
zipWith
- title: zipWith
- tags: array,advanced
Creates an array of elements, grouped based on the position in the original arrays and using a function to specify how grouped values should be combined.
- Check if the last argument provided is a function.
- Use
Math.max()
to get the longest array in the arguments. - Use
Array.from()
to create an array with appropriate length and a mapping function to create array of grouped elements. - If lengths of the argument arrays vary,
undefined
is used where no value could be found. - The function is invoked with the elements of each group.
const zipWith = (...array) => {
const fn =
typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
return Array.from({ length: Math.max(...array.map(a => a.length)) }, (_, i) =>
fn ? fn(...array.map(a => a[i])) : array.map(a => a[i])
);
};
zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111, 222]
zipWith(
[1, 2, 3],
[10, 20],
[100, 200],
(a, b, c) =>
(a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']
join
- title: join
- tags: array,intermediate
Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.
- Use
Array.prototype.reduce()
to combine elements into a string. - Omit the second argument,
separator
, to use a default separator of','
. - Omit the third argument,
end
, to use the same value asseparator
by default.
const join = (arr, separator = ',', end = separator) =>
arr.reduce(
(acc, val, i) =>
i === arr.length - 2
? acc + val + end
: i === arr.length - 1
? acc + val
: acc + val + separator,
''
);
join(['pen', 'pineapple', 'apple', 'pen'],',','&'); // 'pen,pineapple,apple&pen'
join(['pen', 'pineapple', 'apple', 'pen'], ','); // 'pen,pineapple,apple,pen'
join(['pen', 'pineapple', 'apple', 'pen']); // 'pen,pineapple,apple,pen'
juxt
- title: juxt
- tags: function,advanced
Takes several functions as argument and returns a function that is the juxtaposition of those functions.
- Use
Array.prototype.map()
to return afn
that can take a variable number ofargs
. - When
fn
is called, return an array containing the result of applying eachfn
to theargs
.
const juxt = (...fns) => (...args) => [...fns].map(fn => [...args].map(fn));
juxt(
x => x + 1,
x => x - 1,
x => x * 10
)(1, 2, 3); // [[2, 3, 4], [0, 1, 2], [10, 20, 30]]
juxt(
s => s.length,
s => s.split(' ').join('-')
)('30 seconds of code'); // [[18], ['30-seconds-of-code']]
kMeans
- title: kMeans
- tags: algorithm,array,advanced
Groups the given data into k
clusters, using the k-means clustering algorithm.
- Use
Array.from()
andArray.prototype.slice()
to initialize appropriate variables for the clustercentroids
,distances
andclasses
. - Use a
while
loop to repeat the assignment and update steps as long as there are changes in the previous iteration, as indicated byitr
. - Calculate the euclidean distance between each data point and centroid using
Math.hypot()
,Object.keys()
andArray.prototype.map()
. - Use
Array.prototype.indexOf()
andMath.min()
to find the closest centroid. - Use
Array.from()
andArray.prototype.reduce()
, as well asparseFloat()
andNumber.prototype.toFixed()
to calculate the new centroids.
const kMeans = (data, k = 1) => {
const centroids = data.slice(0, k);
const distances = Array.from({ length: data.length }, () =>
Array.from({ length: k }, () => 0)
);
const classes = Array.from({ length: data.length }, () => -1);
let itr = true;
while (itr) {
itr = false;
for (let d in data) {
for (let c = 0; c < k; c++) {
distances[d][c] = Math.hypot(
...Object.keys(data[0]).map(key => data[d][key] - centroids[c][key])
);
}
const m = distances[d].indexOf(Math.min(...distances[d]));
if (classes[d] !== m) itr = true;
classes[d] = m;
}
for (let c = 0; c < k; c++) {
centroids[c] = Array.from({ length: data[0].length }, () => 0);
const size = data.reduce((acc, _, d) => {
if (classes[d] === c) {
acc++;
for (let i in data[0]) centroids[c][i] += data[d][i];
}
return acc;
}, 0);
for (let i in data[0]) {
centroids[c][i] = parseFloat(Number(centroids[c][i] / size).toFixed(2));
}
}
}
return classes;
};
kMeans([[0, 0], [0, 1], [1, 3], [2, 0]], 2); // [0, 1, 1, 0]
kNearestNeighbors
- title: kNearestNeighbors
- tags: algorithm,array,advanced
Classifies a data point relative to a labelled data set, using the k-nearest neighbors algorithm.
- Use
Array.prototype.map()
to map thedata
to objects containing the euclidean distance of each element frompoint
, calculated usingMath.hypot()
,Object.keys()
and itslabel
. - Use
Array.prototype.sort()
andArray.prototype.slice()
to get thek
nearest neighbors ofpoint
. - Use
Array.prototype.reduce()
in combination withObject.keys()
andArray.prototype.indexOf()
to find the most frequentlabel
among them.
const kNearestNeighbors = (data, labels, point, k = 3) => {
const kNearest = data
.map((el, i) => ({
dist: Math.hypot(...Object.keys(el).map(key => point[key] - el[key])),
label: labels[i]
}))
.sort((a, b) => a.dist - b.dist)
.slice(0, k);
return kNearest.reduce(
(acc, { label }, i) => {
acc.classCounts[label] =
Object.keys(acc.classCounts).indexOf(label) !== -1
? acc.classCounts[label] + 1
: 1;
if (acc.classCounts[label] > acc.topClassCount) {
acc.topClassCount = acc.classCounts[label];
acc.topClass = label;
}
return acc;
},
{
classCounts: {},
topClass: kNearest[0].label,
topClassCount: 0
}
).topClass;
};
const data = [[0, 0], [0, 1], [1, 3], [2, 0]];
const labels = [0, 1, 1, 0];
kNearestNeighbors(data, labels, [1, 2], 2); // 1
kNearestNeighbors(data, labels, [1, 0], 2); // 0
kmToMiles
- title: kmToMiles
- tags: math,beginner unlisted: true
Converts kilometers to miles.
- Follow the conversion formula
mi = km * 0.621371
.
const kmToMiles = km => km * 0.621371;
kmToMiles(8.1) // 5.0331051
last
- title: last
- tags: array,beginner
Returns the last element in an array.
- Check if
arr
is truthy and has alength
property. - Use
Array.prototype.length - 1
to compute the index of the last element of the given array and return it, otherwise returnundefined
.
const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
last([1, 2, 3]); // 3
last([]); // undefined
last(null); // undefined
last(undefined); // undefined
lastDateOfMonth
- title: lastDateOfMonth
- tags: date,intermediate
Returns the string representation of the last date in the given date's month.
- Use
Date.prototype.getFullYear()
,Date.prototype.getMonth()
to get the current year and month from the given date. - Use the
new Date()
constructor to create a new date with the given year and month incremented by1
, and the day set to0
(last day of previous month). - Omit the argument,
date
, to use the current date by default.
const lastDateOfMonth = (date = new Date()) => {
let d = new Date(date.getFullYear(), date.getMonth() + 1, 0);
return d.toISOString().split('T')[0];
};
lastDateOfMonth(new Date('2015-08-11')); // '2015-08-30'
lcm
- title: lcm
- tags: math,algorithm,recursion,intermediate
Calculates the least common multiple of two or more numbers.
- Use the greatest common divisor (GCD) formula and the fact that
lcm(x, y) = x * y / gcd(x, y)
to determine the least common multiple. - The GCD formula uses recursion.
const lcm = (...arr) => {
const gcd = (x, y) => (!y ? x : gcd(y, x % y));
const _lcm = (x, y) => (x * y) / gcd(x, y);
return [...arr].reduce((a, b) => _lcm(a, b));
};
lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60
levenshteinDistance
- title: levenshteinDistance
- tags: string,algorithm,intermediate
Calculates the difference between two strings, using the Levenshtein distance algorithm.
- If either of the two strings has a
length
of zero, return thelength
of the other one. - Use a
for
loop to iterate over the letters of the target string and a nestedfor
loop to iterate over the letters of the source string. - Calculate the cost of substituting the letters corresponding to
i - 1
andj - 1
in the target and source respectively (0
if they are the same,1
otherwise). - Use
Math.min()
to populate each element in the 2D array with the minimum of the cell above incremented by one, the cell to the left incremented by one or the cell to the top left incremented by the previously calculated cost. - Return the last element of the last row of the produced array.
const levenshteinDistance = (s, t) => {
if (!s.length) return t.length;
if (!t.length) return s.length;
const arr = [];
for (let i = 0; i <= t.length; i++) {
arr[i] = [i];
for (let j = 1; j <= s.length; j++) {
arr[i][j] =
i === 0
? j
: Math.min(
arr[i - 1][j] + 1,
arr[i][j - 1] + 1,
arr[i - 1][j - 1] + (s[j - 1] === t[i - 1] ? 0 : 1)
);
}
}
return arr[t.length][s.length];
};
levenshteinDistance('duck', 'dark'); // 2
linearSearch
- title: linearSearch
- tags: algorithm,array,beginner
Finds the first index of a given element in an array using the linear search algorithm.
- Use a
for...in
loop to iterate over the indexes of the given array. - Check if the element in the corresponding index is equal to
item
. - If the element is found, return the index, using the unary
+
operator to convert it from a string to a number. - If the element is not found after iterating over the whole array, return
-1
.
const linearSearch = (arr, item) => {
for (const i in arr) {
if (arr[i] === item) return +i;
}
return -1;
};
linearSearch([2, 9, 9], 9); // 1
linearSearch([2, 9, 9], 7); // -1
listenOnce
- title: listenOnce
- tags: browser,event,beginner
Adds an event listener to an element that will only run the callback the first time the event is triggered.
- Use
EventTarget.addEventListener()
to add an event listener to an element. - Use
{ once: true }
as options to only run the given callback once.
const listenOnce = (el, evt, fn) =>
el.addEventListener(evt, fn, { once: true });
listenOnce(
document.getElementById('my-id'),
'click',
() => console.log('Hello world')
); // 'Hello world' will only be logged on the first click
logBase
- title: logBase
- tags: math,beginner
Calculates the logarithm of the given number in the given base.
- Use
Math.log()
to get the logarithm from the value and the base and divide them.
const logBase = (n, base) => Math.log(n) / Math.log(base);
logBase(10, 10); // 1
logBase(100, 10); // 2
longestItem
- title: longestItem
- tags: array,intermediate
Takes any number of iterable objects or objects with a length
property and returns the longest one.
- Use
Array.prototype.reduce()
, comparing the length of objects to find the longest one. - If multiple objects have the same length, the first one will be returned.
- Returns
undefined
if no arguments are provided.
const longestItem = (...vals) =>
vals.reduce((a, x) => (x.length > a.length ? x : a));
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'
lowercaseKeys
- title: lowercaseKeys
- tags: object,intermediate
Creates a new object from the specified object, where all the keys are in lowercase.
- Use
Object.keys()
andArray.prototype.reduce()
to create a new object from the specified object. - Convert each key in the original object to lowercase, using
String.prototype.toLowerCase()
.
const lowercaseKeys = obj =>
Object.keys(obj).reduce((acc, key) => {
acc[key.toLowerCase()] = obj[key];
return acc;
}, {});
const myObj = { Name: 'Adam', sUrnAME: 'Smith' };
const myObjLower = lowercaseKeys(myObj); // {name: 'Adam', surname: 'Smith'};
luhnCheck
- title: luhnCheck
- tags: math,algorithm,advanced
Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
- Use
String.prototype.split('')
,Array.prototype.reverse()
andArray.prototype.map()
in combination withparseInt()
to obtain an array of digits. - Use
Array.prototype.splice(0, 1)
to obtain the last digit. - Use
Array.prototype.reduce()
to implement the Luhn Algorithm. - Return
true
ifsum
is divisible by10
,false
otherwise.
const luhnCheck = num => {
let arr = (num + '')
.split('')
.reverse()
.map(x => parseInt(x));
let lastDigit = arr.splice(0, 1)[0];
let sum = arr.reduce(
(acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9),
0
);
sum += lastDigit;
return sum % 10 === 0;
};
luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); // false
luhnCheck(123456789); // false
mapKeys
- title: mapKeys
- tags: object,intermediate
Maps the keys of an object using the provided function, generating a new object.
- Use
Object.keys()
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same values and mapped keys usingfn
.
const mapKeys = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[fn(obj[k], k, obj)] = obj[k];
return acc;
}, {});
mapKeys({ a: 1, b: 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }
mapNumRange
- title: mapNumRange
- tags: math,beginner
Maps a number from one range to another range.
- Return
num
mapped betweenoutMin
-outMax
frominMin
-inMax
.
const mapNumRange = (num, inMin, inMax, outMin, outMax) =>
((num - inMin) * (outMax - outMin)) / (inMax - inMin) + outMin;
mapNumRange(5, 0, 10, 0, 100); // 50
mapObject
- title: mapObject
- tags: array,object,intermediate
Maps the values of an array to an object using a function.
- Use
Array.prototype.reduce()
to applyfn
to each element inarr
and combine the results into an object. - Use
el
as the key for each property and the result offn
as the value.
const mapObject = (arr, fn) =>
arr.reduce((acc, el, i) => {
acc[el] = fn(el, i, arr);
return acc;
}, {});
mapObject([1, 2, 3], a => a * a); // { 1: 1, 2: 4, 3: 9 }
mapString
- title: mapString
- tags: string,intermediate
Creates a new string with the results of calling a provided function on every character in the given string.
- Use
String.prototype.split('')
andArray.prototype.map()
to call the provided function,fn
, for each character instr
. - Use
Array.prototype.join('')
to recombine the array of characters into a string. - The callback function,
fn
, takes three arguments (the current character, the index of the current character and the stringmapString
was called upon).
const mapString = (str, fn) =>
str
.split('')
.map((c, i) => fn(c, i, str))
.join('');
mapString('lorem ipsum', c => c.toUpperCase()); // 'LOREM IPSUM'
mapValues
- title: mapValues
- tags: object,intermediate
Maps the values of an object using the provided function, generating a new object with the same keys.
- Use
Object.keys()
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same keys and mapped values usingfn
.
const mapValues = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[k] = fn(obj[k], k, obj);
return acc;
}, {});
const users = {
fred: { user: 'fred', age: 40 },
pebbles: { user: 'pebbles', age: 1 }
};
mapValues(users, u => u.age); // { fred: 40, pebbles: 1 }
mask
- title: mask
- tags: string,intermediate
Replaces all but the last num
of characters with the specified mask character.
- Use
String.prototype.slice()
to grab the portion of the characters that will remain unmasked. - Use
String.padStart()
to fill the beginning of the string with themask
character up to the original length. - If
num
is negative, the unmasked characters will be at the start of the string. - Omit the second argument,
num
, to keep a default of4
characters unmasked. - Omit the third argument,
mask
, to use a default character of'*'
for the mask.
const mask = (cc, num = 4, mask = '*') =>
`${cc}`.slice(-num).padStart(`${cc}`.length, mask);
mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'
matches
- title: matches
- tags: object,intermediate
Compares two objects to determine if the first one contains equivalent property values to the second one.
- Use
Object.keys()
to get all the keys of the second object. - Use
Array.prototype.every()
,Object.prototype.hasOwnProperty()
and strict comparison to determine if all keys exist in the first object and have the same values.
const matches = (obj, source) =>
Object.keys(source).every(
key => obj.hasOwnProperty(key) && obj[key] === source[key]
);
matches({ age: 25, hair: 'long', beard: true }, { hair: 'long', beard: true });
// true
matches({ hair: 'long', beard: true }, { age: 25, hair: 'long', beard: true });
// false
matchesWith
- title: matchesWith
- tags: object,intermediate
Compares two objects to determine if the first one contains equivalent property values to the second one, based on a provided function.
- Use
Object.keys()
to get all the keys of the second object. - Use
Array.prototype.every()
,Object.prototype.hasOwnProperty()
and the provided function to determine if all keys exist in the first object and have equivalent values. - If no function is provided, the values will be compared using the equality operator.
const matchesWith = (obj, source, fn) =>
Object.keys(source).every(key =>
obj.hasOwnProperty(key) && fn
? fn(obj[key], source[key], key, obj, source)
: obj[key] == source[key]
);
const isGreeting = val => /^h(?:i|ello)$/.test(val);
matchesWith(
{ greeting: 'hello' },
{ greeting: 'hi' },
(oV, sV) => isGreeting(oV) && isGreeting(sV)
); // true
maxBy
- title: maxBy
- tags: math,array,beginner
Returns the maximum value of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Math.max()
to get the maximum value.
const maxBy = (arr, fn) =>
Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8
maxDate
- title: maxDate
- tags: date,intermediate
Returns the maximum of the given dates.
- Use the ES6 spread syntax with
Math.max()
to find the maximum date value. - Use
new Date()
to convert it to aDate
object.
const maxDate = (...dates) => new Date(Math.max(...dates));
const dates = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
maxDate(...dates); // 2018-03-11T22:00:00.000Z
maxN
- title: maxN
- tags: array,math,intermediate
Returns the n
maximum elements from the provided array.
- Use
Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in descending order. - Use
Array.prototype.slice()
to get the specified number of elements. - Omit the second argument,
n
, to get a one-element array. - If
n
is greater than or equal to the provided array's length, then return the original array (sorted in descending order).
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3, 2]
median
- title: median
- tags: math,array,intermediate
Calculates the median of an array of numbers.
- Find the middle of the array, use
Array.prototype.sort()
to sort the values. - Return the number at the midpoint if
Array.prototype.length
is odd, otherwise the average of the two middle numbers.
const median = arr => {
const mid = Math.floor(arr.length / 2),
nums = [...arr].sort((a, b) => a - b);
return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};
median([5, 6, 50, 1, -5]); // 5
memoize
- title: memoize
- tags: function,advanced
Returns the memoized (cached) function.
- Create an empty cache by instantiating a new
Map
object. - Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not.
- The
function
keyword must be used in order to allow the memoized function to have itsthis
context changed if necessary. - Allow access to the
cache
by setting it as a property on the returned function.
const memoize = fn => {
const cache = new Map();
const cached = function (val) {
return cache.has(val)
? cache.get(val)
: cache.set(val, fn.call(this, val)) && cache.get(val);
};
cached.cache = cache;
return cached;
};
// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's cached
console.log(anagramsCached.cache); // The cached anagrams map
merge
- title: merge
- tags: object,array,intermediate
Creates a new object from the combination of two or more objects.
- Use
Array.prototype.reduce()
combined withObject.keys()
to iterate over all objects and keys. - Use
Object.prototype.hasOwnProperty()
andArray.prototype.concat()
to append values for keys existing in multiple objects.
const merge = (...objs) =>
[...objs].reduce(
(acc, obj) =>
Object.keys(obj).reduce((a, k) => {
acc[k] = acc.hasOwnProperty(k)
? [].concat(acc[k]).concat(obj[k])
: obj[k];
return acc;
}, {}),
{}
);
const object = {
a: [{ x: 2 }, { y: 4 }],
b: 1
};
const other = {
a: { z: 3 },
b: [2, 3],
c: 'foo'
};
merge(object, other);
// { a: [ { x: 2 }, { y: 4 }, { z: 3 } ], b: [ 1, 2, 3 ], c: 'foo' }
mergeSort
- title: mergeSort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the merge sort algorithm.
- Use recursion.
- If the
length
of the array is less than2
, return the array. - Use
Math.floor()
to calculate the middle point of the array. - Use
Array.prototype.slice()
to slice the array in two and recursively callmergeSort()
on the created subarrays. - Finally, use
Array.from()
andArray.prototype.shift()
to combine the two sorted subarrays into one.
const mergeSort = arr => {
if (arr.length < 2) return arr;
const mid = Math.floor(arr.length / 2);
const l = mergeSort(arr.slice(0, mid));
const r = mergeSort(arr.slice(mid, arr.length));
return Array.from({ length: l.length + r.length }, () => {
if (!l.length) return r.shift();
else if (!r.length) return l.shift();
else return l[0] > r[0] ? r.shift() : l.shift();
});
};
mergeSort([5, 1, 4, 2, 3]); // [1, 2, 3, 4, 5]
mergeSortedArrays
- title: mergeSortedArrays
- tags: array,intermediate
Merges two sorted arrays into one.
- Use the spread operator (
...
) to clone both of the given arrays. - Use
Array.from()
to create an array of the appropriate length based on the given arrays. - Use
Array.prototype.shift()
to populate the newly created array from the removed elements of the cloned arrays.
const mergeSortedArrays = (a, b) => {
const _a = [...a],
_b = [...b];
return Array.from({ length: _a.length + _b.length }, () => {
if (!_a.length) return _b.shift();
else if (!_b.length) return _a.shift();
else return _a[0] > _b[0] ? _b.shift() : _a.shift();
});
};
mergeSortedArrays([1, 4, 5], [2, 3, 6]); // [1, 2, 3, 4, 5, 6]
merged
CSVToArray
- title: CSVToArray
- tags: string,array,intermediate
Converts a comma-separated values (CSV) string to a 2D array.
- Use
Array.prototype.slice()
andArray.prototype.indexOf('\n')
to remove the first row (title row) ifomitFirstRow
istrue
. - Use
String.prototype.split('\n')
to create a string for each row, thenString.prototype.split(delimiter)
to separate the values in each row. - Omit the second argument,
delimiter
, to use a default delimiter of,
. - Omit the third argument,
omitFirstRow
, to include the first row (title row) of the CSV string.
const CSVToArray = (data, delimiter = ',', omitFirstRow = false) =>
data
.slice(omitFirstRow ? data.indexOf('\n') + 1 : 0)
.split('\n')
.map(v => v.split(delimiter));
CSVToArray('a,b\nc,d'); // [['a', 'b'], ['c', 'd']];
CSVToArray('a;b\nc;d', ';'); // [['a', 'b'], ['c', 'd']];
CSVToArray('col1,col2\na,b\nc,d', ',', true); // [['a', 'b'], ['c', 'd']];
CSVToJSON
- title: CSVToJSON
- tags: string,object,advanced
Converts a comma-separated values (CSV) string to a 2D array of objects. The first row of the string is used as the title row.
- Use
Array.prototype.slice()
andArray.prototype.indexOf('\n')
andString.prototype.split(delimiter)
to separate the first row (title row) into values. - Use
String.prototype.split('\n')
to create a string for each row, thenArray.prototype.map()
andString.prototype.split(delimiter)
to separate the values in each row. - Use
Array.prototype.reduce()
to create an object for each row's values, with the keys parsed from the title row. - Omit the second argument,
delimiter
, to use a default delimiter of,
.
const CSVToJSON = (data, delimiter = ',') => {
const titles = data.slice(0, data.indexOf('\n')).split(delimiter);
return data
.slice(data.indexOf('\n') + 1)
.split('\n')
.map(v => {
const values = v.split(delimiter);
return titles.reduce(
(obj, title, index) => ((obj[title] = values[index]), obj),
{}
);
});
};
CSVToJSON('col1,col2\na,b\nc,d');
// [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
CSVToJSON('col1;col2\na;b\nc;d', ';');
// [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
HSBToRGB
- title: HSBToRGB
- tags: math,intermediate
Converts a HSB color tuple to RGB format.
- Use the HSB to RGB conversion formula to convert to the appropriate format.
- The range of the input parameters is H: [0, 360], S: [0, 100], B: [0, 100].
- The range of all output values is [0, 255].
const HSBToRGB = (h, s, b) => {
s /= 100;
b /= 100;
const k = (n) => (n + h / 60) % 6;
const f = (n) => b * (1 - s * Math.max(0, Math.min(k(n), 4 - k(n), 1)));
return [255 * f(5), 255 * f(3), 255 * f(1)];
};
HSBToRGB(18, 81, 99); // [252.45, 109.31084999999996, 47.965499999999984]
HSLToRGB
- title: HSLToRGB
- tags: math,intermediate
Converts a HSL color tuple to RGB format.
- Use the HSL to RGB conversion formula to convert to the appropriate format.
- The range of the input parameters is H: [0, 360], S: [0, 100], L: [0, 100].
- The range of all output values is [0, 255].
const HSLToRGB = (h, s, l) => {
s /= 100;
l /= 100;
const k = n => (n + h / 30) % 12;
const a = s * Math.min(l, 1 - l);
const f = n =>
l - a * Math.max(-1, Math.min(k(n) - 3, Math.min(9 - k(n), 1)));
return [255 * f(0), 255 * f(8), 255 * f(4)];
};
HSLToRGB(13, 100, 11); // [56.1, 12.155, 0]
JSONToFile
- title: JSONToFile
- tags: node,intermediate
Writes a JSON object to a file.
- Use
fs.writeFileSync()
, template literals andJSON.stringify()
to write ajson
object to a.json
file.
const fs = require('fs');
const JSONToFile = (obj, filename) =>
fs.writeFileSync(`${filename}.json`, JSON.stringify(obj, null, 2));
JSONToFile({ test: 'is passed' }, 'testJsonFile');
// writes the object to 'testJsonFile.json'
JSONtoCSV
- title: JSONtoCSV
- tags: array,string,object,advanced
Converts an array of objects to a comma-separated values (CSV) string that contains only the columns
specified.
- Use
Array.prototype.join(delimiter)
to combine all the names incolumns
to create the first row. - Use
Array.prototype.map()
andArray.prototype.reduce()
to create a row for each object, substituting non-existent values with empty strings and only mapping values incolumns
. - Use
Array.prototype.join('\n')
to combine all rows into a string. - Omit the third argument,
delimiter
, to use a default delimiter of,
.
const JSONtoCSV = (arr, columns, delimiter = ',') =>
[
columns.join(delimiter),
...arr.map(obj =>
columns.reduce(
(acc, key) =>
`${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
''
)
),
].join('\n');
JSONtoCSV(
[{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
['a', 'b']
); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV(
[{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
['a', 'b'],
';'
); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'
RGBToHSB
- title: RGBToHSB
- tags: math,intermediate
Converts a RGB color tuple to HSB format.
- Use the RGB to HSB conversion formula to convert to the appropriate format.
- The range of all input parameters is [0, 255].
- The range of the resulting values is H: [0, 360], S: [0, 100], B: [0, 100].
const RGBToHSB = (r, g, b) => {
r /= 255;
g /= 255;
b /= 255;
const v = Math.max(r, g, b),
n = v - Math.min(r, g, b);
const h =
n && v === r ? (g - b) / n : v === g ? 2 + (b - r) / n : 4 + (r - g) / n;
return [60 * (h < 0 ? h + 6 : h), v && (n / v) * 100, v * 100];
};
RGBToHSB(252, 111, 48);
// [18.529411764705856, 80.95238095238095, 98.82352941176471]
RGBToHSL
- title: RGBToHSL
- tags: math,intermediate
Converts a RGB color tuple to HSL format.
- Use the RGB to HSL conversion formula to convert to the appropriate format.
- The range of all input parameters is [0, 255].
- The range of the resulting values is H: [0, 360], S: [0, 100], L: [0, 100].
const RGBToHSL = (r, g, b) => {
r /= 255;
g /= 255;
b /= 255;
const l = Math.max(r, g, b);
const s = l - Math.min(r, g, b);
const h = s
? l === r
? (g - b) / s
: l === g
? 2 + (b - r) / s
: 4 + (r - g) / s
: 0;
return [
60 * h < 0 ? 60 * h + 360 : 60 * h,
100 * (s ? (l <= 0.5 ? s / (2 * l - s) : s / (2 - (2 * l - s))) : 0),
(100 * (2 * l - s)) / 2,
];
};
RGBToHSL(45, 23, 11); // [21.17647, 60.71428, 10.98039]
RGBToHex
- title: RGBToHex
- tags: string,math,intermediate
Converts the values of RGB components to a hexadecimal color code.
- Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (
<<
) andNumber.prototype.toString(16)
. - Use
String.prototype.padStart(6, '0')
to get a 6-digit hexadecimal value.
const RGBToHex = (r, g, b) =>
((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');
RGBToHex(255, 165, 1); // 'ffa501'
URLJoin
- title: URLJoin
- tags: string,regexp,advanced
Joins all given URL segments together, then normalizes the resulting URL.
- Use
String.prototype.join('/')
to combine URL segments. - Use a series of
String.prototype.replace()
calls with various regexps to normalize the resulting URL (remove double slashes, add proper slashes for protocol, remove slashes before parameters, combine parameters with'&'
and normalize first parameter delimiter).
const URLJoin = (...args) =>
args
.join('/')
.replace(/[\/]+/g, '/')
.replace(/^(.+):\//, '$1://')
.replace(/^file:/, 'file:/')
.replace(/\/(\?|&|##[^!])/g, '$1')
.replace(/\?/g, '&')
.replace('&', '?');
URLJoin('http://www.google.com', 'a', '/b/cd', '?foo=123', '?bar=foo');
// 'http://www.google.com/a/b/cd?foo=123&bar=foo'
UUIDGeneratorBrowser
- title: UUIDGeneratorBrowser
- tags: browser,random,intermediate
Generates a UUID in a browser.
- Use
Crypto.getRandomValues()
to generate a UUID, compliant with RFC4122 version 4. - Use
Number.prototype.toString(16)
to convert it to a proper UUID.
const UUIDGeneratorBrowser = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(
c ^
(crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))
).toString(16)
);
UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'
UUIDGeneratorNode
- title: UUIDGeneratorNode
- tags: node,random,intermediate
Generates a UUID in Node.JS.
- Use
crypto.randomBytes()
to generate a UUID, compliant with RFC4122 version 4. - Use
Number.prototype.toString(16)
to convert it to a proper UUID.
const crypto = require('crypto');
const UUIDGeneratorNode = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
);
UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'
accumulate
- title: accumulate
- tags: math,array,intermediate
Creates an array of partial sums.
- Use
Array.prototype.reduce()
, initialized with an empty array accumulator to iterate overnums
. - Use
Array.prototype.slice(-1)
, the spread operator (...
) and the unary+
operator to add each value to the accumulator array containing the previous sums.
const accumulate = (...nums) =>
nums.reduce((acc, n) => [...acc, n + +acc.slice(-1)], []);
accumulate(1, 2, 3, 4); // [1, 3, 6, 10]
accumulate(...[1, 2, 3, 4]); // [1, 3, 6, 10]
addClass
- title: addClass
- tags: browser,beginner
Adds a class to an HTML element.
- Use
Element.classList
andDOMTokenList.add()
to add the specified class to the element.
const addClass = (el, className) => el.classList.add(className);
addClass(document.querySelector('p'), 'special');
// The paragraph will now have the 'special' class
addDaysToDate
- title: addDaysToDate
- tags: date,intermediate
Calculates the date of n
days from the given date, returning its string representation.
- Use
new Date()
to create a date object from the first argument. - Use
Date.prototype.getDate()
andDate.prototype.setDate()
to addn
days to the given date. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const addDaysToDate = (date, n) => {
const d = new Date(date);
d.setDate(d.getDate() + n);
return d.toISOString().split('T')[0];
};
addDaysToDate('2020-10-15', 10); // '2020-10-25'
addDaysToDate('2020-10-15', -10); // '2020-10-05'
addMinutesToDate
- title: addMinutesToDate
- tags: date,intermediate
Calculates the date of n
minutes from the given date, returning its string representation.
- Use
new Date()
to create a date object from the first argument. - Use
Date.prototype.getTime()
andDate.prototype.setTime()
to addn
minutes to the given date. - Use
Date.prototype.toISOString()
,String.prototype.split()
andString.prototype.replace()
to return a string inyyyy-mm-dd HH:MM:SS
format.
const addMinutesToDate = (date, n) => {
const d = new Date(date);
d.setTime(d.getTime() + n * 60000);
return d.toISOString().split('.')[0].replace('T',' ');
};
addMinutesToDate('2020-10-19 12:00:00', 10); // '2020-10-19 12:10:00'
addMinutesToDate('2020-10-19', -10); // '2020-10-18 23:50:00'
addMultipleEvents
- title: addMultipleListeners
- tags: browser,event,intermediate
Adds multiple event listeners with the same handler to an element.
- Use
Array.prototype.forEach()
andEventTarget.addEventListener()
to add multiple event listeners with an assigned callback function to an element.
const addMultipleListeners = (el, types, listener, options, useCapture) => {
types.forEach(type =>
el.addEventListener(type, listener, options, useCapture)
);
};
addMultipleListeners(
document.querySelector('.my-element'),
['click', 'mousedown'],
() => { console.log('hello!') }
);
addStyles
- title: addStyles
- tags: browser,beginner
Adds the provided styles to the given element.
- Use
Object.assign()
andElementCSSInlineStyle.style
to merge the providedstyles
object into the style of the given element.
const addStyles = (el, styles) => Object.assign(el.style, styles);
addStyles(document.getElementById('my-element'), {
background: 'red',
color: '##ffff00',
fontSize: '3rem'
});
addWeekDays
- title: addWeekDays
- tags: date,intermediate
Calculates the date after adding the given number of business days.
- Use
Array.from()
to construct an array withlength
equal to thecount
of business days to be added. - Use
Array.prototype.reduce()
to iterate over the array, starting fromstartDate
and incrementing, usingDate.prototype.getDate()
andDate.prototype.setDate()
. - If the current
date
is on a weekend, update it again by adding either one day or two days to make it a weekday. - NOTE: Does not take official holidays into account.
const addWeekDays = (startDate, count) =>
Array.from({ length: count }).reduce(date => {
date = new Date(date.setDate(date.getDate() + 1));
if (date.getDay() % 6 === 0)
date = new Date(date.setDate(date.getDate() + (date.getDay() / 6 + 1)));
return date;
}, startDate);
addWeekDays(new Date('Oct 09, 2020'), 5); // 'Oct 16, 2020'
addWeekDays(new Date('Oct 12, 2020'), 5); // 'Oct 19, 2020'
all
- title: all
- tags: array,beginner
Checks if the provided predicate function returns true
for all elements in a collection.
- Use
Array.prototype.every()
to test if all elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const all = (arr, fn = Boolean) => arr.every(fn);
all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // true
allEqual
- title: allEqual
- tags: array,beginner
Checks if all elements in an array are equal.
- Use
Array.prototype.every()
to check if all the elements of the array are the same as the first one. - Elements in the array are compared using the strict comparison operator, which does not account for
NaN
self-inequality.
const allEqual = arr => arr.every(val => val === arr[0]);
allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // true
allEqualBy
- title: allEqualBy
- tags: array,intermediate
Checks if all elements in an array are equal, based on the provided mapping function.
- Apply
fn
to the first element ofarr
. - Use
Array.prototype.every()
to check iffn
returns the same value for all elements in the array as it did for the first one. - Elements in the array are compared using the strict comparison operator, which does not account for
NaN
self-inequality.
const allEqualBy = (arr, fn) => {
const eql = fn(arr[0]);
return arr.every(val => fn(val) === eql);
};
allEqualBy([1.1, 1.2, 1.3], Math.round); // true
allEqualBy([1.1, 1.3, 1.6], Math.round); // false
allUnique
- title: allUnique
- tags: array,beginner
Checks if all elements in an array are unique.
- Create a new
Set
from the mapped values to keep only unique occurrences. - Use
Array.prototype.length
andSet.prototype.size
to compare the length of the unique values to the original array.
const allUnique = arr => arr.length === new Set(arr).size;
allUnique([1, 2, 3, 4]); // true
allUnique([1, 1, 2, 3]); // false
allUniqueBy
- title: allUniqueBy
- tags: array,intermediate
Checks if all elements in an array are unique, based on the provided mapping function.
- Use
Array.prototype.map()
to applyfn
to all elements inarr
. - Create a new
Set
from the mapped values to keep only unique occurrences. - Use
Array.prototype.length
andSet.prototype.size
to compare the length of the unique mapped values to the original array.
const allUniqueBy = (arr, fn) => arr.length === new Set(arr.map(fn)).size;
allUniqueBy([1.2, 2.4, 2.9], Math.round); // true
allUniqueBy([1.2, 2.3, 2.4], Math.round); // false
and
- title: and
- tags: math,logic,beginner unlisted: true
Checks if both arguments are true
.
- Use the logical and (
&&
) operator on the two given values.
const and = (a, b) => a && b;
and(true, true); // true
and(true, false); // false
and(false, false); // false
any
- title: any
- tags: array,beginner
Checks if the provided predicate function returns true
for at least one element in a collection.
- Use
Array.prototype.some()
to test if any elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const any = (arr, fn = Boolean) => arr.some(fn);
any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // true
aperture
- title: aperture
- tags: array,intermediate
Creates an array of n
-tuples of consecutive elements.
- Use
Array.prototype.slice()
andArray.prototype.map()
to create an array of appropriate length. - Populate the array with
n
-tuples of consecutive elements fromarr
. - If
n
is greater than the length ofarr
, return an empty array.
const aperture = (n, arr) =>
n > arr.length
? []
: arr.slice(n - 1).map((v, i) => arr.slice(i, i + n));
aperture(2, [1, 2, 3, 4]); // [[1, 2], [2, 3], [3, 4]]
aperture(3, [1, 2, 3, 4]); // [[1, 2, 3], [2, 3, 4]]
aperture(5, [1, 2, 3, 4]); // []
approximatelyEqual
- title: approximatelyEqual
- tags: math,beginner
Checks if two numbers are approximately equal to each other.
- Use
Math.abs()
to compare the absolute difference of the two values toepsilon
. - Omit the third argument,
epsilon
, to use a default value of0.001
.
const approximatelyEqual = (v1, v2, epsilon = 0.001) =>
Math.abs(v1 - v2) < epsilon;
approximatelyEqual(Math.PI / 2.0, 1.5708); // true
arithmeticProgression
- title: arithmeticProgression
- tags: math,algorithm,beginner
Creates an array of numbers in the arithmetic progression, starting with the given positive integer and up to the specified limit.
- Use
Array.from()
to create an array of the desired length,lim/n
, and a map function to fill it with the desired values in the given range.
const arithmeticProgression = (n, lim) =>
Array.from({ length: Math.ceil(lim / n) }, (_, i) => (i + 1) * n );
arithmeticProgression(5, 25); // [5, 10, 15, 20, 25]
arrayToCSV
- title: arrayToCSV
- tags: array,string,intermediate
Converts a 2D array to a comma-separated values (CSV) string.
- Use
Array.prototype.map()
andArray.prototype.join(delimiter)
to combine individual 1D arrays (rows) into strings. - Use
Array.prototype.join('\n')
to combine all rows into a CSV string, separating each row with a newline. - Omit the second argument,
delimiter
, to use a default delimiter of,
.
const arrayToCSV = (arr, delimiter = ',') =>
arr
.map(v =>
v.map(x => (isNaN(x) ? `"${x.replace(/"/g, '""')}"` : x)).join(delimiter)
)
.join('\n');
arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'
arrayToCSV([['a', '"b" great'], ['c', 3.1415]]);
// '"a","""b"" great"\n"c",3.1415'
arrayToHTMLList
- title: arrayToHTMLList
- tags: browser,array,intermediate
Converts the given array elements into <li>
tags and appends them to the list of the given id.
- Use
Array.prototype.map()
andDocument.querySelector()
to create a list of html tags.
const arrayToHTMLList = (arr, listID) =>
document.querySelector(`##${listID}`).innerHTML += arr
.map(item => `<li>${item}</li>`)
.join('');
arrayToHTMLList(['item 1', 'item 2'], 'myListID');
ary
- title: ary
- tags: function,advanced
Creates a function that accepts up to n
arguments, ignoring any additional arguments.
- Call the provided function,
fn
, with up ton
arguments, usingArray.prototype.slice(0, n)
and the spread operator (...
).
const ary = (fn, n) => (...args) => fn(...args.slice(0, n));
const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [6, 4, 8], [10]].map(x => firstTwoMax(...x)); // [6, 6, 10]
atob
- title: atob
- tags: node,string,beginner
Decodes a string of data which has been encoded using base-64 encoding.
- Create a
Buffer
for the given string with base-64 encoding and useBuffer.toString('binary')
to return the decoded string.
const atob = str => Buffer.from(str, 'base64').toString('binary');
atob('Zm9vYmFy'); // 'foobar'
attempt
- title: attempt
- tags: function,intermediate
Attempts to invoke a function with the provided arguments, returning either the result or the caught error object.
- Use a
try... catch
block to return either the result of the function or an appropriate error. - If the caught object is not an
Error
, use it to create a newError
.
const attempt = (fn, ...args) => {
try {
return fn(...args);
} catch (e) {
return e instanceof Error ? e : new Error(e);
}
};
var elements = attempt(function(selector) {
return document.querySelectorAll(selector);
}, '>_>');
if (elements instanceof Error) elements = []; // elements = []
average
- title: average
- tags: math,array,beginner
Calculates the average of two or more numbers.
- Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
. - Divide the resulting array by its length.
const average = (...nums) =>
nums.reduce((acc, val) => acc + val, 0) / nums.length;
average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2
averageBy
- title: averageBy
- tags: math,array,intermediate
Calculates the average of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
. - Divide the resulting array by its length.
const averageBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val) => acc + val, 0) / arr.length;
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5
bifurcate
- title: bifurcate
- tags: array,intermediate
Splits values into two groups, based on the result of the given filter
array.
- Use
Array.prototype.reduce()
andArray.prototype.push()
to add elements to groups, based onfilter
. - If
filter
has a truthy value for any element, add it to the first group, otherwise add it to the second group.
const bifurcate = (arr, filter) =>
arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [
[],
[],
]);
bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]);
// [ ['beep', 'boop', 'bar'], ['foo'] ]
bifurcateBy
- title: bifurcateBy
- tags: array,intermediate
Splits values into two groups, based on the result of the given filtering function.
- Use
Array.prototype.reduce()
andArray.prototype.push()
to add elements to groups, based on the value returned byfn
for each element. - If
fn
returns a truthy value for any element, add it to the first group, otherwise add it to the second group.
const bifurcateBy = (arr, fn) =>
arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [
[],
[],
]);
bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b');
// [ ['beep', 'boop', 'bar'], ['foo'] ]
binary
- title: binary
- tags: function,intermediate
Creates a function that accepts up to two arguments, ignoring any additional arguments.
- Call the provided function,
fn
, with the first two arguments given.
const binary = fn => (a, b) => fn(a, b);
['2', '1', '0'].map(binary(Math.max)); // [2, 1, 2]
binarySearch
- title: binarySearch
- tags: algorithm,array,beginner
Finds the index of a given element in a sorted array using the binary search algorithm.
- Declare the left and right search boundaries,
l
andr
, initialized to0
and thelength
of the array respectively. - Use a
while
loop to repeatedly narrow down the search subarray, usingMath.floor()
to cut it in half. - Return the index of the element if found, otherwise return
-1
. - Note: Does not account for duplicate values in the array.
const binarySearch = (arr, item) => {
let l = 0,
r = arr.length - 1;
while (l <= r) {
const mid = Math.floor((l + r) / 2);
const guess = arr[mid];
if (guess === item) return mid;
if (guess > item) r = mid - 1;
else l = mid + 1;
}
return -1;
};
binarySearch([1, 2, 3, 4, 5], 1); // 0
binarySearch([1, 2, 3, 4, 5], 5); // 4
binarySearch([1, 2, 3, 4, 5], 6); // -1
bind
- title: bind
- tags: function,object,advanced
Creates a function that invokes fn
with a given context, optionally prepending any additional supplied parameters to the arguments.
- Return a
function
that usesFunction.prototype.apply()
to apply the givencontext
tofn
. - Use the spread operator (
...
) to prepend any additional supplied parameters to the arguments.
const bind = (fn, context, ...boundArgs) => (...args) =>
fn.apply(context, [...boundArgs, ...args]);
function greet(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
const freddy = { user: 'fred' };
const freddyBound = bind(greet, freddy);
console.log(freddyBound('hi', '!')); // 'hi fred!'
bindAll
- title: bindAll
- tags: object,function,intermediate
Binds methods of an object to the object itself, overwriting the existing method.
- Use
Array.prototype.forEach()
to iterate over the givenfns
. - Return a function for each one, using
Function.prototype.apply()
to apply the given context (obj
) tofn
.
const bindAll = (obj, ...fns) =>
fns.forEach(
fn => (
(f = obj[fn]),
(obj[fn] = function() {
return f.apply(obj);
})
)
);
var view = {
label: 'docs',
click: function() {
console.log('clicked ' + this.label);
}
};
bindAll(view, 'click');
document.body.addEventListener('click', view.click);
// Log 'clicked docs' when clicked.
bindKey
- title: bindKey
- tags: function,object,advanced
Creates a function that invokes the method at a given key of an object, optionally prepending any additional supplied parameters to the arguments.
- Return a
function
that usesFunction.prototype.apply()
to bindcontext[fn]
tocontext
. - Use the spread operator (
...
) to prepend any additional supplied parameters to the arguments.
const bindKey = (context, fn, ...boundArgs) => (...args) =>
context[fn].apply(context, [...boundArgs, ...args]);
const freddy = {
user: 'fred',
greet: function(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
};
const freddyBound = bindKey(freddy, 'greet');
console.log(freddyBound('hi', '!')); // 'hi fred!'
binomialCoefficient
- title: binomialCoefficient
- tags: math,algorithm,beginner
Calculates the number of ways to choose k
items from n
items without repetition and without order.
- Use
Number.isNaN()
to check if any of the two values isNaN
. - Check if
k
is less than0
, greater than or equal ton
, equal to1
orn - 1
and return the appropriate result. - Check if
n - k
is less thank
and switch their values accordingly. - Loop from
2
throughk
and calculate the binomial coefficient. - Use
Math.round()
to account for rounding errors in the calculation.
const binomialCoefficient = (n, k) => {
if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
if (k < 0 || k > n) return 0;
if (k === 0 || k === n) return 1;
if (k === 1 || k === n - 1) return n;
if (n - k < k) k = n - k;
let res = n;
for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
return Math.round(res);
};
binomialCoefficient(8, 2); // 28
both
- title: both
- tags: function,logic,beginner unlisted: true
Checks if both of the given functions return true
for a given set of arguments.
- Use the logical and (
&&
) operator on the result of calling the two functions with the suppliedargs
.
const both = (f, g) => (...args) => f(...args) && g(...args);
const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveEven = both(isEven, isPositive);
isPositiveEven(4); // true
isPositiveEven(-2); // false
bottomVisible
- title: bottomVisible
- tags: browser,beginner
Checks if the bottom of the page is visible.
- Use
scrollY
,scrollHeight
andclientHeight
to determine if the bottom of the page is visible.
const bottomVisible = () =>
document.documentElement.clientHeight + window.scrollY >=
(document.documentElement.scrollHeight ||
document.documentElement.clientHeight);
bottomVisible(); // true
btoa
- title: btoa
- tags: node,string,beginner
Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.
- Create a
Buffer
for the given string with binary encoding and useBuffer.toString('base64')
to return the encoded string.
const btoa = str => Buffer.from(str, 'binary').toString('base64');
btoa('foobar'); // 'Zm9vYmFy'
bubbleSort
- title: bubbleSort
- tags: algorithm,array,beginner
Sorts an array of numbers, using the bubble sort algorithm.
- Declare a variable,
swapped
, that indicates if any values were swapped during the current iteration. - Use the spread operator (
...
) to clone the original array,arr
. - Use a
for
loop to iterate over the elements of the cloned array, terminating before the last element. - Use a nested
for
loop to iterate over the segment of the array between0
andi
, swapping any adjacent out of order elements and settingswapped
totrue
. - If
swapped
isfalse
after an iteration, no more changes are needed, so the cloned array is returned.
const bubbleSort = arr => {
let swapped = false;
const a = [...arr];
for (let i = 1; i < a.length - 1; i++) {
swapped = false;
for (let j = 0; j < a.length - i; j++) {
if (a[j + 1] < a[j]) {
[a[j], a[j + 1]] = [a[j + 1], a[j]];
swapped = true;
}
}
if (!swapped) return a;
}
return a;
};
bubbleSort([2, 1, 4, 3]); // [1, 2, 3, 4]
bucketSort
- title: bucketSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the bucket sort algorithm.
- Use
Math.min(),
Math.max()
and the spread operator (...
) to find the minimum and maximum values of the given array. - Use
Array.from()
andMath.floor()
to create the appropriate number ofbuckets
(empty arrays). - Use
Array.prototype.forEach()
to populate each bucket with the appropriate elements from the array. - Use
Array.prototype.reduce()
, the spread operator (...
) andArray.prototype.sort()
to sort each bucket and append it to the result.
const bucketSort = (arr, size = 5) => {
const min = Math.min(...arr);
const max = Math.max(...arr);
const buckets = Array.from(
{ length: Math.floor((max - min) / size) + 1 },
() => []
);
arr.forEach(val => {
buckets[Math.floor((val - min) / size)].push(val);
});
return buckets.reduce((acc, b) => [...acc, ...b.sort((a, b) => a - b)], []);
};
bucketSort([6, 3, 4, 1]); // [1, 3, 4, 6]
byteSize
- title: byteSize
- tags: string,beginner
Returns the length of a string in bytes.
- Convert a given string to a
Blob
Object. - Use
Blob.size
to get the length of the string in bytes.
const byteSize = str => new Blob([str]).size;
byteSize('😀'); // 4
byteSize('Hello World'); // 11
caesarCipher
- title: caesarCipher
- tags: algorithm,string,beginner
Encrypts or decrypts a given string using the Caesar cipher.
- Use the modulo (
%
) operator and the ternary operator (?
) to calculate the correct encryption/decryption key. - Use the spread operator (
...
) andArray.prototype.map()
to iterate over the letters of the given string. - Use
String.prototype.charCodeAt()
andString.fromCharCode()
to convert each letter appropriately, ignoring special characters, spaces etc. - Use
Array.prototype.join()
to combine all the letters into a string. - Pass
true
to the last parameter,decrypt
, to decrypt an encrypted string.
const caesarCipher = (str, shift, decrypt = false) => {
const s = decrypt ? (26 - shift) % 26 : shift;
const n = s > 0 ? s : 26 + (s % 26);
return [...str]
.map((l, i) => {
const c = str.charCodeAt(i);
if (c >= 65 && c <= 90)
return String.fromCharCode(((c - 65 + n) % 26) + 65);
if (c >= 97 && c <= 122)
return String.fromCharCode(((c - 97 + n) % 26) + 97);
return l;
})
.join('');
};
caesarCipher('Hello World!', -3); // 'Ebiil Tloia!'
caesarCipher('Ebiil Tloia!', 23, true); // 'Hello World!'
call
- title: call
- tags: function,advanced
Given a key and a set of arguments, call them when given a context.
- Use a closure to call
key
withargs
for the givencontext
.
const call = (key, ...args) => context => context[key](...args);
Promise.resolve([1, 2, 3])
.then(call('map', x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
.then(map(x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
capitalize
- title: capitalize
- tags: string,intermediate
Capitalizes the first letter of a string.
- Use array destructuring and
String.prototype.toUpperCase()
to capitalize the first letter of the string. - Use
Array.prototype.join('')
to combine the capitalizedfirst
with the...rest
of the characters. - Omit the
lowerRest
argument to keep the rest of the string intact, or set it totrue
to convert to lowercase.
const capitalize = ([first, ...rest], lowerRest = false) =>
first.toUpperCase() +
(lowerRest ? rest.join('').toLowerCase() : rest.join(''));
capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'
capitalizeEveryWord
- title: capitalizeEveryWord
- tags: string,regexp,intermediate
Capitalizes the first letter of every word in a string.
- Use
String.prototype.replace()
to match the first character of each word andString.prototype.toUpperCase()
to capitalize it.
const capitalizeEveryWord = str =>
str.replace(/\b[a-z]/g, char => char.toUpperCase());
capitalizeEveryWord('hello world!'); // 'Hello World!'
cartesianProduct
- title: cartesianProduct
- tags: math,array,beginner
Calculates the cartesian product of two arrays.
- Use
Array.prototype.reduce()
,Array.prototype.map()
and the spread operator (...
) to generate all possible element pairs from the two arrays.
const cartesianProduct = (a, b) =>
a.reduce((p, x) => [...p, ...b.map(y => [x, y])], []);
cartesianProduct(['x', 'y'], [1, 2]);
// [['x', 1], ['x', 2], ['y', 1], ['y', 2]]
castArray
- title: castArray
- tags: type,array,beginner
Casts the provided value as an array if it's not one.
- Use
Array.prototype.isArray()
to determine ifval
is an array and return it as-is or encapsulated in an array accordingly.
const castArray = val => (Array.isArray(val) ? val : [val]);
castArray('foo'); // ['foo']
castArray([1]); // [1]
celsiusToFahrenheit
- title: celsiusToFahrenheit
- tags: math,beginner unlisted: true
Converts Celsius to Fahrenheit.
- Follow the conversion formula
F = 1.8 * C + 32
.
const celsiusToFahrenheit = degrees => 1.8 * degrees + 32;
celsiusToFahrenheit(33); // 91.4
chainAsync
- title: chainAsync
- tags: function,intermediate
Chains asynchronous functions.
- Loop through an array of functions containing asynchronous events, calling
next
when each asynchronous event has completed.
const chainAsync = fns => {
let curr = 0;
const last = fns[fns.length - 1];
const next = () => {
const fn = fns[curr++];
fn === last ? fn() : fn(next);
};
next();
};
chainAsync([
next => {
console.log('0 seconds');
setTimeout(next, 1000);
},
next => {
console.log('1 second');
setTimeout(next, 1000);
},
() => {
console.log('2 second');
}
]);
changeLightness
- title: changeLightness
- tags: string,browser,regexp,intermediate
Changes the lightness value of an hsl()
color string.
- Use
String.prototype.match()
to get an array of 3 strings with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Make sure the lightness is within the valid range (between
0
and100
), usingMath.max()
andMath.min()
. - Use a template literal to create a new
hsl()
string with the updated value.
const changeLightness = (delta, hslStr) => {
const [hue, saturation, lightness] = hslStr.match(/\d+/g).map(Number);
const newLightness = Math.max(
0,
Math.min(100, lightness + parseFloat(delta))
);
return `hsl(${hue}, ${saturation}%, ${newLightness}%)`;
};
changeLightness(10, 'hsl(330, 50%, 50%)'); // 'hsl(330, 50%, 60%)'
changeLightness(-10, 'hsl(330, 50%, 50%)'); // 'hsl(330, 50%, 40%)'
checkProp
- title: checkProp
- tags: function,object,intermediate
Creates a function that will invoke a predicate function for the specified property on a given object.
- Return a curried function, that will invoke
predicate
for the specifiedprop
onobj
and return a boolean.
const checkProp = (predicate, prop) => obj => !!predicate(obj[prop]);
const lengthIs4 = checkProp(l => l === 4, 'length');
lengthIs4([]); // false
lengthIs4([1, 2, 3, 4]); // true
lengthIs4(new Set([1, 2, 3, 4])); // false (Set uses Size, not length)
const session = { user: {} };
const validUserSession = checkProp(u => u.active && !u.disabled, 'user');
validUserSession(session); // false
session.user.active = true;
validUserSession(session); // true
const noLength = checkProp(l => l === undefined, 'length');
noLength([]); // false
noLength({}); // true
noLength(new Set()); // true
chunk
- title: chunk
- tags: array,intermediate
Chunks an array into smaller arrays of a specified size.
- Use
Array.from()
to create a new array, that fits the number of chunks that will be produced. - Use
Array.prototype.slice()
to map each element of the new array to a chunk the length ofsize
. - If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunk = (arr, size) =>
Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
arr.slice(i * size, i * size + size)
);
chunk([1, 2, 3, 4, 5], 2); // [[1, 2], [3, 4], [5]]
chunkIntoN
- title: chunkIntoN
- tags: array,intermediate
Chunks an array into n
smaller arrays.
- Use
Math.ceil()
andArray.prototype.length
to get the size of each chunk. - Use
Array.from()
to create a new array of sizen
. - Use
Array.prototype.slice()
to map each element of the new array to a chunk the length ofsize
. - If the original array can't be split evenly, the final chunk will contain the remaining elements.
const chunkIntoN = (arr, n) => {
const size = Math.ceil(arr.length / n);
return Array.from({ length: n }, (v, i) =>
arr.slice(i * size, i * size + size)
);
}
chunkIntoN([1, 2, 3, 4, 5, 6, 7], 4); // [[1, 2], [3, 4], [5, 6], [7]]
clampNumber
- title: clampNumber
- tags: math,beginner
Clamps num
within the inclusive range specified by the boundary values a
and b
.
- If
num
falls within the range, returnnum
. - Otherwise, return the nearest number in the range.
const clampNumber = (num, a, b) =>
Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1
cloneRegExp
- title: cloneRegExp
- tags: type,intermediate
Clones a regular expression.
- Use
new RegExp()
,RegExp.prototype.source
andRegExp.prototype.flags
to clone the given regular expression.
const cloneRegExp = regExp => new RegExp(regExp.source, regExp.flags);
const regExp = /lorem ipsum/gi;
const regExp2 = cloneRegExp(regExp); // regExp !== regExp2
coalesce
- title: coalesce
- tags: type,beginner
Returns the first defined, non-null argument.
- Use
Array.prototype.find()
andArray.prototype.includes()
to find the first value that is not equal toundefined
ornull
.
const coalesce = (...args) => args.find(v => ![undefined, null].includes(v));
coalesce(null, undefined, '', NaN, 'Waldo'); // ''
coalesceFactory
- title: coalesceFactory
- tags: function,type,intermediate
Customizes a coalesce function that returns the first argument which is true based on the given validator.
- Use
Array.prototype.find()
to return the first argument that returnstrue
from the provided argument validation function,valid
.
const coalesceFactory = valid => (...args) => args.find(valid);
const customCoalesce = coalesceFactory(
v => ![null, undefined, '', NaN].includes(v)
);
customCoalesce(undefined, null, NaN, '', 'Waldo'); // 'Waldo'
collectInto
- title: collectInto
- tags: function,array,intermediate
Changes a function that accepts an array into a variadic function.
- Given a function, return a closure that collects all inputs into an array-accepting function.
const collectInto = fn => (...args) => fn(args);
const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)
colorize
- title: colorize
- tags: node,string,intermediate
Adds special characters to text to print in color in the console (combined with console.log()
).
- Use template literals and special characters to add the appropriate color code to the string output.
- For background colors, add a special character that resets the background color at the end of the string.
const colorize = (...args) => ({
black: `\x1b[30m${args.join(' ')}`,
red: `\x1b[31m${args.join(' ')}`,
green: `\x1b[32m${args.join(' ')}`,
yellow: `\x1b[33m${args.join(' ')}`,
blue: `\x1b[34m${args.join(' ')}`,
magenta: `\x1b[35m${args.join(' ')}`,
cyan: `\x1b[36m${args.join(' ')}`,
white: `\x1b[37m${args.join(' ')}`,
bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});
console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite);
// 'foo bar' (first word in yellow letters, second word in green letters, white background for both)
combine
- title: combine
- tags: array,object,intermediate
Combines two arrays of objects, using the specified key to match objects.
- Use
Array.prototype.reduce()
with an object accumulator to combine all objects in both arrays based on the givenprop
. - Use
Object.values()
to convert the resulting object to an array and return it.
const combine = (a, b, prop) =>
Object.values(
[...a, ...b].reduce((acc, v) => {
if (v[prop])
acc[v[prop]] = acc[v[prop]]
? { ...acc[v[prop]], ...v }
: { ...v };
return acc;
}, {})
);
const x = [
{ id: 1, name: 'John' },
{ id: 2, name: 'Maria' }
];
const y = [
{ id: 1, age: 28 },
{ id: 3, age: 26 },
{ age: 3}
];
combine(x, y, 'id');
// [
// { id: 1, name: 'John', age: 28 },
// { id: 2, name: 'Maria' },
// { id: 3, age: 26 }
// ]
compact
- title: compact
- tags: array,beginner
Removes falsy values from an array.
- Use
Array.prototype.filter()
to filter out falsy values (false
,null
,0
,""
,undefined
, andNaN
).
const compact = arr => arr.filter(Boolean);
compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]);
// [ 1, 2, 3, 'a', 's', 34 ]
compactObject
- title: compactObject
- tags: object,array,recursion,advanced
Deeply removes all falsy values from an object or array.
- Use recursion.
- Initialize the iterable data, using
Array.isArray()
,Array.prototype.filter()
andBoolean
for arrays in order to avoid sparse arrays. - Use
Object.keys()
andArray.prototype.reduce()
to iterate over each key with an appropriate initial value. - Use
Boolean
to determine the truthiness of each key's value and add it to the accumulator if it's truthy. - Use
typeof
to determine if a given value is anobject
and call the function again to deeply compact it.
const compactObject = val => {
const data = Array.isArray(val) ? val.filter(Boolean) : val;
return Object.keys(data).reduce(
(acc, key) => {
const value = data[key];
if (Boolean(value))
acc[key] = typeof value === 'object' ? compactObject(value) : value;
return acc;
},
Array.isArray(val) ? [] : {}
);
};
const obj = {
a: null,
b: false,
c: true,
d: 0,
e: 1,
f: '',
g: 'a',
h: [null, false, '', true, 1, 'a'],
i: { j: 0, k: false, l: 'a' }
};
compactObject(obj);
// { c: true, e: 1, g: 'a', h: [ true, 1, 'a' ], i: { l: 'a' } }
compactWhitespace
- title: compactWhitespace
- tags: string,regexp,beginner
Compacts whitespaces in a string.
- Use
String.prototype.replace()
with a regular expression to replace all occurrences of 2 or more whitespace characters with a single space.
const compactWhitespace = str => str.replace(/\s{2,}/g, ' ');
compactWhitespace('Lorem Ipsum'); // 'Lorem Ipsum'
compactWhitespace('Lorem \n Ipsum'); // 'Lorem Ipsum'
complement
- title: complement
- tags: function,logic,beginner
Returns a function that is the logical complement of the given function, fn
.
- Use the logical not (
!
) operator on the result of callingfn
with any suppliedargs
.
const complement = fn => (...args) => !fn(...args);
const isEven = num => num % 2 === 0;
const isOdd = complement(isEven);
isOdd(2); // false
isOdd(3); // true
compose
- title: compose
- tags: function,intermediate
Performs right-to-left function composition.
- Use
Array.prototype.reduce()
to perform right-to-left function composition. - The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.
const compose = (...fns) =>
fns.reduce((f, g) => (...args) => f(g(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(
add5,
multiply
);
multiplyAndAdd5(5, 2); // 15
composeRight
- title: composeRight
- tags: function,intermediate
Performs left-to-right function composition.
- Use
Array.prototype.reduce()
to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const composeRight = (...fns) =>
fns.reduce((f, g) => (...args) => g(f(...args)));
const add = (x, y) => x + y;
const square = x => x * x;
const addAndSquare = composeRight(add, square);
addAndSquare(1, 2); // 9
containsWhitespace
- title: containsWhitespace
- tags: string,regexp,beginner
Checks if the given string contains any whitespace characters.
- Use
RegExp.prototype.test()
with an appropriate regular expression to check if the given string contains any whitespace characters.
const containsWhitespace = str => /\s/.test(str);
containsWhitespace('lorem'); // false
containsWhitespace('lorem ipsum'); // true
converge
- title: converge
- tags: function,intermediate
Accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.
- Use
Array.prototype.map()
andFunction.prototype.apply()
to apply each function to the given arguments. - Use the spread operator (
...
) to callconverger
with the results of all other functions.
const converge = (converger, fns) => (...args) =>
converger(...fns.map(fn => fn.apply(null, args)));
const average = converge((a, b) => a / b, [
arr => arr.reduce((a, v) => a + v, 0),
arr => arr.length
]);
average([1, 2, 3, 4, 5, 6, 7]); // 4
copySign
- title: copySign
- tags: math,beginner
Returns the absolute value of the first number, but the sign of the second.
- Use
Math.sign()
to check if the two numbers have the same sign. - Return
x
if they do,-x
otherwise.
const copySign = (x, y) => Math.sign(x) === Math.sign(y) ? x : -x;
copySign(2, 3); // 2
copySign(2, -3); // -2
copySign(-2, 3); // 2
copySign(-2, -3); // -2
copyToClipboard
- title: copyToClipboard
- tags: browser,string,event,advanced
Copies a string to the clipboard.
Only works as a result of user action (i.e. inside a click
event listener).
- Create a new
<textarea>
element, fill it with the supplied data and add it to the HTML document. - Use
Selection.getRangeAt()
to store the selected range (if any). - Use
Document.execCommand('copy')
to copy to the clipboard. - Remove the
<textarea>
element from the HTML document. - Finally, use
Selection().addRange()
to recover the original selected range (if any). - ⚠️ NOTICE: The same functionality can be easily implemented by using the new asynchronous Clipboard API, which is still experimental but should be used in the future instead of this snippet. Find out more about it here.
const copyToClipboard = str => {
const el = document.createElement('textarea');
el.value = str;
el.setAttribute('readonly', '');
el.style.position = 'absolute';
el.style.left = '-9999px';
document.body.appendChild(el);
const selected =
document.getSelection().rangeCount > 0
? document.getSelection().getRangeAt(0)
: false;
el.select();
document.execCommand('copy');
document.body.removeChild(el);
if (selected) {
document.getSelection().removeAllRanges();
document.getSelection().addRange(selected);
}
};
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.
countBy
- title: countBy
- tags: array,object,intermediate
Groups the elements of an array based on the given function and returns the count of elements in each group.
- Use
Array.prototype.map()
to map the values of an array to a function or property name. - Use
Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const countBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
acc[val] = (acc[val] || 0) + 1;
return acc;
}, {});
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
countBy([{ count: 5 }, { count: 10 }, { count: 5 }], x => x.count)
// {5: 2, 10: 1}
countOccurrences
- title: countOccurrences
- tags: array,intermediate
Counts the occurrences of a value in an array.
- Use
Array.prototype.reduce()
to increment a counter each time the specific value is encountered inside the array.
const countOccurrences = (arr, val) =>
arr.reduce((a, v) => (v === val ? a + 1 : a), 0);
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3
countSubstrings
- title: countSubstrings
- tags: string,algorithm,beginner
Counts the occurrences of a substring in a given string.
- Use
Array.prototype.indexOf()
to look forsearchValue
instr
. - Increment a counter if the value is found and update the index,
i
. - Use a
while
loop that will return as soon as the value returned fromArray.prototype.indexOf()
is-1
.
const countSubstrings = (str, searchValue) => {
let count = 0,
i = 0;
while (true) {
const r = str.indexOf(searchValue, i);
if (r !== -1) [count, i] = [count + 1, r + 1];
else return count;
}
};
countSubstrings('tiktok tok tok tik tok tik', 'tik'); // 3
countSubstrings('tutut tut tut', 'tut'); // 4
countWeekDaysBetween
- title: countWeekDaysBetween
- tags: date,intermediate
Counts the weekdays between two dates.
- Use
Array.from()
to construct an array withlength
equal to the number of days betweenstartDate
andendDate
. - Use
Array.prototype.reduce()
to iterate over the array, checking if each date is a weekday and incrementingcount
. - Update
startDate
with the next day each loop usingDate.prototype.getDate()
andDate.prototype.setDate()
to advance it by one day. - NOTE: Does not take official holidays into account.
const countWeekDaysBetween = (startDate, endDate) =>
Array
.from({ length: (endDate - startDate) / (1000 * 3600 * 24) })
.reduce(count => {
if (startDate.getDay() % 6 !== 0) count++;
startDate = new Date(startDate.setDate(startDate.getDate() + 1));
return count;
}, 0);
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 06, 2020')); // 1
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 14, 2020')); // 7
counter
- title: counter
- tags: browser,advanced
Creates a counter with the specified range, step and duration for the specified selector.
- Check if
step
has the proper sign and change it accordingly. - Use
setInterval()
in combination withMath.abs()
andMath.floor()
to calculate the time between each new text draw. - Use
Document.querySelector()
,Element.innerHTML
to update the value of the selected element. - Omit the fourth argument,
step
, to use a default step of1
. - Omit the fifth argument,
duration
, to use a default duration of2000
ms.
const counter = (selector, start, end, step = 1, duration = 2000) => {
let current = start,
_step = (end - start) * step < 0 ? -step : step,
timer = setInterval(() => {
current += _step;
document.querySelector(selector).innerHTML = current;
if (current >= end) document.querySelector(selector).innerHTML = end;
if (current >= end) clearInterval(timer);
}, Math.abs(Math.floor(duration / (end - start))));
return timer;
};
counter('##my-id', 1, 1000, 5, 2000);
// Creates a 2-second timer for the element with id="my-id"
createDirIfNotExists
- title: createDirIfNotExists
- tags: node,beginner
Creates a directory, if it does not exist.
- Use
fs.existsSync()
to check if the directory exists,fs.mkdirSync()
to create it.
const fs = require('fs');
const createDirIfNotExists = dir => (!fs.existsSync(dir) ? fs.mkdirSync(dir) : undefined);
createDirIfNotExists('test');
// creates the directory 'test', if it doesn't exist
createElement
- title: createElement
- tags: browser,beginner
Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.
- Use
Document.createElement()
to create a new element. - Use
Element.innerHTML
to set its inner HTML to the string supplied as the argument. - Use
ParentNode.firstElementChild
to return the element version of the string.
const createElement = str => {
const el = document.createElement('div');
el.innerHTML = str;
return el.firstElementChild;
};
const el = createElement(
`<div class="container">
<p>Hello!</p>
</div>`
);
console.log(el.className); // 'container'
createEventHub
- title: createEventHub
- tags: browser,event,advanced
Creates a pub/sub (publish–subscribe) event hub with emit
, on
, and off
methods.
- Use
Object.create(null)
to create an emptyhub
object that does not inherit properties fromObject.prototype
. - For
emit
, resolve the array of handlers based on theevent
argument and then run each one withArray.prototype.forEach()
by passing in the data as an argument. - For
on
, create an array for the event if it does not yet exist, then useArray.prototype.push()
to add the handler - to the array.
- For
off
, useArray.prototype.findIndex()
to find the index of the handler in the event array and remove it usingArray.prototype.splice()
.
const createEventHub = () => ({
hub: Object.create(null),
emit(event, data) {
(this.hub[event] || []).forEach(handler => handler(data));
},
on(event, handler) {
if (!this.hub[event]) this.hub[event] = [];
this.hub[event].push(handler);
},
off(event, handler) {
const i = (this.hub[event] || []).findIndex(h => h === handler);
if (i > -1) this.hub[event].splice(i, 1);
if (this.hub[event].length === 0) delete this.hub[event];
}
});
const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;
// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);
// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1
// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);
currentURL
- title: currentURL
- tags: browser,beginner
Returns the current URL.
- Use
Window.location.href
to get the current URL.
const currentURL = () => window.location.href;
currentURL(); // 'https://www.google.com/'
curry
- title: curry
- tags: function,recursion,advanced
Curries a function.
- Use recursion.
- If the number of provided arguments (
args
) is sufficient, call the passed functionfn
. - Otherwise, use
Function.prototype.bind()
to return a curried functionfn
that expects the rest of the arguments. - If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g.
Math.min()
), you can optionally pass the number of arguments to the second parameterarity
.
const curry = (fn, arity = fn.length, ...args) =>
arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);
curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2
cycleGenerator
- title: cycleGenerator
- tags: function,generator,advanced
Creates a generator, looping over the given array indefinitely.
- Use a non-terminating
while
loop, that willyield
a value every timeGenerator.prototype.next()
is called. - Use the module operator (
%
) withArray.prototype.length
to get the next value's index and increment the counter after eachyield
statement.
const cycleGenerator = function* (arr) {
let i = 0;
while (true) {
yield arr[i % arr.length];
i++;
}
};
const binaryCycle = cycleGenerator([0, 1]);
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
binaryCycle.next(); // { value: 0, done: false }
binaryCycle.next(); // { value: 1, done: false }
dayName
- title: dayName
- tags: date,beginner
Gets the name of the weekday from a Date
object.
- Use
Date.prototype.toLocaleDateString()
with the{ weekday: 'long' }
option to retrieve the weekday. - Use the optional second argument to get a language-specific name or omit it to use the default locale.
const dayName = (date, locale) =>
date.toLocaleDateString(locale, { weekday: 'long' });
dayName(new Date()); // 'Saturday'
dayName(new Date('09/23/2020'), 'de-DE'); // 'Samstag'
dayOfYear
- title: dayOfYear
- tags: date,beginner
Gets the day of the year (number in the range 1-366) from a Date
object.
- Use
new Date()
andDate.prototype.getFullYear()
to get the first day of the year as aDate
object. - Subtract the first day of the year from
date
and divide with the milliseconds in each day to get the result. - Use
Math.floor()
to appropriately round the resulting day count to an integer.
const dayOfYear = date =>
Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);
dayOfYear(new Date()); // 272
daysAgo
- title: daysAgo
- tags: date,beginner
Calculates the date of n
days ago from today as a string representation.
- Use
new Date()
to get the current date,Math.abs()
andDate.prototype.getDate()
to update the date accordingly and set to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const daysAgo = n => {
let d = new Date();
d.setDate(d.getDate() - Math.abs(n));
return d.toISOString().split('T')[0];
};
daysAgo(20); // 2020-09-16 (if current date is 2020-10-06)
daysFromNow
- title: daysFromNow
- tags: date,beginner
Calculates the date of n
days from today as a string representation.
- Use
new Date()
to get the current date,Math.abs()
andDate.prototype.getDate()
to update the date accordingly and set to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const daysFromNow = n => {
let d = new Date();
d.setDate(d.getDate() + Math.abs(n));
return d.toISOString().split('T')[0];
};
daysFromNow(5); // 2020-10-13 (if current date is 2020-10-08)
debounce
- title: debounce
- tags: function,intermediate
Creates a debounced function that delays invoking the provided function until at least ms
milliseconds have elapsed since the last time it was invoked.
- Each time the debounced function is invoked, clear the current pending timeout with
clearTimeout()
and usesetTimeout()
to create a new timeout that delays invoking the function until at leastms
milliseconds has elapsed. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessary arguments. - Omit the second argument,
ms
, to set the timeout at a default of0
ms.
const debounce = (fn, ms = 0) => {
let timeoutId;
return function(...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => fn.apply(this, args), ms);
};
};
window.addEventListener(
'resize',
debounce(() => {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250ms
debouncePromise
- title: debouncePromise
- tags: function,promise,advanced
Creates a debounced function that returns a promise, but delays invoking the provided function until at least ms
milliseconds have elapsed since the last time it was invoked.
All promises returned during this time will return the same data.
- Each time the debounced function is invoked, clear the current pending timeout with
clearTimeout()
and usesetTimeout()
to create a new timeout that delays invoking the function until at leastms
milliseconds has elapsed. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessary arguments. - Create a new
Promise
and add itsresolve
andreject
callbacks to thepending
promises stack. - When
setTimeout
is called, copy the current stack (as it can change between the provided function call and its resolution), clear it and call the provided function. - When the provided function resolves/rejects, resolve/reject all promises in the stack (copied when the function was called) with the returned data.
- Omit the second argument,
ms
, to set the timeout at a default of0
ms.
const debouncePromise = (fn, ms = 0) => {
let timeoutId;
const pending = [];
return (...args) =>
new Promise((res, rej) => {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => {
const currentPending = [...pending];
pending.length = 0;
Promise.resolve(fn.apply(this, args)).then(
data => {
currentPending.forEach(({ resolve }) => resolve(data));
},
error => {
currentPending.forEach(({ reject }) => reject(error));
}
);
}, ms);
pending.push({ resolve: res, reject: rej });
});
};
const fn = arg => new Promise(resolve => {
setTimeout(resolve, 1000, ['resolved', arg]);
});
const debounced = debouncePromise(fn, 200);
debounced('foo').then(console.log);
debounced('bar').then(console.log);
// Will log ['resolved', 'bar'] both times
decapitalize
- title: decapitalize
- tags: string,intermediate
Decapitalizes the first letter of a string.
- Use array destructuring and
String.prototype.toLowerCase()
to decapitalize first letter,...rest
to get array of characters after first letter and thenArray.prototype.join('')
to make it a string again. - Omit the
upperRest
argument to keep the rest of the string intact, or set it totrue
to convert to uppercase.
const decapitalize = ([first, ...rest], upperRest = false) =>
first.toLowerCase() +
(upperRest ? rest.join('').toUpperCase() : rest.join(''));
decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'
deepClone
- title: deepClone
- tags: object,recursion,advanced
Creates a deep clone of an object. Clones primitives, arrays and objects, excluding class instances.
- Use recursion.
- Check if the passed object is
null
and, if so, returnnull
. - Use
Object.assign()
and an empty object ({}
) to create a shallow clone of the original. - Use
Object.keys()
andArray.prototype.forEach()
to determine which key-value pairs need to be deep cloned. - If the object is an
Array
, set theclone
'slength
to that of the original and useArray.from(clone)
to create a clone.
const deepClone = obj => {
if (obj === null) return null;
let clone = Object.assign({}, obj);
Object.keys(clone).forEach(
key =>
(clone[key] =
typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
);
if (Array.isArray(obj)) {
clone.length = obj.length;
return Array.from(clone);
}
return clone;
};
const a = { foo: 'bar', obj: { a: 1, b: 2 } };
const b = deepClone(a); // a !== b, a.obj !== b.obj
deepFlatten
- title: deepFlatten
- tags: array,recursion,intermediate
Deep flattens an array.
- Use recursion.
- Use
Array.prototype.concat()
with an empty array ([]
) and the spread operator (...
) to flatten an array. - Recursively flatten each element that is an array.
const deepFlatten = arr =>
[].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));
deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]
deepFreeze
- title: deepFreeze
- tags: object,recursion,intermediate
Deep freezes an object.
- Use
Object.keys()
to get all the properties of the passed object,Array.prototype.forEach()
to iterate over them. - Call
Object.freeze(obj)
recursively on all properties, applyingdeepFreeze()
as necessary. - Finally, use
Object.freeze()
to freeze the given object.
const deepFreeze = obj => {
Object.keys(obj).forEach(prop => {
if (typeof obj[prop] === 'object') deepFreeze(obj[prop]);
});
return Object.freeze(obj);
};
'use strict';
const val = deepFreeze([1, [2, 3]]);
val[0] = 3; // not allowed
val[1][0] = 4; // not allowed as well
deepGet
- title: deepGet
- tags: object,intermediate
Gets the target value in a nested JSON object, based on the keys
array.
- Compare the keys you want in the nested JSON object as an
Array
. - Use
Array.prototype.reduce()
to get the values in the nested JSON object one by one. - If the key exists in the object, return the target value, otherwise return
null
.
const deepGet = (obj, keys) =>
keys.reduce(
(xs, x) => (xs && xs[x] !== null && xs[x] !== undefined ? xs[x] : null),
obj
);
let index = 2;
const data = {
foo: {
foz: [1, 2, 3],
bar: {
baz: ['a', 'b', 'c']
}
}
};
deepGet(data, ['foo', 'foz', index]); // get 3
deepGet(data, ['foo', 'bar', 'baz', 8, 'foz']); // null
deepMapKeys
- title: deepMapKeys
- tags: object,recursion,advanced
Deep maps an object's keys.
- Creates an object with the same values as the provided object and keys generated by running the provided function for each key.
- Use
Object.keys(obj)
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same values and mapped keys usingfn
.
const deepMapKeys = (obj, fn) =>
Array.isArray(obj)
? obj.map(val => deepMapKeys(val, fn))
: typeof obj === 'object'
? Object.keys(obj).reduce((acc, current) => {
const key = fn(current);
const val = obj[current];
acc[key] =
val !== null && typeof val === 'object' ? deepMapKeys(val, fn) : val;
return acc;
}, {})
: obj;
const obj = {
foo: '1',
nested: {
child: {
withArray: [
{
grandChild: ['hello']
}
]
}
}
};
const upperKeysObj = deepMapKeys(obj, key => key.toUpperCase());
/*
{
"FOO":"1",
"NESTED":{
"CHILD":{
"WITHARRAY":[
{
"GRANDCHILD":[ 'hello' ]
}
]
}
}
}
*/
defaults
- title: defaults
- tags: object,intermediate
Assigns default values for all properties in an object that are undefined
.
- Use
Object.assign()
to create a new empty object and copy the original one to maintain key order. - Use
Array.prototype.reverse()
and the spread operator (...
) to combine the default values from left to right. - Finally, use
obj
again to overwrite properties that originally had a value.
const defaults = (obj, ...defs) =>
Object.assign({}, obj, ...defs.reverse(), obj);
defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }
defer
- title: defer
- tags: function,intermediate
Defers invoking a function until the current call stack has cleared.
- Use
setTimeout()
with a timeout of1
ms to add a new event to the event queue and allow the rendering engine to complete its work. - Use the spread (
...
) operator to supply the function with an arbitrary number of arguments.
const defer = (fn, ...args) => setTimeout(fn, 1, ...args);
// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'
// Example B:
document.querySelector('##someElement').innerHTML = 'Hello';
longRunningFunction();
// Browser will not update the HTML until this has finished
defer(longRunningFunction);
// Browser will update the HTML then run the function
degreesToRads
- title: degreesToRads
- tags: math,beginner
Converts an angle from degrees to radians.
- Use
Math.PI
and the degree to radian formula to convert the angle from degrees to radians.
const degreesToRads = deg => (deg * Math.PI) / 180.0;
degreesToRads(90.0); // ~1.5708
delay
- title: delay
- tags: function,intermediate
Invokes the provided function after ms
milliseconds.
- Use
setTimeout()
to delay execution offn
. - Use the spread (
...
) operator to supply the function with an arbitrary number of arguments.
const delay = (fn, ms, ...args) => setTimeout(fn, ms, ...args);
delay(
function(text) {
console.log(text);
},
1000,
'later'
); // Logs 'later' after one second.
detectDeviceType
- title: detectDeviceType
- tags: browser,regexp,intermediate
Detects whether the page is being viewed on a mobile device or a desktop.
- Use a regular expression to test the
navigator.userAgent
property to figure out if the device is a mobile device or a desktop.
const detectDeviceType = () =>
/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(
navigator.userAgent
)
? 'Mobile'
: 'Desktop';
detectDeviceType(); // 'Mobile' or 'Desktop'
detectLanguage
- title: detectLanguage
- tags: browser,intermediate
Detects the preferred language of the current user.
- Use
NavigationLanguage.language
or the firstNavigationLanguage.languages
if available, otherwise returndefaultLang
. - Omit the second argument,
defaultLang
, to use'en-US'
as the default language code.
const detectLanguage = (defaultLang = 'en-US') =>
navigator.language ||
(Array.isArray(navigator.languages) && navigator.languages[0]) ||
defaultLang;
detectLanguage(); // 'nl-NL'
difference
- title: difference
- tags: array,beginner
Calculates the difference between two arrays, without filtering duplicate values.
- Create a
Set
fromb
to get the unique values inb
. - Use
Array.prototype.filter()
ona
to only keep values not contained inb
, usingSet.prototype.has()
.
const difference = (a, b) => {
const s = new Set(b);
return a.filter(x => !s.has(x));
};
difference([1, 2, 3, 3], [1, 2, 4]); // [3, 3]
differenceBy
- title: differenceBy
- tags: array,intermediate
Returns the difference between two arrays, after applying the provided function to each array element of both.
- Create a
Set
by applyingfn
to each element inb
. - Use
Array.prototype.map()
to applyfn
to each element ina
. - Use
Array.prototype.filter()
in combination withfn
ona
to only keep values not contained inb
, usingSet.prototype.has()
.
const differenceBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.map(fn).filter(el => !s.has(el));
};
differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]
differenceWith
- title: differenceWith
- tags: array,intermediate
Filters out all values from an array for which the comparator function does not return true
.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
to find the appropriate values. - Omit the last argument,
comp
, to use a default strict equality comparator.
const differenceWith = (arr, val, comp = (a, b) => a === b) =>
arr.filter(a => val.findIndex(b => comp(a, b)) === -1);
differenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2]
differenceWith([1, 1.2, 1.3], [1, 1.3, 1.5]); // [1.2]
dig
- title: dig
- tags: object,recursion,intermediate
Gets the target value in a nested JSON object, based on the given key.
- Use the
in
operator to check iftarget
exists inobj
. - If found, return the value of
obj[target]
. - Otherwise use
Object.values(obj)
andArray.prototype.reduce()
to recursively calldig
on each nested object until the first matching key/value pair is found.
const dig = (obj, target) =>
target in obj
? obj[target]
: Object.values(obj).reduce((acc, val) => {
if (acc !== undefined) return acc;
if (typeof val === 'object') return dig(val, target);
}, undefined);
const data = {
level1: {
level2: {
level3: 'some data'
}
}
};
dig(data, 'level3'); // 'some data'
dig(data, 'level4'); // undefined
digitize
- title: digitize
- tags: math,beginner
Converts a number to an array of digits, removing its sign if necessary.
- Use
Math.abs()
to strip the number's sign. - Convert the number to a string, using the spread operator (
...
) to build an array. - Use
Array.prototype.map()
andparseInt()
to transform each value to an integer.
const digitize = n => [...`${Math.abs(n)}`].map(i => parseInt(i));
digitize(123); // [1, 2, 3]
digitize(-123); // [1, 2, 3]
distance
- title: distance
- tags: math,algorithm,beginner
Calculates the distance between two points.
- Use
Math.hypot()
to calculate the Euclidean distance between two points.
const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);
distance(1, 1, 2, 3); // ~2.2361
divmod
- title: divmod
- tags: math,beginner
Returns an array consisting of the quotient and remainder of the given numbers.
- Use
Math.floor()
to get the quotient of the divisionx / y
. - Use the modulo operator (
%
) to get the remainder of the divisionx / y
.
const divmod = (x, y) => [Math.floor(x / y), x % y];
divmod(8, 3); // [2, 2]
divmod(3, 8); // [0, 3]
divmod(5, 5); // [1, 0]
drop
- title: drop
- tags: array,beginner
Creates a new array with n
elements removed from the left.
- Use
Array.prototype.slice()
to remove the specified number of elements from the left. - Omit the last argument,
n
, to use a default value of1
.
const drop = (arr, n = 1) => arr.slice(n);
drop([1, 2, 3]); // [2, 3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []
dropRight
- title: dropRight
- tags: array,beginner
Creates a new array with n
elements removed from the right.
- Use
Array.prototype.slice()
to remove the specified number of elements from the right. - Omit the last argument,
n
, to use a default value of1
.
const dropRight = (arr, n = 1) => arr.slice(0, -n);
dropRight([1, 2, 3]); // [1, 2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []
dropRightWhile
- title: dropRightWhile
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns true
.
Returns the remaining elements in the array.
- Loop through the array, using
Array.prototype.slice()
to drop the last element of the array until the value returned fromfunc
istrue
. - Return the remaining elements.
const dropRightWhile = (arr, func) => {
let rightIndex = arr.length;
while (rightIndex-- && !func(arr[rightIndex]));
return arr.slice(0, rightIndex + 1);
};
dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
dropWhile
- title: dropWhile
- tags: array,intermediate
Removes elements in an array until the passed function returns true
.
Returns the remaining elements in the array.
- Loop through the array, using
Array.prototype.slice()
to drop the first element of the array until the value returned fromfunc
istrue
. - Return the remaining elements.
const dropWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
return arr;
};
dropWhile([1, 2, 3, 4], n => n >= 3); // [3, 4]
either
- title: either
- tags: function,logic,beginner
Checks if at least one function returns true
for a given set of arguments.
- Use the logical or (
||
) operator on the result of calling the two functions with the suppliedargs
.
const either = (f, g) => (...args) => f(...args) || g(...args);
const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveOrEven = either(isPositive, isEven);
isPositiveOrEven(4); // true
isPositiveOrEven(3); // true
elementContains
- title: elementContains
- tags: browser,intermediate
Checks if the parent
element contains the child
element.
- Check that
parent
is not the same element aschild
. - Use
Node.contains()
to check if theparent
element contains thechild
element.
const elementContains = (parent, child) =>
parent !== child && parent.contains(child);
elementContains(
document.querySelector('head'),
document.querySelector('title')
);
// true
elementContains(document.querySelector('body'), document.querySelector('body'));
// false
elementIsFocused
- title: elementIsFocused
- tags: browser,beginner
Checks if the given element is focused.
- Use
Document.activeElement
to determine if the given element is focused.
const elementIsFocused = el => (el === document.activeElement);
elementIsFocused(el); // true if the element is focused
elementIsVisibleInViewport
- title: elementIsVisibleInViewport
- tags: browser,intermediate
Checks if the element specified is visible in the viewport.
- Use
Element.getBoundingClientRect()
and theWindow.inner(Width|Height)
values to determine if a given element is visible in the viewport. - Omit the second argument to determine if the element is entirely visible, or specify
true
to determine if it is partially visible.
const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
const { top, left, bottom, right } = el.getBoundingClientRect();
const { innerHeight, innerWidth } = window;
return partiallyVisible
? ((top > 0 && top < innerHeight) ||
(bottom > 0 && bottom < innerHeight)) &&
((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
: top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)
equals
- title: equals
- tags: object,array,type,advanced
Performs a deep comparison between two values to determine if they are equivalent.
- Check if the two values are identical, if they are both
Date
objects with the same time, usingDate.prototype.getTime()
or if they are both non-object values with an equivalent value (strict comparison). - Check if only one value is
null
orundefined
or if their prototypes differ. - If none of the above conditions are met, use
Object.keys()
to check if both values have the same number of keys. - Use
Array.prototype.every()
to check if every key ina
exists inb
and if they are equivalent by callingequals()
recursively.
const equals = (a, b) => {
if (a === b) return true;
if (a instanceof Date && b instanceof Date)
return a.getTime() === b.getTime();
if (!a || !b || (typeof a !== 'object' && typeof b !== 'object'))
return a === b;
if (a.prototype !== b.prototype) return false;
let keys = Object.keys(a);
if (keys.length !== Object.keys(b).length) return false;
return keys.every(k => equals(a[k], b[k]));
};
equals(
{ a: [2, { e: 3 }], b: [4], c: 'foo' },
{ a: [2, { e: 3 }], b: [4], c: 'foo' }
); // true
equals([1, 2, 3], { 0: 1, 1: 2, 2: 3 }); // true
escapeHTML
- title: escapeHTML
- tags: string,browser,regexp,intermediate
Escapes a string for use in HTML.
- Use
String.prototype.replace()
with a regexp that matches the characters that need to be escaped. - Use the callback function to replace each character instance with its associated escaped character using a dictionary (object).
const escapeHTML = str =>
str.replace(
/[&<>'"]/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
"'": '&##39;',
'"': '"'
}[tag] || tag)
);
escapeHTML('<a href="##">Me & you</a>');
// '<a href="##">Me & you</a>'
escapeRegExp
- title: escapeRegExp
- tags: string,regexp,intermediate
Escapes a string to use in a regular expression.
- Use
String.prototype.replace()
to escape special characters.
const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
escapeRegExp('(test)'); // \\(test\\)
euclideanDistance
- title: euclideanDistance
- tags: math,algorithm,intermediate
Calculates the distance between two points in any number of dimensions.
- Use
Object.keys()
andArray.prototype.map()
to map each coordinate to its difference between the two points. - Use
Math.hypot()
to calculate the Euclidean distance between the two points.
const euclideanDistance = (a, b) =>
Math.hypot(...Object.keys(a).map(k => b[k] - a[k]));
euclideanDistance([1, 1], [2, 3]); // ~2.2361
euclideanDistance([1, 1, 1], [2, 3, 2]); // ~2.4495
everyNth
- title: everyNth
- tags: array,beginner
Returns every nth
element in an array.
- Use
Array.prototype.filter()
to create a new array that contains everynth
element of a given array.
const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]
expandTabs
- title: expandTabs
- tags: string,regexp,beginner
Convert tabs to spaces, where each tab corresponds to count
spaces.
- Use
String.prototype.replace()
with a regular expression andString.prototype.repeat()
to replace each tab character withcount
spaces.
const expandTabs = (str, count) => str.replace(/\t/g, ' '.repeat(count));
expandTabs('\t\tlorem', 3); // ' lorem'
extendHex
- title: extendHex
- tags: string,intermediate
Extends a 3-digit color code to a 6-digit color code.
- Use
Array.prototype.map()
,String.prototype.split()
andArray.prototype.join()
to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form. Array.prototype.slice()
is used to remove##
from string start since it's added once.
const extendHex = shortHex =>
'##' +
shortHex
.slice(shortHex.startsWith('##') ? 1 : 0)
.split('')
.map(x => x + x)
.join('');
extendHex('##03f'); // '##0033ff'
extendHex('05a'); // '##0055aa'
factorial
- title: factorial
- tags: math,algorithm,recursion,beginner
Calculates the factorial of a number.
- Use recursion.
- If
n
is less than or equal to1
, return1
. - Otherwise, return the product of
n
and the factorial ofn - 1
. - Throw a
TypeError
ifn
is a negative number.
const factorial = n =>
n < 0
? (() => {
throw new TypeError('Negative numbers are not allowed!');
})()
: n <= 1
? 1
: n * factorial(n - 1);
factorial(6); // 720
fahrenheitToCelsius
- title: fahrenheitToCelsius
- tags: math,beginner unlisted: true
Converts Fahrenheit to Celsius.
- Follow the conversion formula
C = (F - 32) * 5/9
.
const fahrenheitToCelsius = degrees => (degrees - 32) * 5 / 9;
fahrenheitToCelsius(32); // 0
fibonacci
- title: fibonacci
- tags: math,algorithm,intermediate
Generates an array, containing the Fibonacci sequence, up until the nth term.
- Use
Array.from()
to create an empty array of the specific length, initializing the first two values (0
and1
). - Use
Array.prototype.reduce()
andArray.prototype.concat()
to add values into the array, using the sum of the last two values, except for the first two.
const fibonacci = n =>
Array.from({ length: n }).reduce(
(acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
[]
);
fibonacci(6); // [0, 1, 1, 2, 3, 5]
filterNonUnique
- title: filterNonUnique
- tags: array,beginner
Creates an array with the non-unique values filtered out.
- Use
new Set()
and the spread operator (...
) to create an array of the unique values inarr
. - Use
Array.prototype.filter()
to create an array containing only the unique values.
const filterNonUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]
filterNonUniqueBy
- title: filterNonUniqueBy
- tags: array,intermediate
Creates an array with the non-unique values filtered out, based on a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.every()
to create an array containing only the unique values, based on the comparator function,fn
. - The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterNonUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));
filterNonUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id === b.id
); // [ { id: 2, value: 'c' } ]
filterUnique
- title: filterUnique
- tags: array,beginner
Creates an array with the unique values filtered out.
- Use
new Set()
and the spread operator (...
) to create an array of the unique values inarr
. - Use
Array.prototype.filter()
to create an array containing only the non-unique values.
const filterUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) !== arr.lastIndexOf(i));
filterUnique([1, 2, 2, 3, 4, 4, 5]); // [2, 4]
filterUniqueBy
- title: filterUniqueBy
- tags: array,intermediate
Creates an array with the unique values filtered out, based on a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.every()
to create an array containing only the non-unique values, based on the comparator function,fn
. - The comparator function takes four arguments: the values of the two elements being compared and their indexes.
const filterUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.some((x, j) => (i !== j) === fn(v, x, i, j)));
filterUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 3, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 0, value: 'e' } ]
findKey
- title: findKey
- tags: object,intermediate
Finds the first key that satisfies the provided testing function.
Otherwise undefined
is returned.
- Use
Object.keys(obj)
to get all the properties of the object,Array.prototype.find()
to test each key-value pair usingfn
. - The callback receives three arguments - the value, the key and the object.
const findKey = (obj, fn) =>
Object.keys(obj).find(key => fn(obj[key], key, obj));
findKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
x => x['active']
); // 'barney'
findKeys
- title: findKeys
- tags: object,beginner
Finds all the keys in the provided object that match the given value.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.filter()
to test each key-value pair and return all keys that are equal to the given value.
const findKeys = (obj, val) =>
Object.keys(obj).filter(key => obj[key] === val);
const ages = {
Leo: 20,
Zoey: 21,
Jane: 20,
};
findKeys(ages, 20); // [ 'Leo', 'Jane' ]
findLast
- title: findLast
- tags: array,beginner
Finds the last element for which the provided function returns a truthy value.
- Use
Array.prototype.filter()
to remove elements for whichfn
returns falsy values. - Use
Array.prototype.pop()
to get the last element in the filtered array.
const findLast = (arr, fn) => arr.filter(fn).pop();
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3
findLastIndex
- title: findLastIndex
- tags: array,intermediate
Finds the index of the last element for which the provided function returns a truthy value.
- Use
Array.prototype.map()
to map each element to an array with its index and value. - Use
Array.prototype.filter()
to remove elements for whichfn
returns falsy values - Use
Array.prototype.pop()
to get the last element in the filtered array. - Return
-1
if there are no matching elements.
const findLastIndex = (arr, fn) =>
(arr
.map((val, i) => [i, val])
.filter(([i, val]) => fn(val, i, arr))
.pop() || [-1])[0];
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)
findLastKey
- title: findLastKey
- tags: object,intermediate
Finds the last key that satisfies the provided testing function.
Otherwise undefined
is returned.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.reverse()
to reverse the order andArray.prototype.find()
to test the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const findLastKey = (obj, fn) =>
Object.keys(obj)
.reverse()
.find(key => fn(obj[key], key, obj));
findLastKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
x => x['active']
); // 'pebbles'
flatten
- title: flatten
- tags: array,recursion,intermediate
Flattens an array up to the specified depth.
- Use recursion, decrementing
depth
by1
for each level of depth. - Use
Array.prototype.reduce()
andArray.prototype.concat()
to merge elements or arrays. - Base case, for
depth
equal to1
stops recursion. - Omit the second argument,
depth
, to flatten only to a depth of1
(single flatten).
const flatten = (arr, depth = 1) =>
arr.reduce(
(a, v) =>
a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v),
[]
);
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]
flattenObject
- title: flattenObject
- tags: object,recursion,advanced
Flattens an object with the paths for keys.
- Use recursion.
- Use
Object.keys(obj)
combined withArray.prototype.reduce()
to convert every leaf node to a flattened path node. - If the value of a key is an object, the function calls itself with the appropriate
prefix
to create the path usingObject.assign()
. - Otherwise, it adds the appropriate prefixed key-value pair to the accumulator object.
- You should always omit the second argument,
prefix
, unless you want every key to have a prefix.
const flattenObject = (obj, prefix = '') =>
Object.keys(obj).reduce((acc, k) => {
const pre = prefix.length ? `${prefix}.` : '';
if (
typeof obj[k] === 'object' &&
obj[k] !== null &&
Object.keys(obj[k]).length > 0
)
Object.assign(acc, flattenObject(obj[k], pre + k));
else acc[pre + k] = obj[k];
return acc;
}, {});
flattenObject({ a: { b: { c: 1 } }, d: 1 }); // { 'a.b.c': 1, d: 1 }
flip
- title: flip
- tags: function,intermediate
Takes a function as an argument, then makes the first argument the last.
- Use argument destructuring and a closure with variadic arguments.
- Splice the first argument, using the spread operator (
...
), to make it the last before applying the rest.
const flip = fn => (first, ...rest) => fn(...rest, first);
let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b
forEachRight
- title: forEachRight
- tags: array,intermediate
Executes a provided function once for each array element, starting from the array's last element.
- Use
Array.prototype.slice()
to clone the given array andArray.prototype.reverse()
to reverse it. - Use
Array.prototype.forEach()
to iterate over the reversed array.
const forEachRight = (arr, callback) =>
arr
.slice()
.reverse()
.forEach(callback);
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'
forOwn
- title: forOwn
- tags: object,intermediate
Iterates over all own properties of an object, running a callback for each one.
- Use
Object.keys(obj)
to get all the properties of the object. - Use
Array.prototype.forEach()
to run the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const forOwn = (obj, fn) =>
Object.keys(obj).forEach(key => fn(obj[key], key, obj));
forOwn({ foo: 'bar', a: 1 }, v => console.log(v)); // 'bar', 1
forOwnRight
- title: forOwnRight
- tags: object,intermediate
Iterates over all own properties of an object in reverse, running a callback for each one.
- Use
Object.keys(obj)
to get all the properties of the object,Array.prototype.reverse()
to reverse their order. - Use
Array.prototype.forEach()
to run the provided function for each key-value pair. - The callback receives three arguments - the value, the key and the object.
const forOwnRight = (obj, fn) =>
Object.keys(obj)
.reverse()
.forEach(key => fn(obj[key], key, obj));
forOwnRight({ foo: 'bar', a: 1 }, v => console.log(v)); // 1, 'bar'
formToObject
- title: formToObject
- tags: browser,object,intermediate
Encodes a set of form elements as an object
.
- Use the
Foata
constructor to convert the HTMLform
toFoata
andArray.from()
to convert to an array. - Collect the object from the array using
Array.prototype.reduce()
.
const formToObject = form =>
Array.from(new Foata(form)).reduce(
(acc, [key, value]) => ({
...acc,
[key]: value
}),
{}
);
formToObject(document.querySelector('##form'));
// { email: 'test@email.com', name: 'Test Name' }
formatDuration
- title: formatDuration
- tags: date,math,string,intermediate
Returns the human-readable format of the given number of milliseconds.
- Divide
ms
with the appropriate values to obtain the appropriate values forday
,hour
,minute
,second
andmillisecond
. - Use
Object.entries()
withArray.prototype.filter()
to keep only non-zero values. - Use
Array.prototype.map()
to create the string for each value, pluralizing appropriately. - Use
String.prototype.join(', ')
to combine the values into a string.
const formatDuration = ms => {
if (ms < 0) ms = -ms;
const time = {
day: Math.floor(ms / 86400000),
hour: Math.floor(ms / 3600000) % 24,
minute: Math.floor(ms / 60000) % 60,
second: Math.floor(ms / 1000) % 60,
millisecond: Math.floor(ms) % 1000
};
return Object.entries(time)
.filter(val => val[1] !== 0)
.map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
.join(', ');
};
formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574);
// '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'
formatNumber
- title: formatNumber
- tags: string,math,beginner
Formats a number using the local number format order.
- Use
Number.prototype.toLocaleString()
to convert a number to using the local number format separators.
const formatNumber = num => num.toLocaleString();
formatNumber(123456); // '123,456' in `en-US`
formatNumber(15675436903); // '15.675.436.903' in `de-DE`
frequencies
- title: frequencies
- tags: array,object,intermediate
Creates an object with the unique values of an array as keys and their frequencies as the values.
- Use
Array.prototype.reduce()
to map unique values to an object's keys, adding to existing keys every time the same value is encountered.
const frequencies = arr =>
arr.reduce((a, v) => {
a[v] = a[v] ? a[v] + 1 : 1;
return a;
}, {});
frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // { a: 4, b: 2, c: 1 }
frequencies([...'ball']); // { b: 1, a: 1, l: 2 }
fromCamelCase
- title: fromCamelCase
- tags: string,intermediate
Converts a string from camelcase.
- Use
String.prototype.replace()
to break the string into words and add aseparator
between them. - Omit the second argument to use a default
separator
of_
.
const fromCamelCase = (str, separator = '_') =>
str
.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
.replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
.toLowerCase();
fromCamelCase('someDatabaseFieldName', ' '); // 'some database field name'
fromCamelCase('someLabelThatNeedsToBeDecamelized', '-');
// 'some-label-that-needs-to-be-decamelized'
fromCamelCase('someJavascriptProperty', '_'); // 'some_javascript_property'
fromCamelCase('JSONToCSV', '.'); // 'json.to.csv'
fromTimestamp
- title: fromTimestamp
- tags: date,beginner
Creates a Date
object from a Unix timestamp.
- Convert the timestamp to milliseconds by multiplying with
1000
. - Use
new Date()
to create a newDate
object.
const fromTimestamp = timestamp => new Date(timestamp * 1000);
fromTimestamp(1602162242); // 2020-10-08T13:04:02.000Z
frozenSet
- title: frozenSet
- tags: array,intermediate
Creates a frozen Set
object.
- Use the
new Set()
constructor to create a newSet
object fromiterable
. - Set the
add
,delete
andclear
methods of the newly created object toundefined
, so that they cannot be used, practically freezing the object.
const frozenSet = iterable => {
const s = new Set(iterable);
s.add = undefined;
s.delete = undefined;
s.clear = undefined;
return s;
};
frozenSet([1, 2, 3, 1, 2]);
// Set { 1, 2, 3, add: undefined, delete: undefined, clear: undefined }
fullscreen
- title: fullscreen
- tags: browser,intermediate
Opens or closes an element in fullscreen mode.
- Use
Document.querySelector()
andElement.requestFullscreen()
to open the given element in fullscreen. - Use
Document.exitFullscreen()
to exit fullscreen mode. - Omit the second argument,
el
, to usebody
as the default element. - Omit the first element,
mode
, to open the element in fullscreen mode by default.
const fullscreen = (mode = true, el = 'body') =>
mode
? document.querySelector(el).requestFullscreen()
: document.exitFullscreen();
fullscreen(); // Opens `body` in fullscreen mode
fullscreen(false); // Exits fullscreen mode
functionName
- title: functionName
- tags: function,beginner
Logs the name of a function.
- Use
console.debug()
and thename
property of the passed function to log the function's name to thedebug
channel of the console. - Return the given function
fn
.
const functionName = fn => (console.debug(fn.name), fn);
let m = functionName(Math.max)(5, 6);
// max (logged in debug channel of console)
// m = 6
functions
- title: functions
- tags: object,function,advanced
Gets an array of function property names from own (and optionally inherited) enumerable properties of an object.
- Use
Object.keys(obj)
to iterate over the object's own properties. - If
inherited
istrue
, useObject.getPrototypeOf(obj)
to also get the object's inherited properties. - Use
Array.prototype.filter()
to keep only those properties that are functions. - Omit the second argument,
inherited
, to not include inherited properties by default.
const functions = (obj, inherited = false) =>
(inherited
? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
: Object.keys(obj)
).filter(key => typeof obj[key] === 'function');
function Foo() {
this.a = () => 1;
this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']
gcd
- title: gcd
- tags: math,algorithm,recursion,intermediate
Calculates the greatest common divisor between two or more numbers/arrays.
- The inner
_gcd
function uses recursion. - Base case is when
y
equals0
. In this case, returnx
. - Otherwise, return the GCD of
y
and the remainder of the divisionx/y
.
const gcd = (...arr) => {
const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
return [...arr].reduce((a, b) => _gcd(a, b));
};
gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4
generateItems
- title: generateItems
- tags: array,function,intermediate
Generates an array with the given amount of items, using the given function.
- Use
Array.from()
to create an empty array of the specific length, callingfn
with the index of each newly created element. - The callback takes one argument - the index of each element.
const generateItems = (n, fn) => Array.from({ length: n }, (_, i) => fn(i));
generateItems(10, Math.random);
// [0.21, 0.08, 0.40, 0.96, 0.96, 0.24, 0.19, 0.96, 0.42, 0.70]
generatorToArray
- title: generatorToArray
- tags: function,array,generator,beginner
Converts the output of a generator function to an array.
- Use the spread operator (
...
) to convert the output of the generator function to an array.
const generatorToArray = gen => [...gen];
const s = new Set([1, 2, 1, 3, 1, 4]);
generatorToArray(s.entries()); // [[ 1, 1 ], [ 2, 2 ], [ 3, 3 ], [ 4, 4 ]]
geometricProgression
- title: geometricProgression
- tags: math,algorithm,intermediate
Initializes an array containing the numbers in the specified range where start
and end
are inclusive and the ratio between two terms is step
.
Returns an error if step
equals 1
.
- Use
Array.from()
,Math.log()
andMath.floor()
to create an array of the desired length,Array.prototype.map()
to fill with the desired values in a range. - Omit the second argument,
start
, to use a default value of1
. - Omit the third argument,
step
, to use a default value of2
.
const geometricProgression = (end, start = 1, step = 2) =>
Array.from({
length: Math.floor(Math.log(end / start) / Math.log(step)) + 1,
}).map((_, i) => start * step ** i);
geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]
get
- title: get
- tags: object,regexp,intermediate
Retrieves a set of properties indicated by the given selectors from an object.
- Use
Array.prototype.map()
for each selector,String.prototype.replace()
to replace square brackets with dots. - Use
String.prototype.split('.')
to split each selector. - Use
Array.prototype.filter()
to remove empty values andArray.prototype.reduce()
to get the value indicated by each selector.
const get = (from, ...selectors) =>
[...selectors].map(s =>
s
.replace(/\[([^\[\]]*)\]/g, '.$1.')
.split('.')
.filter(t => t !== '')
.reduce((prev, cur) => prev && prev[cur], from)
);
const obj = {
selector: { to: { val: 'val to select' } },
target: [1, 2, { a: 'test' }],
};
get(obj, 'selector.to.val', 'target[0]', 'target[2].a');
// ['val to select', 1, 'test']
getAncestors
- title: getAncestors
- tags: browser,beginner
Returns all the ancestors of an element from the document root to the given element.
- Use
Node.parentNode
and awhile
loop to move up the ancestor tree of the element. - Use
Array.prototype.unshift()
to add each new ancestor to the start of the array.
const getAncestors = el => {
let ancestors = [];
while (el) {
ancestors.unshift(el);
el = el.parentNode;
}
return ancestors;
};
getAncestors(document.querySelector('nav'));
// [document, html, body, header, nav]
getBaseURL
- title: getBaseURL
- tags: string,browser,regexp,beginner
Gets the current URL without any parameters or fragment identifiers.
- Use
String.prototype.replace()
with an appropriate regular expression to remove everything after either'?'
or'##'
, if found.
const getBaseURL = url => url.replace(/[?##].*$/, '');
getBaseURL('http://url.com/page?name=Adam&surname=Smith');
// 'http://url.com/page'
getColonTimeFrate
- title: getColonTimeFrate
- tags: date,string,beginner
Returns a string of the form HH:MM:SS
from a Date
object.
- Use
Date.prototype.toTimeString()
andString.prototype.slice()
to get theHH:MM:SS
part of a givenDate
object.
const getColonTimeFrate = date => date.toTimeString().slice(0, 8);
getColonTimeFrate(new Date()); // '08:38:00'
getDaysDiffBetweenDates
- title: getDaysDiffBetweenDates
- tags: date,intermediate
Calculates the difference (in days) between two dates.
- Subtract the two
Date
object and divide by the number of milliseconds in a day to get the difference (in days) between them.
const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
(dateFinal - dateInitial) / (1000 * 3600 * 24);
getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9
getElementsBiggerThanViewport
- title: getElementsBiggerThanViewport
- tags: browser,intermediate
Returns an array of HTML elements whose width is larger than that of the viewport's.
- Use
HTMLElement.offsetWidth
to get the width of thedocument
. - Use
Array.prototype.filter()
on the result ofDocument.querySelectorAll()
to check the width of all elements in the document.
const getElementsBiggerThanViewport = () => {
const docWidth = document.documentElement.offsetWidth;
return [...document.querySelectorAll('*')].filter(
el => el.offsetWidth > docWidth
);
};
getElementsBiggerThanViewport(); // <div id="ultra-wide-item" />
getImages
- title: getImages
- tags: browser,intermediate
Fetches all images from within an element and puts them into an array.
- Use
Element.getElementsByTagName()
to get all<img>
elements inside the provided element. - Use
Array.prototype.map()
to map everysrc
attribute of each<img>
element. - If
includeDuplicates
isfalse
, create a newSet
to eliminate duplicates and return it after spreading into an array. - Omit the second argument,
includeDuplicates
, to discard duplicates by default.
const getImages = (el, includeDuplicates = false) => {
const images = [...el.getElementsByTagName('img')].map(img =>
img.getAttribute('src')
);
return includeDuplicates ? images : [...new Set(images)];
};
getImages(document, true); // ['image1.jpg', 'image2.png', 'image1.png', '...']
getImages(document, false); // ['image1.jpg', 'image2.png', '...']
getMeridiemSuffixOfInteger
- title: getMeridiemSuffixOfInteger
- tags: date,beginner
Converts an integer to a suffixed string, adding am
or pm
based on its value.
- Use the modulo operator (
%
) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.
const getMeridiemSuffixOfInteger = num =>
num === 0 || num === 24
? 12 + 'am'
: num === 12
? 12 + 'pm'
: num < 12
? (num % 12) + 'am'
: (num % 12) + 'pm';
getMeridiemSuffixOfInteger(0); // '12am'
getMeridiemSuffixOfInteger(11); // '11am'
getMeridiemSuffixOfInteger(13); // '1pm'
getMeridiemSuffixOfInteger(25); // '1pm'
getMonthsDiffBetweenDates
- title: getMonthsDiffBetweenDates
- tags: date,intermediate
Calculates the difference (in months) between two dates.
- Use
Date.prototype.getFullYear()
andDate.prototype.getMonth()
to calculate the difference (in months) between twoDate
objects.
const getMonthsDiffBetweenDates = (dateInitial, dateFinal) =>
Math.max(
(dateFinal.getFullYear() - dateInitial.getFullYear()) * 12 +
dateFinal.getMonth() -
dateInitial.getMonth(),
0
);
getMonthsDiffBetweenDates(new Date('2017-12-13'), new Date('2018-04-29')); // 4
getParentsUntil
- title: getParentsUntil
- tags: browser,intermediate
Finds all the ancestors of an element up until the element matched by the specified selector.
- Use
Node.parentNode
and awhile
loop to move up the ancestor tree of the element. - Use
Array.prototype.unshift()
to add each new ancestor to the start of the array. - Use
Element.matches()
to check if the current element matches the specifiedselector
.
const getParentsUntil = (el, selector) => {
let parents = [],
_el = el.parentNode;
while (_el && typeof _el.matches === 'function') {
parents.unshift(_el);
if (_el.matches(selector)) return parents;
else _el = _el.parentNode;
}
return [];
};
getParentsUntil(document.querySelector('##home-link'), 'header');
// [header, nav, ul, li]
getProtocol
- title: getProtocol
- tags: browser,beginner
Gets the protocol being used on the current page.
- Use
Window.location.protocol
to get the protocol (http:
orhttps:
) of the current page.
const getProtocol = () => window.location.protocol;
getProtocol(); // 'https:'
getScrollPosition
- title: getScrollPosition
- tags: browser,intermediate
Returns the scroll position of the current page.
- Use
Window.pageXOffset
andWindow.pageYOffset
if they are defined, otherwiseElement.scrollLeft
andElement.scrollTop
. - Omit the single argument,
el
, to use a default value ofwindow
.
const getScrollPosition = (el = window) => ({
x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});
getScrollPosition(); // {x: 0, y: 200}
getSelectedText
- title: getSelectedText
- tags: browser,beginner
Gets the currently selected text.
- Use
Window.getSelection()
andSelection.toString()
to get the currently selected text.
const getSelectedText = () => window.getSelection().toString();
getSelectedText(); // 'Lorem ipsum'
getSiblings
- title: getSiblings
- tags: browser,intermediate
Returns an array containing all the siblings of the given element.
- Use
Node.parentNode
andNode.childNodes
to get aNodeList
of all the elements contained in the element's parent. - Use the spread operator (
...
) andArray.prototype.filter()
to convert to an array and remove the given element from it.
const getSiblings = el =>
[...el.parentNode.childNodes].filter(node => node !== el);
getSiblings(document.querySelector('head')); // ['body']
getStyle
- title: getStyle
- tags: browser,css,beginner
Retrieves the value of a CSS rule for the specified element.
- Use
Window.getComputedStyle()
to get the value of the CSS rule for the specified element.
const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];
getStyle(document.querySelector('p'), 'font-size'); // '16px'
getTimestamp
- title: getTimestamp
- tags: date,beginner
Gets the Unix timestamp from a Date
object.
- Use
Date.prototype.getTime()
to get the timestamp in milliseconds and divide by1000
to get the timestamp in seconds. - Use
Math.floor()
to appropriately round the resulting timestamp to an integer. - Omit the argument,
date
, to use the current date.
const getTimestamp = (date = new Date()) => Math.floor(date.getTime() / 1000);
getTimestamp(); // 1602162242
getType
- title: getType
- tags: type,beginner
Returns the native type of a value.
- Return
'undefined'
or'null'
if the value isundefined
ornull
. - Otherwise, use
Object.prototype.constructor.name
to get the name of the constructor.
const getType = v =>
(v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name);
getType(new Set([1, 2, 3])); // 'Set'
getURLParameters
- title: getURLParameters
- tags: browser,string,regexp,intermediate
Creates an object containing the parameters of the current URL.
- Use
String.prototype.match()
with an appropriate regular expression to get all key-value pairs. - Use
Array.prototype.reduce()
to map and combine them into a single object. - Pass
location.search
as the argument to apply to the currenturl
.
const getURLParameters = url =>
(url.match(/([^?=&]+)(=([^&]*))/g) || []).reduce(
(a, v) => (
(a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a
),
{}
);
getURLParameters('google.com'); // {}
getURLParameters('http://url.com/page?name=Adam&surname=Smith');
// {name: 'Adam', surname: 'Smith'}
getVerticalOffset
- title: getVerticalOffset
- tags: browser,beginner
Finds the distance from a given element to the top of the document.
- Use a
while
loop andHTMLElement.offsetParent
to move up the offset parents of the given element. - Add
HTMLElement.offsetTop
for each element and return the result.
const getVerticalOffset = el => {
let offset = el.offsetTop,
_el = el;
while (_el.offsetParent) {
_el = _el.offsetParent;
offset += _el.offsetTop;
}
return offset;
};
getVerticalOffset('.my-element'); // 120
groupBy
- title: groupBy
- tags: array,object,intermediate
Groups the elements of an array based on the given function.
- Use
Array.prototype.map()
to map the values of the array to a function or property name. - Use
Array.prototype.reduce()
to create an object, where the keys are produced from the mapped results.
const groupBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val, i) => {
acc[val] = (acc[val] || []).concat(arr[i]);
return acc;
}, {});
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}
hammingDistance
- title: hammingDistance
- tags: math,algorithm,intermediate
Calculates the Hamming distance between two values.
- Use the XOR operator (
^
) to find the bit difference between the two numbers. - Convert to a binary string using
Number.prototype.toString(2)
. - Count and return the number of
1
s in the string, usingString.prototype.match(/1/g)
.
const hammingDistance = (num1, num2) =>
((num1 ^ num2).toString(2).match(/1/g) || '').length;
hammingDistance(2, 3); // 1
hasClass
- title: hasClass
- tags: browser,css,beginner
Checks if the given element has the specified class.
- Use
Element.classList
andDOMTokenList.contains()
to check if the element has the specified class.
const hasClass = (el, className) => el.classList.contains(className);
hasClass(document.querySelector('p.special'), 'special'); // true
hasDuplicates
- title: hasDuplicates
- tags: array,beginner
Checks if there are duplicate values in a flat array.
- Use
Set()
to get the unique values in the array. - Use
Set.prototype.size
andArray.prototype.length
to check if the count of the unique values is the same as elements in the original array.
const hasDuplicates = arr => new Set(arr).size !== arr.length;
hasDuplicates([0, 1, 1, 2]); // true
hasDuplicates([0, 1, 2, 3]); // false
hasFlags
- title: hasFlags
- tags: node,intermediate
Checks if the current process's arguments contain the specified flags.
- Use
Array.prototype.every()
andArray.prototype.includes()
to check ifprocess.argv
contains all the specified flags. - Use a regular expression to test if the specified flags are prefixed with
-
or--
and prefix them accordingly.
const hasFlags = (...flags) =>
flags.every(flag =>
process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag)
);
// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // false
hasKey
- title: hasKey
- tags: object,intermediate
Checks if the target value exists in a JSON object.
- Check if
keys
is non-empty and useArray.prototype.every()
to sequentially check its keys to internal depth of the object,obj
. - Use
Object.prototype.hasOwnProperty()
to check ifobj
does not have the current key or is not an object, stop propagation and returnfalse
. - Otherwise assign the key's value to
obj
to use on the next iteration. - Return
false
beforehand if given key list is empty.
const hasKey = (obj, keys) => {
return (
keys.length > 0 &&
keys.every(key => {
if (typeof obj !== 'object' || !obj.hasOwnProperty(key)) return false;
obj = obj[key];
return true;
})
);
};
let obj = {
a: 1,
b: { c: 4 },
'b.d': 5
};
hasKey(obj, ['a']); // true
hasKey(obj, ['b']); // true
hasKey(obj, ['b', 'c']); // true
hasKey(obj, ['b.d']); // true
hasKey(obj, ['d']); // false
hasKey(obj, ['c']); // false
hasKey(obj, ['b', 'f']); // false
hashBrowser
- title: hashBrowser
- tags: browser,promise,advanced
Creates a hash for a value using the SHA-256 algorithm. Returns a promise.
- Use the SubtleCrypto API to create a hash for the given value.
- Create a new
TextEncoder
and use it to encodeval
, passing its value toSubtleCrypto.digest()
to generate a digest of the given data. - Use
DataView.prototype.getUint32()
to read data from the resolvedArrayBuffer
. - Add the data to an array using
Array.prototype.push()
after converting it to its hexadecimal representation usingNumber.prototype.toString(16)
. - Finally, use
Array.prototype.join()
to combine values in the array ofhexes
into a string.
const hashBrowser = val =>
crypto.subtle
.digest('SHA-256', new TextEncoder('utf-8').encode(val))
.then(h => {
let hexes = [],
view = new DataView(h);
for (let i = 0; i < view.byteLength; i += 4)
hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
return hexes.join('');
});
hashBrowser(
JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })
).then(console.log);
// '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'
hashNode
- title: hashNode
- tags: node,promise,advanced
Creates a hash for a value using the SHA-256 algorithm. Returns a promise.
- Use
crypto.createHash()
to create aHash
object with the appropriate algorithm. - Use
hash.update()
to add the data fromval
to theHash
,hash.digest()
to calculate the digest of the data. - Use
setTimeout()
to prevent blocking on a long operation, and return aPromise
to give it a familiar interface.
const crypto = require('crypto');
const hashNode = val =>
new Promise(resolve =>
setTimeout(
() => resolve(crypto.createHash('sha256').update(val).digest('hex')),
0
)
);
hashNode(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(
console.log
);
// '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'
haveSameContents
- title: haveSameContents
- tags: array,intermediate
Checks if two arrays contain the same elements regardless of order.
- Use a
for...of
loop over aSet
created from the values of both arrays. - Use
Array.prototype.filter()
to compare the amount of occurrences of each distinct value in both arrays. - Return
false
if the counts do not match for any element,true
otherwise.
const haveSameContents = (a, b) => {
for (const v of new Set([...a, ...b]))
if (a.filter(e => e === v).length !== b.filter(e => e === v).length)
return false;
return true;
};
haveSameContents([1, 2, 4], [2, 4, 1]); // true
head
- title: head
- tags: array,beginner
Returns the head of an array.
- Check if
arr
is truthy and has alength
property. - Use
arr[0]
if possible to return the first element, otherwise returnundefined
.
const head = arr => (arr && arr.length ? arr[0] : undefined);
head([1, 2, 3]); // 1
head([]); // undefined
head(null); // undefined
head(undefined); // undefined
heapsort
- title: heapsort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the heapsort algorithm.
- Use recursion.
- Use the spread operator (
...
) to clone the original array,arr
. - Use closures to declare a variable,
l
, and a functionheapify
. - Use a
for
loop andMath.floor()
in combination withheapify
to create a max heap from the array. - Use a
for
loop to repeatedly narrow down the considered range, usingheapify
and swapping values as necessary in order to sort the cloned array.
const heapsort = arr => {
const a = [...arr];
let l = a.length;
const heapify = (a, i) => {
const left = 2 * i + 1;
const right = 2 * i + 2;
let max = i;
if (left < l && a[left] > a[max]) max = left;
if (right < l && a[right] > a[max]) max = right;
if (max !== i) {
[a[max], a[i]] = [a[i], a[max]];
heapify(a, max);
}
};
for (let i = Math.floor(l / 2); i >= 0; i -= 1) heapify(a, i);
for (i = a.length - 1; i > 0; i--) {
[a[0], a[i]] = [a[i], a[0]];
l--;
heapify(a, 0);
}
return a;
};
heapsort([6, 3, 4, 1]); // [1, 3, 4, 6]
hexToRGB
- title: hexToRGB
- tags: string,math,advanced
Converts a color code to an rgb()
or rgba()
string if alpha value is provided.
- Use bitwise right-shift operator and mask bits with
&
(and) operator to convert a hexadecimal color code (with or without prefixed with##
) to a string with the RGB values. - If it's 3-digit color code, first convert to 6-digit version.
- If an alpha value is provided alongside 6-digit hex, give
rgba()
string in return.
const hexToRGB = hex => {
let alpha = false,
h = hex.slice(hex.startsWith('##') ? 1 : 0);
if (h.length === 3) h = [...h].map(x => x + x).join('');
else if (h.length === 8) alpha = true;
h = parseInt(h, 16);
return (
'rgb' +
(alpha ? 'a' : '') +
'(' +
(h >>> (alpha ? 24 : 16)) +
', ' +
((h & (alpha ? 0x00ff0000 : 0x00ff00)) >>> (alpha ? 16 : 8)) +
', ' +
((h & (alpha ? 0x0000ff00 : 0x0000ff)) >>> (alpha ? 8 : 0)) +
(alpha ? `, ${h & 0x000000ff}` : '') +
')'
);
};
hexToRGB('##27ae60ff'); // 'rgba(39, 174, 96, 255)'
hexToRGB('27ae60'); // 'rgb(39, 174, 96)'
hexToRGB('##fff'); // 'rgb(255, 255, 255)'
hide
- title: hide
- tags: browser,css,beginner
Hides all the elements specified.
- Use
NodeList.prototype.forEach()
to applydisplay: none
to each element specified.
const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));
hide(document.querySelectorAll('img')); // Hides all <img> elements on the page
httpDelete
- title: httpDelete
- tags: browser,intermediate
Makes a DELETE
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aDELETE
request to the givenurl
. - Handle the
onload
event, by running the providedcallback
function. - Handle the
onerror
event, by running the providederr
function. - Omit the third argument,
err
to log the request to the console's error stream by default.
const httpDelete = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('DELETE', url, true);
request.onload = () => callback(request);
request.onerror = () => err(request);
request.send();
};
httpDelete('https://jsonplaceholder.typicode.com/posts/1', request => {
console.log(request.responseText);
}); // Logs: {}
httpGet
- title: httpGet
- tags: browser,intermediate
Makes a GET
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aGET
request to the givenurl
. - Handle the
onload
event, by calling the givencallback
theresponseText
. - Handle the
onerror
event, by running the providederr
function. - Omit the third argument,
err
, to log errors to the console'serror
stream by default.
const httpGet = (url, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('GET', url, true);
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send();
};
httpGet(
'https://jsonplaceholder.typicode.com/posts/1',
console.log
); /*
Logs: {
"userId": 1,
"id": 1,
"title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
"body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}
*/
httpPost
- title: httpPost
- tags: browser,intermediate
Makes a POST
request to the passed URL.
- Use the
XMLHttpRequest
web API to make aPOST
request to the givenurl
. - Set the value of an
HTTP
request header withsetRequestHeader
method. - Handle the
onload
event, by calling the givencallback
theresponseText
. - Handle the
onerror
event, by running the providederr
function. - Omit the fourth argument,
err
, to log errors to the console'serror
stream by default.
const httpPost = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('POST', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request.responseText);
request.onerror = () => err(request);
request.send(data);
};
const newPost = {
userId: 1,
id: 1337,
- title: 'Foo',
body: 'bar bar bar'
};
const data = JSON.stringify(newPost);
httpPost(
'https://jsonplaceholder.typicode.com/posts',
data,
console.log
); /*
Logs: {
"userId": 1,
"id": 1337,
"title": "Foo",
"body": "bar bar bar"
}
*/
httpPost(
'https://jsonplaceholder.typicode.com/posts',
null, // does not send a body
console.log
); /*
Logs: {
"id": 101
}
*/
httpPut
- title: httpPut
- tags: browser,intermediate
Makes a PUT
request to the passed URL.
- Use
XMLHttpRequest
web api to make aPUT
request to the givenurl
. - Set the value of an
HTTP
request header withsetRequestHeader
method. - Handle the
onload
event, by running the providedcallback
function. - Handle the
onerror
event, by running the providederr
function. - Omit the last argument,
err
to log the request to the console's error stream by default.
const httpPut = (url, data, callback, err = console.error) => {
const request = new XMLHttpRequest();
request.open('PUT', url, true);
request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
request.onload = () => callback(request);
request.onerror = () => err(request);
request.send(data);
};
const password = 'fooBaz';
const data = JSON.stringify({
id: 1,
- title: 'foo',
body: 'bar',
userId: 1
});
httpPut('https://jsonplaceholder.typicode.com/posts/1', data, request => {
console.log(request.responseText);
}); /*
Logs: {
id: 1,
- title: 'foo',
body: 'bar',
userId: 1
}
*/
httpsRedirect
- title: httpsRedirect
- tags: browser,intermediate
Redirects the page to HTTPS if it's currently in HTTP.
- Use
location.protocol
to get the protocol currently being used. - If it's not HTTPS, use
location.replace()
to replace the existing page with the HTTPS version of the page. - Use
location.href
to get the full address, split it withString.prototype.split()
and remove the protocol part of the URL. - Note that pressing the back button doesn't take it back to the HTTP page as its replaced in the history.
const httpsRedirect = () => {
if (location.protocol !== 'https:')
location.replace('https://' + location.href.split('//')[1]);
};
httpsRedirect();
// If you are on http://mydomain.com, you are redirected to https://mydomain.com
hz
- title: hz
- tags: function,intermediate unlisted: true
Measures the number of times a function is executed per second (hz
/hertz
).
- Use
performance.now()
to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the functioniterations
times. - Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed.
- Omit the second argument,
iterations
, to use the default of 100 iterations.
const hz = (fn, iterations = 100) => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return (1000 * iterations) / (performance.now() - before);
};
const numbers = Array(10000).fill().map((_, i) => i);
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
let sum = 0;
for (let i = 0; i < numbers.length; i++) sum += numbers[i];
return sum;
};
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784
inRange
- title: inRange
- tags: math,beginner
Checks if the given number falls within the given range.
- Use arithmetic comparison to check if the given number is in the specified range.
- If the second argument,
end
, is not specified, the range is considered to be from0
tostart
.
const inRange = (n, start, end = null) => {
if (end && start > end) [end, start] = [start, end];
return end == null ? n >= 0 && n < start : n >= start && n < end;
};
inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // false
includesAll
- title: includesAll
- tags: array,beginner
Checks if all the elements in values
are included in arr
.
- Use
Array.prototype.every()
andArray.prototype.includes()
to check if all elements ofvalues
are included inarr
.
const includesAll = (arr, values) => values.every(v => arr.includes(v));
includesAll([1, 2, 3, 4], [1, 4]); // true
includesAll([1, 2, 3, 4], [1, 5]); // false
includesAny
- title: includesAny
- tags: array,beginner
Checks if at least one element of values
is included in arr
.
- Use
Array.prototype.some()
andArray.prototype.includes()
to check if at least one element ofvalues
is included inarr
.
const includesAny = (arr, values) => values.some(v => arr.includes(v));
includesAny([1, 2, 3, 4], [2, 9]); // true
includesAny([1, 2, 3, 4], [8, 9]); // false
indentString
- title: indentString
- tags: string,beginner
Indents each line in the provided string.
- Use
String.prototype.replace()
and a regular expression to add the character specified byindent
count
times at the start of each line. - Omit the third argument,
indent
, to use a default indentation character of' '
.
const indentString = (str, count, indent = ' ') =>
str.replace(/^/gm, indent.repeat(count));
indentString('Lorem\nIpsum', 2); // ' Lorem\n Ipsum'
indentString('Lorem\nIpsum', 2, '_'); // '__Lorem\n__Ipsum'
indexOfAll
- title: indexOfAll
- tags: array,intermediate
Finds all indexes of val
in an array.
If val
never occurs, returns an empty array.
- Use
Array.prototype.reduce()
to loop over elements and store indexes for matching elements.
const indexOfAll = (arr, val) =>
arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3]
indexOfAll([1, 2, 3], 4); // []
indexOfSubstrings
- title: indexOfSubstrings
- tags: string,algorithm,generator,intermediate
Finds all the indexes of a substring in a given string.
- Use
Array.prototype.indexOf()
to look forsearchValue
instr
. - Use
yield
to return the index if the value is found and update the index,i
. - Use a
while
loop that will terminate the generator as soon as the value returned fromArray.prototype.indexOf()
is-1
.
const indexOfSubstrings = function* (str, searchValue) {
let i = 0;
while (true) {
const r = str.indexOf(searchValue, i);
if (r !== -1) {
yield r;
i = r + 1;
} else return;
}
};
[...indexOfSubstrings('tiktok tok tok tik tok tik', 'tik')]; // [0, 15, 23]
[...indexOfSubstrings('tutut tut tut', 'tut')]; // [0, 2, 6, 10]
[...indexOfSubstrings('hello', 'hi')]; // []
initial
- title: initial
- tags: array,beginner
Returns all the elements of an array except the last one.
- Use
Array.prototype.slice(0, -1)
to return all but the last element of the array.
const initial = arr => arr.slice(0, -1);
initial([1, 2, 3]); // [1, 2]
initialize2DArray
- title: initialize2DArray
- tags: array,intermediate
Initializes a 2D array of given width and height and value.
- Use
Array.from()
andArray.prototype.map()
to generateh
rows where each is a new array of sizew
. - Use
Array.prototype.fill()
to initialize all items with valueval
. - Omit the last argument,
val
, to use a default value ofnull
.
const initialize2DArray = (w, h, val = null) =>
Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));
initialize2DArray(2, 2, 0); // [[0, 0], [0, 0]]
initializeArrayWithRange
- title: initializeArrayWithRange
- tags: array,intermediate
Initializes an array containing the numbers in the specified range where start
and end
are inclusive with their common difference step
.
- Use
Array.from()
to create an array of the desired length. - Use
(end - start + 1)/step
and a map function to fill the array with the desired values in the given range. - Omit the second argument,
start
, to use a default value of0
. - Omit the last argument,
step
, to use a default value of1
.
const initializeArrayWithRange = (end, start = 0, step = 1) =>
Array.from(
{ length: Math.ceil((end - start + 1) / step) },
(_, i) => i * step + start
);
initializeArrayWithRange(5); // [0, 1, 2, 3, 4, 5]
initializeArrayWithRange(7, 3); // [3, 4, 5, 6, 7]
initializeArrayWithRange(9, 0, 2); // [0, 2, 4, 6, 8]
initializeArrayWithRangeRight
- title: initializeArrayWithRangeRight
- tags: array,intermediate
Initializes an array containing the numbers in the specified range (in reverse) where start
and end
are inclusive with their common difference step
.
- Use
Array.from(Math.ceil((end+1-start)/step))
to create an array of the desired length(the amounts of elements is equal to(end-start)/step
or(end+1-start)/step
for inclusive end),Array.prototype.map()
to fill with the desired values in a range. - Omit the second argument,
start
, to use a default value of0
. - Omit the last argument,
step
, to use a default value of1
.
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
(v, i, arr) => (arr.length - i - 1) * step + start
);
initializeArrayWithRangeRight(5); // [5, 4, 3, 2, 1, 0]
initializeArrayWithRangeRight(7, 3); // [7, 6, 5, 4, 3]
initializeArrayWithRangeRight(9, 0, 2); // [8, 6, 4, 2, 0]
initializeArrayWithValues
- title: initializeArrayWithValues
- tags: array,intermediate
Initializes and fills an array with the specified values.
- Use
Array.from()
to create an array of the desired length,Array.prototype.fill()
to fill it with the desired values. - Omit the last argument,
val
, to use a default value of0
.
const initializeArrayWithValues = (n, val = 0) =>
Array.from({ length: n }).fill(val);
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]
initializeNDArray
- title: initializeNDArray
- tags: array,recursion,intermediate
Create a n-dimensional array with given value.
- Use recursion.
- Use
Array.from()
,Array.prototype.map()
to generate rows where each is a new array initialized usinginitializeNDArray()
.
const initializeNDArray = (val, ...args) =>
args.length === 0
? val
: Array.from({ length: args[0] }).map(() =>
initializeNDArray(val, ...args.slice(1))
);
initializeNDArray(1, 3); // [1, 1, 1]
initializeNDArray(5, 2, 2, 2); // [[[5, 5], [5, 5]], [[5, 5], [5, 5]]]
injectCSS
- title: injectCSS
- tags: browser,css,intermediate
Injects the given CSS code into the current document
- Use
Document.createElement()
to create a newstyle
element and set its type totext/css
. - Use
Element.innerText
to set the value to the given CSS string. - Use
Document.head
andElement.appendChild()
to append the new element to the document head. - Return the newly created
style
element.
const injectCSS = css => {
let el = document.createElement('style');
el.type = 'text/css';
el.innerText = css;
document.head.appendChild(el);
return el;
};
injectCSS('body { background-color: ##000 }');
// '<style type="text/css">body { background-color: ##000 }</style>'
insertAfter
- title: insertAfter
- tags: browser,beginner
Inserts an HTML string after the end of the specified element.
- Use
Element.insertAdjacentHTML()
with a position of'afterend'
to parsehtmlString
and insert it after the end ofel
.
const insertAfter = (el, htmlString) =>
el.insertAdjacentHTML('afterend', htmlString);
insertAfter(document.getElementById('myId'), '<p>after</p>');
// <div id="myId">...</div> <p>after</p>
insertAt
- title: insertAt
- tags: array,intermediate
Mutates the original array to insert the given values after the specified index.
- Use
Array.prototype.splice()
with an appropriate index and a delete count of0
, spreading the given values to be inserted.
const insertAt = (arr, i, ...v) => {
arr.splice(i + 1, 0, ...v);
return arr;
};
let myArray = [1, 2, 3, 4];
insertAt(myArray, 2, 5); // myArray = [1, 2, 3, 5, 4]
let otherArray = [2, 10];
insertAt(otherArray, 0, 4, 6, 8); // otherArray = [2, 4, 6, 8, 10]
insertBefore
- title: insertBefore
- tags: browser,beginner
Inserts an HTML string before the start of the specified element.
- Use
Element.insertAdjacentHTML()
with a position of'beforebegin'
to parsehtmlString
and insert it before the start ofel
.
const insertBefore = (el, htmlString) =>
el.insertAdjacentHTML('beforebegin', htmlString);
insertBefore(document.getElementById('myId'), '<p>before</p>');
// <p>before</p> <div id="myId">...</div>
insertionSort
- title: insertionSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the insertion sort algorithm.
- Use
Array.prototype.reduce()
to iterate over all the elements in the given array. - If the
length
of the accumulator is0
, add the current element to it. - Use
Array.prototype.some()
to iterate over the results in the accumulator until the correct position is found. - Use
Array.prototype.splice()
to insert the current element into the accumulator.
const insertionSort = arr =>
arr.reduce((acc, x) => {
if (!acc.length) return [x];
acc.some((y, j) => {
if (x <= y) {
acc.splice(j, 0, x);
return true;
}
if (x > y && j === acc.length - 1) {
acc.splice(j + 1, 0, x);
return true;
}
return false;
});
return acc;
}, []);
insertionSort([6, 3, 4, 1]); // [1, 3, 4, 6]
intersection
- title: intersection
- tags: array,intermediate
Returns the elements that exist in both arrays, filtering duplicate values.
- Create a
Set
fromb
, then useArray.prototype.filter()
ona
to only keep values contained inb
.
const intersection = (a, b) => {
const s = new Set(b);
return [...new Set(a)].filter(x => s.has(x));
};
intersection([1, 2, 3], [4, 3, 2]); // [2, 3]
intersectionBy
- title: intersectionBy
- tags: array,intermediate
Returns the elements that exist in both arrays, after applying the provided function to each array element of both.
- Create a
Set
by applyingfn
to all elements inb
. - Use
Array.prototype.filter()
ona
to only keep elements, which produce values contained inb
whenfn
is applied to them.
const intersectionBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return [...new Set(a)].filter(x => s.has(fn(x)));
};
intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]
intersectionBy(
[{ - title: 'Apple' }, { - title: 'Orange' }],
[{ - title: 'Orange' }, { - title: 'Melon' }],
x => x.title
); // [{ - title: 'Orange' }]
intersectionWith
- title: intersectionWith
- tags: array,intermediate
Returns the elements that exist in both arrays, using a provided comparator function.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
in combination with the provided comparator to determine intersecting values.
const intersectionWith = (a, b, comp) =>
a.filter(x => b.findIndex(y => comp(x, y)) !== -1);
intersectionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1.5, 3, 0]
invertKeyValues
- title: invertKeyValues
- tags: object,advanced
Inverts the key-value pairs of an object, without mutating it.
- Use
Object.keys()
andArray.prototype.reduce()
to invert the key-value pairs of an object and apply the function provided (if any). - Omit the second argument,
fn
, to get the inverted keys without applying a function to them. - The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.
const invertKeyValues = (obj, fn) =>
Object.keys(obj).reduce((acc, key) => {
const val = fn ? fn(obj[key]) : obj[key];
acc[val] = acc[val] || [];
acc[val].push(key);
return acc;
}, {});
invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value);
// { group1: [ 'a', 'c' ], group2: [ 'b' ] }
is
- title: is
- tags: type,array,intermediate
Checks if the provided value is of the specified type.
- Ensure the value is not
undefined
ornull
usingArray.prototype.includes()
. - Compare the
constructor
property on the value withtype
to check if the provided value is of the specifiedtype
.
const is = (type, val) => ![, null].includes(val) && val.constructor === type;
is(Array, [1]); // true
is(ArrayBuffer, new ArrayBuffer()); // true
is(Map, new Map()); // true
is(RegExp, /./g); // true
is(Set, new Set()); // true
is(WeakMap, new WeakMap()); // true
is(WeakSet, new WeakSet()); // true
is(String, ''); // true
is(String, new String('')); // true
is(Number, 1); // true
is(Number, new Number(1)); // true
is(Boolean, true); // true
is(Boolean, new Boolean(true)); // true
isAbsoluteURL
- title: isAbsoluteURL
- tags: string,browser,regexp,intermediate
Checks if the given string is an absolute URL.
- Use
RegExp.prototype.test()
to test if the string is an absolute URL.
const isAbsoluteURL = str => /^[a-z][a-z0-9+.-]*:/.test(str);
isAbsoluteURL('https://google.com'); // true
isAbsoluteURL('ftp://www.myserver.net'); // true
isAbsoluteURL('/foo/bar'); // false
isAfterDate
- title: isAfterDate
- tags: date,beginner
Checks if a date is after another date.
- Use the greater than operator (
>
) to check if the first date comes after the second one.
const isAfterDate = (dateA, dateB) => dateA > dateB;
isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // true
isAlpha
- title: isAlpha
- tags: string,regexp,beginner
Checks if a string contains only alpha characters.
- Use
RegExp.prototype.test()
to check if the given string matches against the alphabetic regexp pattern.
const isAlpha = str => /^[a-zA-Z]*$/.test(str);
isAlpha('sampleInput'); // true
isAlpha('this Will fail'); // false
isAlpha('123'); // false
isAlphaNumeric
- title: isAlphaNumeric
- tags: string,regexp,beginner
Checks if a string contains only alphanumeric characters.
- Use
RegExp.prototype.test()
to check if the input string matches against the alphanumeric regexp pattern.
const isAlphaNumeric = str => /^[a-z0-9]+$/gi.test(str);
isAlphaNumeric('hello123'); // true
isAlphaNumeric('123'); // true
isAlphaNumeric('hello 123'); // false (space character is not alphanumeric)
isAlphaNumeric('##$hello'); // false
isAnagram
- title: isAnagram
- tags: string,regexp,intermediate
Checks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).
- Use
String.prototype.toLowerCase()
andString.prototype.replace()
with an appropriate regular expression to remove unnecessary characters. - Use
String.prototype.split('')
,Array.prototype.sort()
andArray.prototype.join('')
on both strings to normalize them, then check if their normalized forms are equal.
const isAnagram = (str1, str2) => {
const normalize = str =>
str
.toLowerCase()
.replace(/[^a-z0-9]/gi, '')
.split('')
.sort()
.join('');
return normalize(str1) === normalize(str2);
};
isAnagram('iceman', 'cinema'); // true
isArrayLike
- title: isArrayLike
- tags: type,array,intermediate
Checks if the provided argument is array-like (i.e. is iterable).
- Check if the provided argument is not
null
and that itsSymbol.iterator
property is a function.
const isArrayLike = obj =>
obj != null && typeof obj[Symbol.iterator] === 'function';
isArrayLike([1, 2, 3]); // true
isArrayLike(document.querySelectorAll('.className')); // true
isArrayLike('abc'); // true
isArrayLike(null); // false
isAsyncFunction
- title: isAsyncFunction
- tags: type,function,intermediate
Checks if the given argument is an async
function.
- Use
Object.prototype.toString()
andFunction.prototype.call()
and check if the result is'[object AsyncFunction]'
.
const isAsyncFunction = val =>
Object.prototype.toString.call(val) === '[object AsyncFunction]';
isAsyncFunction(function() {}); // false
isAsyncFunction(async function() {}); // true
isBeforeDate
- title: isBeforeDate
- tags: date,beginner
Checks if a date is before another date.
- Use the less than operator (
<
) to check if the first date comes before the second one.
const isBeforeDate = (dateA, dateB) => dateA < dateB;
isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // true
isBetweenDates
- title: isBetweenDates
- tags: date,beginner
Checks if a date is between two other dates.
- Use the greater than (
>
) and less than (<
) operators to check ifdate
is betweendateStart
anddateEnd
.
const isBetweenDates = (dateStart, dateEnd, date) =>
date > dateStart && date < dateEnd;
isBetweenDates(
new Date(2010, 11, 20),
new Date(2010, 11, 30),
new Date(2010, 11, 19)
); // false
isBetweenDates(
new Date(2010, 11, 20),
new Date(2010, 11, 30),
new Date(2010, 11, 25)
); // true
isBoolean
- title: isBoolean
- tags: type,beginner
Checks if the given argument is a native boolean element.
- Use
typeof
to check if a value is classified as a boolean primitive.
const isBoolean = val => typeof val === 'boolean';
isBoolean(null); // false
isBoolean(false); // true
isBrowser
- title: isBrowser
- tags: browser,node,intermediate
Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.
- Use
Array.prototype.includes()
on thetypeof
values of bothwindow
anddocument
(globals usually only available in a browser environment unless they were explicitly defined), which will returntrue
if one of them isundefined
. typeof
allows globals to be checked for existence without throwing aReferenceError
.- If both of them are not
undefined
, then the current environment is assumed to be a browser.
const isBrowser = () => ![typeof window, typeof document].includes('undefined');
isBrowser(); // true (browser)
isBrowser(); // false (Node)
isBrowserTabFocused
- title: isBrowserTabFocused
- tags: browser,beginner
Checks if the browser tab of the page is focused.
- Use the
Document.hidden
property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.
const isBrowserTabFocused = () => !document.hidden;
isBrowserTabFocused(); // true
isContainedIn
- title: isContainedIn
- tags: array,intermediate
Checks if the elements of the first array are contained in the second one regardless of order.
- Use a
for...of
loop over aSet
created from the first array. - Use
Array.prototype.some()
to check if all distinct values are contained in the second array. - Use
Array.prototype.filter()
to compare the number of occurrences of each distinct value in both arrays. - Return
false
if the count of any element is greater in the first array than the second one,true
otherwise.
const isContainedIn = (a, b) => {
for (const v of new Set(a)) {
if (
!b.some(e => e === v) ||
a.filter(e => e === v).length > b.filter(e => e === v).length
)
return false;
}
return true;
};
isContainedIn([1, 4], [2, 4, 1]); // true
isDateValid
- title: isDateValid
- tags: date,intermediate
Checks if a valid date object can be created from the given values.
- Use the spread operator (
...
) to pass the array of arguments to theDate
constructor. - Use
Date.prototype.valueOf()
andNumber.isNaN()
to check if a validDate
object can be created from the given values.
const isDateValid = (...val) => !Number.isNaN(new Date(...val).valueOf());
isDateValid('December 17, 1995 03:24:00'); // true
isDateValid('1995-12-17T03:24:00'); // true
isDateValid('1995-12-17 T03:24:00'); // false
isDateValid('Duck'); // false
isDateValid(1995, 11, 17); // true
isDateValid(1995, 11, 17, 'Duck'); // false
isDateValid({}); // false
isDeepFrozen
- title: isDeepFrozen
- tags: object,recursion,intermediate
Checks if an object is deeply frozen.
- Use recursion.
- Use
Object.isFrozen()
on the given object. - Use
Object.keys()
,Array.prototype.every()
to check that all keys are either deeply frozen objects or non-object values.
const isDeepFrozen = obj =>
Object.isFrozen(obj) &&
Object.keys(obj).every(
prop => typeof obj[prop] !== 'object' || isDeepFrozen(obj[prop])
);
const x = Object.freeze({ a: 1 });
const y = Object.freeze({ b: { c: 2 } });
isDeepFrozen(x); // true
isDeepFrozen(y); // false
isDisjoint
- title: isDisjoint
- tags: array,intermediate
Checks if the two iterables are disjointed (have no common values).
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that the two iterables have no common values.
const isDisjoint = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sA].every(v => !sB.has(v));
};
isDisjoint(new Set([1, 2]), new Set([3, 4])); // true
isDisjoint(new Set([1, 2]), new Set([1, 3])); // false
isDivisible
- title: isDivisible
- tags: math,beginner
Checks if the first numeric argument is divisible by the second one.
- Use the modulo operator (
%
) to check if the remainder is equal to0
.
const isDivisible = (dividend, divisor) => dividend % divisor === 0;
isDivisible(6, 3); // true
isDuplexStream
- title: isDuplexStream
- tags: node,type,intermediate
Checks if the given argument is a duplex (readable and writable) stream.
- Check if the value is different from
null
. - Use
typeof
to check if a value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_read
,_write
and_readableState
,_writableState
properties arefunction
andobject
respectively.
const isDuplexStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';
const Stream = require('stream');
isDuplexStream(new Stream.Duplex()); // true
isEmpty
- title: isEmpty
- tags: type,array,object,string,beginner
Checks if the a value is an empty object/collection, has no enumerable properties or is any type that is not considered a collection.
- Check if the provided value is
null
or if itslength
is equal to0
.
const isEmpty = val => val == null || !(Object.keys(val) || val).length;
isEmpty([]); // true
isEmpty({}); // true
isEmpty(''); // true
isEmpty([1, 2]); // false
isEmpty({ a: 1, b: 2 }); // false
isEmpty('text'); // false
isEmpty(123); // true - type is not considered a collection
isEmpty(true); // true - type is not considered a collection
isEven
- title: isEven
- tags: math,beginner
Checks if the given number is even.
- Checks whether a number is odd or even using the modulo (
%
) operator. - Returns
true
if the number is even,false
if the number is odd.
const isEven = num => num % 2 === 0;
isEven(3); // false
isFunction
- title: isFunction
- tags: type,function,beginner
Checks if the given argument is a function.
- Use
typeof
to check if a value is classified as a function primitive.
const isFunction = val => typeof val === 'function';
isFunction('x'); // false
isFunction(x => x); // true
isGeneratorFunction
- title: isGeneratorFunction
- tags: type,function,intermediate
Checks if the given argument is a generator function.
- Use
Object.prototype.toString()
andFunction.prototype.call()
and check if the result is'[object GeneratorFunction]'
.
const isGeneratorFunction = val =>
Object.prototype.toString.call(val) === '[object GeneratorFunction]';
isGeneratorFunction(function() {}); // false
isGeneratorFunction(function*() {}); // true
isISOString
- title: isISOString
- tags: date,intermediate
Checks if the given string is valid in the simplified extended ISO format (ISO 8601).
- Use
new Date()
to create a date object from the given string. - Use
Date.prototype.valueOf()
andNumber.isNaN()
to check if the produced date object is valid. - Use
Date.prototype.toISOString()
to compare the ISO formatted string representation of the date with the original string.
const isISOString = val => {
const d = new Date(val);
return !Number.isNaN(d.valueOf()) && d.toISOString() === val;
};
isISOString('2020-10-12T10:10:10.000Z'); // true
isISOString('2020-10-12'); // false
isLeapYear
- title: isLeapYear
- tags: date,beginner
Checks if the given year
is a leap year.
- Use
new Date()
, setting the date to February 29th of the givenyear
. - Use
Date.prototype.getMonth()
to check if the month is equal to1
.
const isLeapYear = year => new Date(year, 1, 29).getMonth() === 1;
isLeapYear(2019); // false
isLeapYear(2020); // true
isLocalStorageEnabled
- title: isLocalStorageEnabled
- tags: browser,intermediate
Checks if localStorage
is enabled.
- Use a
try...catch
block to returntrue
if all operations complete successfully,false
otherwise. - Use
Storage.setItem()
andStorage.removeItem()
to test storing and deleting a value inwindow.localStorage
.
const isLocalStorageEnabled = () => {
try {
const key = `__storage__test`;
window.localStorage.setItem(key, null);
window.localStorage.removeItem(key);
return true;
} catch (e) {
return false;
}
};
isLocalStorageEnabled(); // true, if localStorage is accessible
isLowerCase
- title: isLowerCase
- tags: string,beginner
Checks if a string is lower case.
- Convert the given string to lower case, using
String.prototype.toLowerCase()
and compare it to the original.
const isLowerCase = str => str === str.toLowerCase();
isLowerCase('abc'); // true
isLowerCase('a3@$'); // true
isLowerCase('Ab4'); // false
isNegativeZero
- title: isNegativeZero
- tags: math,intermediate
Checks if the given value is equal to negative zero (-0
).
- Check whether a passed value is equal to
0
and if1
divided by the value equals-Infinity
.
const isNegativeZero = val => val === 0 && 1 / val === -Infinity;
isNegativeZero(-0); // true
isNegativeZero(0); // false
isNil
- title: isNil
- tags: type,beginner
Checks if the specified value is null
or undefined
.
- Use the strict equality operator to check if the value of
val
is equal tonull
orundefined
.
const isNil = val => val === undefined || val === null;
isNil(null); // true
isNil(undefined); // true
isNil(''); // false
isNode
- title: isNode
- tags: node,browser,intermediate
Determines if the current runtime environment is Node.js.
- Use the
process
global object that provides information about the current Node.js process. - Check if
process
is defined andprocess.versions
,process.versions.node
are notnull
.
const isNode = () =>
typeof process !== 'undefined' &&
process.versions !== null &&
process.versions.node !== null;
isNode(); // true (Node)
isNode(); // false (browser)
isNull
- title: isNull
- tags: type,beginner
Checks if the specified value is null
.
- Use the strict equality operator to check if the value of
val
is equal tonull
.
const isNull = val => val === null;
isNull(null); // true
isNumber
- title: isNumber
- tags: type,math,beginner
Checks if the given argument is a number.
- Use
typeof
to check if a value is classified as a number primitive. - To safeguard against
NaN
, check ifval === val
(asNaN
has atypeof
equal tonumber
and is the only value not equal to itself).
const isNumber = val => typeof val === 'number' && val === val;
isNumber(1); // true
isNumber('1'); // false
isNumber(NaN); // false
isObject
- title: isObject
- tags: type,object,beginner
Checks if the passed value is an object or not.
- Uses the
Object
constructor to create an object wrapper for the given value. - If the value is
null
orundefined
, create and return an empty object. - Otherwise, return an object of a type that corresponds to the given value.
const isObject = obj => obj === Object(obj);
isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // false
isObjectLike
- title: isObjectLike
- tags: type,object,beginner
Checks if a value is object-like.
- Check if the provided value is not
null
and itstypeof
is equal to'object'
.
const isObjectLike = val => val !== null && typeof val === 'object';
isObjectLike({}); // true
isObjectLike([1, 2, 3]); // true
isObjectLike(x => x); // false
isObjectLike(null); // false
isOdd
- title: isOdd
- tags: math,beginner
Checks if the given number is odd.
- Check whether a number is odd or even using the modulo (
%
) operator. - Return
true
if the number is odd,false
if the number is even.
const isOdd = num => num % 2 === 1;
isOdd(3); // true
isPlainObject
- title: isPlainObject
- tags: type,object,intermediate
Checks if the provided value is an object created by the Object constructor.
- Check if the provided value is truthy.
- Use
typeof
to check if it is an object andObject.prototype.constructor
to make sure the constructor is equal toObject
.
const isPlainObject = val =>
!!val && typeof val === 'object' && val.constructor === Object;
isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // false
isPowerOfTen
- title: isPowerOfTen
- tags: math,beginner
Checks if the given number is a power of 10
.
- Use
Math.log10()
and the modulo operator (%
) to determine ifn
is a power of10
.
const isPowerOfTen = n => Math.log10(n) % 1 === 0;
isPowerOfTen(1); // true
isPowerOfTen(10); // true
isPowerOfTen(20); // false
isPowerOfTwo
- title: isPowerOfTwo
- tags: math,beginner
Checks if the given number is a power of 2
.
- Use the bitwise binary AND operator (
&
) to determine ifn
is a power of2
. - Additionally, check that
n
is not falsy.
const isPowerOfTwo = n => !!n && (n & (n - 1)) == 0;
isPowerOfTwo(0); // false
isPowerOfTwo(1); // true
isPowerOfTwo(8); // true
isPrime
- title: isPrime
- tags: math,algorithm,beginner
Checks if the provided integer is a prime number.
- Check numbers from
2
to the square root of the given number. - Return
false
if any of them divides the given number, else returntrue
, unless the number is less than2
.
const isPrime = num => {
const boundary = Math.floor(Math.sqrt(num));
for (let i = 2; i <= boundary; i++) if (num % i === 0) return false;
return num >= 2;
};
isPrime(11); // true
isPrimitive
- title: isPrimitive
- tags: type,intermediate
Checks if the passed value is primitive or not.
- Create an object from
val
and compare it withval
to determine if the passed value is primitive (i.e. not equal to the created object).
const isPrimitive = val => Object(val) !== val;
isPrimitive(null); // true
isPrimitive(undefined); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // false
isPrimitive({}); // false
isPromiseLike
- title: isPromiseLike
- tags: type,function,promise,intermediate
Checks if an object looks like a Promise
.
- Check if the object is not
null
, itstypeof
matches eitherobject
orfunction
and if it has a.then
property, which is also afunction
.
const isPromiseLike = obj =>
obj !== null &&
(typeof obj === 'object' || typeof obj === 'function') &&
typeof obj.then === 'function';
isPromiseLike({
then: function() {
return '';
}
}); // true
isPromiseLike(null); // false
isPromiseLike({}); // false
isReadableStream
- title: isReadableStream
- tags: node,type,intermediate
Checks if the given argument is a readable stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_read
and_readableState
properties arefunction
andobject
respectively.
const isReadableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object';
const fs = require('fs');
isReadableStream(fs.createReadStream('test.txt')); // true
isSameDate
- title: isSameDate
- tags: date,beginner
Checks if a date is the same as another date.
- Use
Date.prototype.toISOString()
and strict equality checking (===
) to check if the first date is the same as the second one.
const isSameDate = (dateA, dateB) =>
dateA.toISOString() === dateB.toISOString();
isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // true
isSessionStorageEnabled
- title: isSessionStorageEnabled
- tags: browser,intermediate
Checks if sessionStorage
is enabled.
- Use a
try...catch
block to returntrue
if all operations complete successfully,false
otherwise. - Use
Storage.setItem()
andStorage.removeItem()
to test storing and deleting a value inwindow.sessionStorage
.
const isSessionStorageEnabled = () => {
try {
const key = `__storage__test`;
window.sessionStorage.setItem(key, null);
window.sessionStorage.removeItem(key);
return true;
} catch (e) {
return false;
}
};
isSessionStorageEnabled(); // true, if sessionStorage is accessible
isSorted
- title: isSorted
- tags: array,intermediate
Checks if a numeric array is sorted.
- Calculate the ordering
direction
for the first pair of adjacent array elements. - Return
0
if the given array is empty, only has one element or thedirection
changes for any pair of adjacent array elements. - Use
Math.sign()
to covert the final value ofdirection
to-1
(descending order) or1
(ascending order).
const isSorted = arr => {
if (arr.length <= 1) return 0;
const direction = arr[1] - arr[0];
for (let i = 2; i < arr.length; i++) {
if ((arr[i] - arr[i - 1]) * direction < 0) return 0;
}
return Math.sign(direction);
};
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0
isSorted([4]); // 0
isStream
- title: isStream
- tags: node,type,intermediate
Checks if the given argument is a stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
.
const isStream = val =>
val !== null && typeof val === 'object' && typeof val.pipe === 'function';
const fs = require('fs');
isStream(fs.createReadStream('test.txt')); // true
isString
- title: isString
- tags: type,string,beginner
Checks if the given argument is a string. Only works for string primitives.
- Use
typeof
to check if a value is classified as a string primitive.
const isString = val => typeof val === 'string';
isString('10'); // true
isSymbol
- title: isSymbol
- tags: type,beginner
Checks if the given argument is a symbol.
- Use
typeof
to check if a value is classified as a symbol primitive.
const isSymbol = val => typeof val === 'symbol';
isSymbol(Symbol('x')); // true
isTravisCI
- title: isTravisCI
- tags: node,intermediate
Checks if the current environment is Travis CI.
- Check if the current environment has the
TRAVIS
andCI
environment variables (reference).
const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;
isTravisCI(); // true (if code is running on Travis CI)
isUndefined
- title: isUndefined
- tags: type,beginner
Checks if the specified value is undefined
.
- Use the strict equality operator to check if
val
is equal toundefined
.
const isUndefined = val => val === undefined;
isUndefined(undefined); // true
isUpperCase
- title: isUpperCase
- tags: string,beginner
Checks if a string is upper case.
- Convert the given string to upper case, using
String.prototype.toUpperCase()
and compare it to the original.
const isUpperCase = str => str === str.toUpperCase();
isUpperCase('ABC'); // true
isUpperCase('A3@$'); // true
isUpperCase('aB4'); // false
isValidJSON
- title: isValidJSON
- tags: type,intermediate
Checks if the provided string is a valid JSON.
- Use
JSON.parse()
and atry... catch
block to check if the provided string is a valid JSON.
const isValidJSON = str => {
try {
JSON.parse(str);
return true;
} catch (e) {
return false;
}
};
isValidJSON('{"name":"Adam","age":20}'); // true
isValidJSON('{"name":"Adam",age:"20"}'); // false
isValidJSON(null); // true
isWeekday
- title: isWeekday
- tags: date,beginner
Checks if the given date is a weekday.
- Use
Date.prototype.getDay()
to check weekday by using a modulo operator (%
). - Omit the argument,
d
, to use the current date as default.
const isWeekday = (d = new Date()) => d.getDay() % 6 !== 0;
isWeekday(); // true (if current date is 2019-07-19)
isWeekend
- title: isWeekend
- tags: date,beginner
Checks if the given date is a weekend.
- Use
Date.prototype.getDay()
to check weekend by using a modulo operator (%
). - Omit the argument,
d
, to use the current date as default.
const isWeekend = (d = new Date()) => d.getDay() % 6 === 0;
isWeekend(); // 2018-10-19 (if current date is 2018-10-18)
isWritableStream
- title: isWritableStream
- tags: node,type,intermediate
Checks if the given argument is a writable stream.
- Check if the value is different from
null
. - Use
typeof
to check if the value is of typeobject
and thepipe
property is of typefunction
. - Additionally check if the
typeof
the_write
and_writableState
properties arefunction
andobject
respectively.
const isWritableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';
const fs = require('fs');
isWritableStream(fs.createWriteStream('test.txt')); // true
join
- title: join
- tags: array,intermediate
Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.
- Use
Array.prototype.reduce()
to combine elements into a string. - Omit the second argument,
separator
, to use a default separator of','
. - Omit the third argument,
end
, to use the same value asseparator
by default.
const join = (arr, separator = ',', end = separator) =>
arr.reduce(
(acc, val, i) =>
i === arr.length - 2
? acc + val + end
: i === arr.length - 1
? acc + val
: acc + val + separator,
''
);
join(['pen', 'pineapple', 'apple', 'pen'],',','&'); // 'pen,pineapple,apple&pen'
join(['pen', 'pineapple', 'apple', 'pen'], ','); // 'pen,pineapple,apple,pen'
join(['pen', 'pineapple', 'apple', 'pen']); // 'pen,pineapple,apple,pen'
juxt
- title: juxt
- tags: function,advanced
Takes several functions as argument and returns a function that is the juxtaposition of those functions.
- Use
Array.prototype.map()
to return afn
that can take a variable number ofargs
. - When
fn
is called, return an array containing the result of applying eachfn
to theargs
.
const juxt = (...fns) => (...args) => [...fns].map(fn => [...args].map(fn));
juxt(
x => x + 1,
x => x - 1,
x => x * 10
)(1, 2, 3); // [[2, 3, 4], [0, 1, 2], [10, 20, 30]]
juxt(
s => s.length,
s => s.split(' ').join('-')
)('30 seconds of code'); // [[18], ['30-seconds-of-code']]
kMeans
- title: kMeans
- tags: algorithm,array,advanced
Groups the given data into k
clusters, using the k-means clustering algorithm.
- Use
Array.from()
andArray.prototype.slice()
to initialize appropriate variables for the clustercentroids
,distances
andclasses
. - Use a
while
loop to repeat the assignment and update steps as long as there are changes in the previous iteration, as indicated byitr
. - Calculate the euclidean distance between each data point and centroid using
Math.hypot()
,Object.keys()
andArray.prototype.map()
. - Use
Array.prototype.indexOf()
andMath.min()
to find the closest centroid. - Use
Array.from()
andArray.prototype.reduce()
, as well asparseFloat()
andNumber.prototype.toFixed()
to calculate the new centroids.
const kMeans = (data, k = 1) => {
const centroids = data.slice(0, k);
const distances = Array.from({ length: data.length }, () =>
Array.from({ length: k }, () => 0)
);
const classes = Array.from({ length: data.length }, () => -1);
let itr = true;
while (itr) {
itr = false;
for (let d in data) {
for (let c = 0; c < k; c++) {
distances[d][c] = Math.hypot(
...Object.keys(data[0]).map(key => data[d][key] - centroids[c][key])
);
}
const m = distances[d].indexOf(Math.min(...distances[d]));
if (classes[d] !== m) itr = true;
classes[d] = m;
}
for (let c = 0; c < k; c++) {
centroids[c] = Array.from({ length: data[0].length }, () => 0);
const size = data.reduce((acc, _, d) => {
if (classes[d] === c) {
acc++;
for (let i in data[0]) centroids[c][i] += data[d][i];
}
return acc;
}, 0);
for (let i in data[0]) {
centroids[c][i] = parseFloat(Number(centroids[c][i] / size).toFixed(2));
}
}
}
return classes;
};
kMeans([[0, 0], [0, 1], [1, 3], [2, 0]], 2); // [0, 1, 1, 0]
kNearestNeighbors
- title: kNearestNeighbors
- tags: algorithm,array,advanced
Classifies a data point relative to a labelled data set, using the k-nearest neighbors algorithm.
- Use
Array.prototype.map()
to map thedata
to objects containing the euclidean distance of each element frompoint
, calculated usingMath.hypot()
,Object.keys()
and itslabel
. - Use
Array.prototype.sort()
andArray.prototype.slice()
to get thek
nearest neighbors ofpoint
. - Use
Array.prototype.reduce()
in combination withObject.keys()
andArray.prototype.indexOf()
to find the most frequentlabel
among them.
const kNearestNeighbors = (data, labels, point, k = 3) => {
const kNearest = data
.map((el, i) => ({
dist: Math.hypot(...Object.keys(el).map(key => point[key] - el[key])),
label: labels[i]
}))
.sort((a, b) => a.dist - b.dist)
.slice(0, k);
return kNearest.reduce(
(acc, { label }, i) => {
acc.classCounts[label] =
Object.keys(acc.classCounts).indexOf(label) !== -1
? acc.classCounts[label] + 1
: 1;
if (acc.classCounts[label] > acc.topClassCount) {
acc.topClassCount = acc.classCounts[label];
acc.topClass = label;
}
return acc;
},
{
classCounts: {},
topClass: kNearest[0].label,
topClassCount: 0
}
).topClass;
};
const data = [[0, 0], [0, 1], [1, 3], [2, 0]];
const labels = [0, 1, 1, 0];
kNearestNeighbors(data, labels, [1, 2], 2); // 1
kNearestNeighbors(data, labels, [1, 0], 2); // 0
kmToMiles
- title: kmToMiles
- tags: math,beginner unlisted: true
Converts kilometers to miles.
- Follow the conversion formula
mi = km * 0.621371
.
const kmToMiles = km => km * 0.621371;
kmToMiles(8.1) // 5.0331051
last
- title: last
- tags: array,beginner
Returns the last element in an array.
- Check if
arr
is truthy and has alength
property. - Use
Array.prototype.length - 1
to compute the index of the last element of the given array and return it, otherwise returnundefined
.
const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
last([1, 2, 3]); // 3
last([]); // undefined
last(null); // undefined
last(undefined); // undefined
lastDateOfMonth
- title: lastDateOfMonth
- tags: date,intermediate
Returns the string representation of the last date in the given date's month.
- Use
Date.prototype.getFullYear()
,Date.prototype.getMonth()
to get the current year and month from the given date. - Use the
new Date()
constructor to create a new date with the given year and month incremented by1
, and the day set to0
(last day of previous month). - Omit the argument,
date
, to use the current date by default.
const lastDateOfMonth = (date = new Date()) => {
let d = new Date(date.getFullYear(), date.getMonth() + 1, 0);
return d.toISOString().split('T')[0];
};
lastDateOfMonth(new Date('2015-08-11')); // '2015-08-30'
lcm
- title: lcm
- tags: math,algorithm,recursion,intermediate
Calculates the least common multiple of two or more numbers.
- Use the greatest common divisor (GCD) formula and the fact that
lcm(x, y) = x * y / gcd(x, y)
to determine the least common multiple. - The GCD formula uses recursion.
const lcm = (...arr) => {
const gcd = (x, y) => (!y ? x : gcd(y, x % y));
const _lcm = (x, y) => (x * y) / gcd(x, y);
return [...arr].reduce((a, b) => _lcm(a, b));
};
lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60
levenshteinDistance
- title: levenshteinDistance
- tags: string,algorithm,intermediate
Calculates the difference between two strings, using the Levenshtein distance algorithm.
- If either of the two strings has a
length
of zero, return thelength
of the other one. - Use a
for
loop to iterate over the letters of the target string and a nestedfor
loop to iterate over the letters of the source string. - Calculate the cost of substituting the letters corresponding to
i - 1
andj - 1
in the target and source respectively (0
if they are the same,1
otherwise). - Use
Math.min()
to populate each element in the 2D array with the minimum of the cell above incremented by one, the cell to the left incremented by one or the cell to the top left incremented by the previously calculated cost. - Return the last element of the last row of the produced array.
const levenshteinDistance = (s, t) => {
if (!s.length) return t.length;
if (!t.length) return s.length;
const arr = [];
for (let i = 0; i <= t.length; i++) {
arr[i] = [i];
for (let j = 1; j <= s.length; j++) {
arr[i][j] =
i === 0
? j
: Math.min(
arr[i - 1][j] + 1,
arr[i][j - 1] + 1,
arr[i - 1][j - 1] + (s[j - 1] === t[i - 1] ? 0 : 1)
);
}
}
return arr[t.length][s.length];
};
levenshteinDistance('duck', 'dark'); // 2
linearSearch
- title: linearSearch
- tags: algorithm,array,beginner
Finds the first index of a given element in an array using the linear search algorithm.
- Use a
for...in
loop to iterate over the indexes of the given array. - Check if the element in the corresponding index is equal to
item
. - If the element is found, return the index, using the unary
+
operator to convert it from a string to a number. - If the element is not found after iterating over the whole array, return
-1
.
const linearSearch = (arr, item) => {
for (const i in arr) {
if (arr[i] === item) return +i;
}
return -1;
};
linearSearch([2, 9, 9], 9); // 1
linearSearch([2, 9, 9], 7); // -1
listenOnce
- title: listenOnce
- tags: browser,event,beginner
Adds an event listener to an element that will only run the callback the first time the event is triggered.
- Use
EventTarget.addEventListener()
to add an event listener to an element. - Use
{ once: true }
as options to only run the given callback once.
const listenOnce = (el, evt, fn) =>
el.addEventListener(evt, fn, { once: true });
listenOnce(
document.getElementById('my-id'),
'click',
() => console.log('Hello world')
); // 'Hello world' will only be logged on the first click
logBase
- title: logBase
- tags: math,beginner
Calculates the logarithm of the given number in the given base.
- Use
Math.log()
to get the logarithm from the value and the base and divide them.
const logBase = (n, base) => Math.log(n) / Math.log(base);
logBase(10, 10); // 1
logBase(100, 10); // 2
longestItem
- title: longestItem
- tags: array,intermediate
Takes any number of iterable objects or objects with a length
property and returns the longest one.
- Use
Array.prototype.reduce()
, comparing the length of objects to find the longest one. - If multiple objects have the same length, the first one will be returned.
- Returns
undefined
if no arguments are provided.
const longestItem = (...vals) =>
vals.reduce((a, x) => (x.length > a.length ? x : a));
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'
lowercaseKeys
- title: lowercaseKeys
- tags: object,intermediate
Creates a new object from the specified object, where all the keys are in lowercase.
- Use
Object.keys()
andArray.prototype.reduce()
to create a new object from the specified object. - Convert each key in the original object to lowercase, using
String.prototype.toLowerCase()
.
const lowercaseKeys = obj =>
Object.keys(obj).reduce((acc, key) => {
acc[key.toLowerCase()] = obj[key];
return acc;
}, {});
const myObj = { Name: 'Adam', sUrnAME: 'Smith' };
const myObjLower = lowercaseKeys(myObj); // {name: 'Adam', surname: 'Smith'};
luhnCheck
- title: luhnCheck
- tags: math,algorithm,advanced
Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
- Use
String.prototype.split('')
,Array.prototype.reverse()
andArray.prototype.map()
in combination withparseInt()
to obtain an array of digits. - Use
Array.prototype.splice(0, 1)
to obtain the last digit. - Use
Array.prototype.reduce()
to implement the Luhn Algorithm. - Return
true
ifsum
is divisible by10
,false
otherwise.
const luhnCheck = num => {
let arr = (num + '')
.split('')
.reverse()
.map(x => parseInt(x));
let lastDigit = arr.splice(0, 1)[0];
let sum = arr.reduce(
(acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9),
0
);
sum += lastDigit;
return sum % 10 === 0;
};
luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); // false
luhnCheck(123456789); // false
mapKeys
- title: mapKeys
- tags: object,intermediate
Maps the keys of an object using the provided function, generating a new object.
- Use
Object.keys()
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same values and mapped keys usingfn
.
const mapKeys = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[fn(obj[k], k, obj)] = obj[k];
return acc;
}, {});
mapKeys({ a: 1, b: 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }
mapNumRange
- title: mapNumRange
- tags: math,beginner
Maps a number from one range to another range.
- Return
num
mapped betweenoutMin
-outMax
frominMin
-inMax
.
const mapNumRange = (num, inMin, inMax, outMin, outMax) =>
((num - inMin) * (outMax - outMin)) / (inMax - inMin) + outMin;
mapNumRange(5, 0, 10, 0, 100); // 50
mapObject
- title: mapObject
- tags: array,object,intermediate
Maps the values of an array to an object using a function.
- Use
Array.prototype.reduce()
to applyfn
to each element inarr
and combine the results into an object. - Use
el
as the key for each property and the result offn
as the value.
const mapObject = (arr, fn) =>
arr.reduce((acc, el, i) => {
acc[el] = fn(el, i, arr);
return acc;
}, {});
mapObject([1, 2, 3], a => a * a); // { 1: 1, 2: 4, 3: 9 }
mapString
- title: mapString
- tags: string,intermediate
Creates a new string with the results of calling a provided function on every character in the given string.
- Use
String.prototype.split('')
andArray.prototype.map()
to call the provided function,fn
, for each character instr
. - Use
Array.prototype.join('')
to recombine the array of characters into a string. - The callback function,
fn
, takes three arguments (the current character, the index of the current character and the stringmapString
was called upon).
const mapString = (str, fn) =>
str
.split('')
.map((c, i) => fn(c, i, str))
.join('');
mapString('lorem ipsum', c => c.toUpperCase()); // 'LOREM IPSUM'
mapValues
- title: mapValues
- tags: object,intermediate
Maps the values of an object using the provided function, generating a new object with the same keys.
- Use
Object.keys()
to iterate over the object's keys. - Use
Array.prototype.reduce()
to create a new object with the same keys and mapped values usingfn
.
const mapValues = (obj, fn) =>
Object.keys(obj).reduce((acc, k) => {
acc[k] = fn(obj[k], k, obj);
return acc;
}, {});
const users = {
fred: { user: 'fred', age: 40 },
pebbles: { user: 'pebbles', age: 1 }
};
mapValues(users, u => u.age); // { fred: 40, pebbles: 1 }
mask
- title: mask
- tags: string,intermediate
Replaces all but the last num
of characters with the specified mask character.
- Use
String.prototype.slice()
to grab the portion of the characters that will remain unmasked. - Use
String.padStart()
to fill the beginning of the string with themask
character up to the original length. - If
num
is negative, the unmasked characters will be at the start of the string. - Omit the second argument,
num
, to keep a default of4
characters unmasked. - Omit the third argument,
mask
, to use a default character of'*'
for the mask.
const mask = (cc, num = 4, mask = '*') =>
`${cc}`.slice(-num).padStart(`${cc}`.length, mask);
mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'
matches
- title: matches
- tags: object,intermediate
Compares two objects to determine if the first one contains equivalent property values to the second one.
- Use
Object.keys()
to get all the keys of the second object. - Use
Array.prototype.every()
,Object.prototype.hasOwnProperty()
and strict comparison to determine if all keys exist in the first object and have the same values.
const matches = (obj, source) =>
Object.keys(source).every(
key => obj.hasOwnProperty(key) && obj[key] === source[key]
);
matches({ age: 25, hair: 'long', beard: true }, { hair: 'long', beard: true });
// true
matches({ hair: 'long', beard: true }, { age: 25, hair: 'long', beard: true });
// false
matchesWith
- title: matchesWith
- tags: object,intermediate
Compares two objects to determine if the first one contains equivalent property values to the second one, based on a provided function.
- Use
Object.keys()
to get all the keys of the second object. - Use
Array.prototype.every()
,Object.prototype.hasOwnProperty()
and the provided function to determine if all keys exist in the first object and have equivalent values. - If no function is provided, the values will be compared using the equality operator.
const matchesWith = (obj, source, fn) =>
Object.keys(source).every(key =>
obj.hasOwnProperty(key) && fn
? fn(obj[key], source[key], key, obj, source)
: obj[key] == source[key]
);
const isGreeting = val => /^h(?:i|ello)$/.test(val);
matchesWith(
{ greeting: 'hello' },
{ greeting: 'hi' },
(oV, sV) => isGreeting(oV) && isGreeting(sV)
); // true
maxBy
- title: maxBy
- tags: math,array,beginner
Returns the maximum value of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Math.max()
to get the maximum value.
const maxBy = (arr, fn) =>
Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8
maxDate
- title: maxDate
- tags: date,intermediate
Returns the maximum of the given dates.
- Use the ES6 spread syntax with
Math.max()
to find the maximum date value. - Use
new Date()
to convert it to aDate
object.
const maxDate = (...dates) => new Date(Math.max(...dates));
const dates = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
maxDate(...dates); // 2018-03-11T22:00:00.000Z
maxN
- title: maxN
- tags: array,math,intermediate
Returns the n
maximum elements from the provided array.
- Use
Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in descending order. - Use
Array.prototype.slice()
to get the specified number of elements. - Omit the second argument,
n
, to get a one-element array. - If
n
is greater than or equal to the provided array's length, then return the original array (sorted in descending order).
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3, 2]
median
- title: median
- tags: math,array,intermediate
Calculates the median of an array of numbers.
- Find the middle of the array, use
Array.prototype.sort()
to sort the values. - Return the number at the midpoint if
Array.prototype.length
is odd, otherwise the average of the two middle numbers.
const median = arr => {
const mid = Math.floor(arr.length / 2),
nums = [...arr].sort((a, b) => a - b);
return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};
median([5, 6, 50, 1, -5]); // 5
memoize
- title: memoize
- tags: function,advanced
Returns the memoized (cached) function.
- Create an empty cache by instantiating a new
Map
object. - Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not.
- The
function
keyword must be used in order to allow the memoized function to have itsthis
context changed if necessary. - Allow access to the
cache
by setting it as a property on the returned function.
const memoize = fn => {
const cache = new Map();
const cached = function (val) {
return cache.has(val)
? cache.get(val)
: cache.set(val, fn.call(this, val)) && cache.get(val);
};
cached.cache = cache;
return cached;
};
// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's cached
console.log(anagramsCached.cache); // The cached anagrams map
merge
- title: merge
- tags: object,array,intermediate
Creates a new object from the combination of two or more objects.
- Use
Array.prototype.reduce()
combined withObject.keys()
to iterate over all objects and keys. - Use
Object.prototype.hasOwnProperty()
andArray.prototype.concat()
to append values for keys existing in multiple objects.
const merge = (...objs) =>
[...objs].reduce(
(acc, obj) =>
Object.keys(obj).reduce((a, k) => {
acc[k] = acc.hasOwnProperty(k)
? [].concat(acc[k]).concat(obj[k])
: obj[k];
return acc;
}, {}),
{}
);
const object = {
a: [{ x: 2 }, { y: 4 }],
b: 1
};
const other = {
a: { z: 3 },
b: [2, 3],
c: 'foo'
};
merge(object, other);
// { a: [ { x: 2 }, { y: 4 }, { z: 3 } ], b: [ 1, 2, 3 ], c: 'foo' }
mergeSort
- title: mergeSort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the merge sort algorithm.
- Use recursion.
- If the
length
of the array is less than2
, return the array. - Use
Math.floor()
to calculate the middle point of the array. - Use
Array.prototype.slice()
to slice the array in two and recursively callmergeSort()
on the created subarrays. - Finally, use
Array.from()
andArray.prototype.shift()
to combine the two sorted subarrays into one.
const mergeSort = arr => {
if (arr.length < 2) return arr;
const mid = Math.floor(arr.length / 2);
const l = mergeSort(arr.slice(0, mid));
const r = mergeSort(arr.slice(mid, arr.length));
return Array.from({ length: l.length + r.length }, () => {
if (!l.length) return r.shift();
else if (!r.length) return l.shift();
else return l[0] > r[0] ? r.shift() : l.shift();
});
};
mergeSort([5, 1, 4, 2, 3]); // [1, 2, 3, 4, 5]
mergeSortedArrays
- title: mergeSortedArrays
- tags: array,intermediate
Merges two sorted arrays into one.
- Use the spread operator (
...
) to clone both of the given arrays. - Use
Array.from()
to create an array of the appropriate length based on the given arrays. - Use
Array.prototype.shift()
to populate the newly created array from the removed elements of the cloned arrays.
const mergeSortedArrays = (a, b) => {
const _a = [...a],
_b = [...b];
return Array.from({ length: _a.length + _b.length }, () => {
if (!_a.length) return _b.shift();
else if (!_b.length) return _a.shift();
else return _a[0] > _b[0] ? _b.shift() : _a.shift();
});
};
mergeSortedArrays([1, 4, 5], [2, 3, 6]); // [1, 2, 3, 4, 5, 6]
midpoint
- title: midpoint
- tags: math,beginner
Calculates the midpoint between two pairs of (x,y) points.
- Destructure the array to get
x1
,y1
,x2
andy2
. - Calculate the midpoint for each dimension by dividing the sum of the two endpoints by
2
.
const midpoint = ([x1, y1], [x2, y2]) => [(x1 + x2) / 2, (y1 + y2) / 2];
midpoint([2, 2], [4, 4]); // [3, 3]
midpoint([4, 4], [6, 6]); // [5, 5]
midpoint([1, 3], [2, 4]); // [1.5, 3.5]
milesToKm
- title: milesToKm
- tags: math,beginner unlisted: true
Converts miles to kilometers.
- Follow the conversion formula
km = mi * 1.609344
.
const milesToKm = miles => miles * 1.609344;
milesToKm(5); // ~8.04672
minBy
- title: minBy
- tags: math,array,beginner
Returns the minimum value of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Math.min()
to get the minimum value.
const minBy = (arr, fn) =>
Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2
minDate
- title: minDate
- tags: date,intermediate
Returns the minimum of the given dates.
- Use the ES6 spread syntax with
Math.min()
to find the minimum date value. - Use
new Date()
to convert it to aDate
object.
const minDate = (...dates) => new Date(Math.min(...dates));
const dates = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
minDate(...dates); // 2016-01-08T22:00:00.000Z
minN
- title: minN
- tags: array,math,intermediate
Returns the n
minimum elements from the provided array.
- Use
Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in ascending order. - Use
Array.prototype.slice()
to get the specified number of elements. - Omit the second argument,
n
, to get a one-element array. - If
n
is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).
const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1, 2]
mostFrequent
- title: mostFrequent
- tags: array,intermediate
Returns the most frequent element in an array.
- Use
Array.prototype.reduce()
to map unique values to an object's keys, adding to existing keys every time the same value is encountered. - Use
Object.entries()
on the result in combination withArray.prototype.reduce()
to get the most frequent value in the array.
const mostFrequent = arr =>
Object.entries(
arr.reduce((a, v) => {
a[v] = a[v] ? a[v] + 1 : 1;
return a;
}, {})
).reduce((a, v) => (v[1] >= a[1] ? v : a), [null, 0])[0];
mostFrequent(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // 'a'
mostPerformant
- title: mostPerformant
- tags: function,advanced
Returns the index of the function in an array of functions which executed the fastest.
- Use
Array.prototype.map()
to generate an array where each value is the total time taken to execute the function afteriterations
times. - Use the difference in
performance.now()
values before and after to get the total time in milliseconds to a high degree of accuracy. - Use
Math.min()
to find the minimum execution time, and return the index of that shortest time which corresponds to the index of the most performant function. - Omit the second argument,
iterations
, to use a default of10000
iterations. - The more iterations, the more reliable the result but the longer it will take.
const mostPerformant = (fns, iterations = 10000) => {
const times = fns.map(fn => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return performance.now() - before;
});
return times.indexOf(Math.min(...times));
};
mostPerformant([
() => {
// Loops through the entire array before returning `false`
[1, 2, 3, 4, 5, 6, 7, 8, 9, '10'].every(el => typeof el === 'number');
},
() => {
// Only needs to reach index `1` before returning `false`
[1, '2', 3, 4, 5, 6, 7, 8, 9, 10].every(el => typeof el === 'number');
}
]); // 1
negate
- title: negate
- tags: function,beginner
Negates a predicate function.
- Take a predicate function and apply the not operator (
!
) to it with its arguments.
const negate = func => (...args) => !func(...args);
[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 === 0)); // [ 1, 3, 5 ]
nest
- title: nest
- tags: object,recursion,intermediate
Nests recursively objects linked to one another in a flat array.
- Use recursion.
- Use
Array.prototype.filter()
to filter the items where theid
matches thelink
. - Use
Array.prototype.map()
to map each item to a new object that has achildren
property which recursively nests the items based on which ones are children of the current item. - Omit the second argument,
id
, to default tonull
which indicates the object is not linked to another one (i.e. it is a top level object). - Omit the third argument,
link
, to use'parent_id'
as the default property which links the object to another one by itsid
.
const nest = (items, id = null, link = 'parent_id') =>
items
.filter(item => item[link] === id)
.map(item => ({ ...item, children: nest(items, item.id, link) }));
const comments = [
{ id: 1, parent_id: null },
{ id: 2, parent_id: 1 },
{ id: 3, parent_id: 1 },
{ id: 4, parent_id: 2 },
{ id: 5, parent_id: 4 }
];
const nestedComments = nest(comments);
// [{ id: 1, parent_id: null, children: [...] }]
nodeListToArray
- title: nodeListToArray
- tags: browser,array,beginner
Converts a NodeList
to an array.
- Use spread operator (
...
) inside new array to convert aNodeList
to an array.
const nodeListToArray = nodeList => [...nodeList];
nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]
none
- title: none
- tags: array,beginner
Checks if the provided predicate function returns false
for all elements in a collection.
- Use
Array.prototype.some()
to test if any elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const none = (arr, fn = Boolean) => !arr.some(fn);
none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // true
normalizeLineEndings
- title: normalizeLineEndings
- tags: string,regexp,intermediate
Normalizes line endings in a string.
- Use
String.prototype.replace()
and a regular expression to match and replace line endings with thenormalized
version. - Omit the second argument,
normalized
, to use the default value of'\r\n'
.
const normalizeLineEndings = (str, normalized = '\r\n') =>
str.replace(/\r?\n/g, normalized);
normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n');
// 'This\r\nis a\r\nmultiline\r\nstring.\r\n'
normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n', '\n');
// 'This\nis a\nmultiline\nstring.\n'
not
- title: not
- tags: math,logic,beginner unlisted: true
Returns the logical inverse of the given value.
- Use the logical not (
!
) operator to return the inverse of the given value.
const not = a => !a;
not(true); // false
not(false); // true
nthArg
- title: nthArg
- tags: function,beginner
Creates a function that gets the argument at index n
.
- Use
Array.prototype.slice()
to get the desired argument at indexn
. - If
n
is negative, the nth argument from the end is returned.
const nthArg = n => (...args) => args.slice(n)[0];
const third = nthArg(2);
third(1, 2, 3); // 3
third(1, 2); // undefined
const last = nthArg(-1);
last(1, 2, 3, 4, 5); // 5
nthElement
- title: nthElement
- tags: array,beginner
Returns the nth element of an array.
- Use
Array.prototype.slice()
to get an array containing the nth element at the first place. - If the index is out of bounds, return
undefined
. - Omit the second argument,
n
, to get the first element of the array.
const nthElement = (arr, n = 0) =>
(n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'
nthRoot
- title: nthRoot
- tags: math,beginner
Calculates the nth root of a given number.
- Use
Math.pow()
to calculatex
to the power of1/n
which is equal to the nth root ofx
.
const nthRoot = (x, n) => Math.pow(x, 1 / n);
nthRoot(32, 5); // 2
objectFromPairs
- title: objectFromPairs
- tags: object,array,beginner
Creates an object from the given key-value pairs.
- Use
Array.prototype.reduce()
to create and combine key-value pairs.
const objectFromPairs = arr =>
arr.reduce((a, [key, val]) => ((a[key] = val), a), {});
objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}
objectToEntries
- title: objectToEntries
- tags: object,array,beginner
Creates an array of key-value pair arrays from an object.
- Use
Object.keys()
andArray.prototype.map()
to iterate over the object's keys and produce an array with key-value pairs.
const objectToEntries = obj => Object.keys(obj).map(k => [k, obj[k]]);
objectToEntries({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]
objectToPairs
- title: objectToPairs
- tags: object,array,beginner
Creates an array of key-value pair arrays from an object.
- Use
Object.entries()
to get an array of key-value pair arrays from the given object.
const objectToPairs = obj => Object.entries(obj);
objectToPairs({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]
objectToQueryString
- title: objectToQueryString
- tags: object,advanced
Generates a query string from the key-value pairs of the given object.
- Use
Array.prototype.reduce()
onObject.entries(queryParameters)
to create the query string. - Determine the
symbol
to be either?
or&
based on the length ofqueryString
. - Concatenate
val
toqueryString
only if it's a string. - Return the
queryString
or an empty string when thequeryParameters
are falsy.
const objectToQueryString = queryParameters => {
return queryParameters
? Object.entries(queryParameters).reduce(
(queryString, [key, val], index) => {
const symbol = queryString.length === 0 ? '?' : '&';
queryString +=
typeof val === 'string' ? `${symbol}${key}=${val}` : '';
return queryString;
},
''
)
: '';
};
objectToQueryString({ page: '1', size: '2kg', key: undefined });
// '?page=1&size=2kg'
observeMutations
- title: observeMutations
- tags: browser,event,advanced
Creates a new MutationObserver
and runs the provided callback for each mutation on the specified element.
- Use a
MutationObserver
to observe mutations on the given element. - Use
Array.prototype.forEach()
to run the callback for each mutation that is observed. - Omit the third argument,
options
, to use the default options (alltrue
).
const observeMutations = (element, callback, options) => {
const observer = new MutationObserver(mutations =>
mutations.forEach(m => callback(m))
);
observer.observe(
element,
Object.assign(
{
childList: true,
attributes: true,
attributeOldValue: true,
characterData: true,
characterDataOldValue: true,
subtree: true,
},
options
)
);
return observer;
};
const obs = observeMutations(document, console.log);
// Logs all mutations that happen on the page
obs.disconnect();
// Disconnects the observer and stops logging mutations on the page
off
- title: off
- tags: browser,event,intermediate
Removes an event listener from an element.
- Use
EventTarget.removeEventListener()
to remove an event listener from an element. - Omit the fourth argument
opts
to usefalse
or specify it based on the options used when the event listener was added.
const off = (el, evt, fn, opts = false) =>
el.removeEventListener(evt, fn, opts);
const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the page
offset
- title: offset
- tags: array,beginner
Moves the specified amount of elements to the end of the array.
- Use
Array.prototype.slice()
twice to get the elements after the specified index and the elements before that. - Use the spread operator (
...
) to combine the two into one array. - If
offset
is negative, the elements will be moved from end to start.
const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];
offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]
omit
- title: omit
- tags: object,intermediate
Omits the key-value pairs corresponding to the given keys from an object.
- Use
Object.keys()
,Array.prototype.filter()
andArray.prototype.includes()
to remove the provided keys. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs.
const omit = (obj, arr) =>
Object.keys(obj)
.filter(k => !arr.includes(k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omit({ a: 1, b: '2', c: 3 }, ['b']); // { 'a': 1, 'c': 3 }
omitBy
- title: omitBy
- tags: object,intermediate
Omits the key-value pairs corresponding to the keys of the object for which the given function returns falsy.
- Use
Object.keys()
andArray.prototype.filter()
to remove the keys for whichfn
returns a truthy value. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs. - The callback function is invoked with two arguments: (value, key).
const omitBy = (obj, fn) =>
Object.keys(obj)
.filter(k => !fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }
on
- title: on
- tags: browser,event,intermediate
Adds an event listener to an element with the ability to use event delegation.
- Use
EventTarget.addEventListener()
to add an event listener to an element. - If there is a
target
property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correctthis
context. - Omit
opts
to default to non-delegation behavior and event bubbling. - Returns a reference to the custom delegator function, in order to be possible to use with
off
.
const on = (el, evt, fn, opts = {}) => {
const delegatorFn = e =>
e.target.matches(opts.target) && fn.call(e.target, e);
el.addEventListener(
evt,
opts.target ? delegatorFn : fn,
opts.options || false
);
if (opts.target) return delegatorFn;
};
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' });
// logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true });
// use capturing instead of bubbling
onClickOutside
- title: onClickOutside
- tags: browser,event,intermediate
Runs the callback whenever the user clicks outside of the specified element.
- Use
EventTarget.addEventListener()
to listen for'click'
events. - Use
Node.contains()
to check ifEvent.target
is a descendant ofelement
and runcallback
if not.
const onClickOutside = (element, callback) => {
document.addEventListener('click', e => {
if (!element.contains(e.target)) callback();
});
};
onClickOutside('##my-element', () => console.log('Hello'));
// Will log 'Hello' whenever the user clicks outside of ##my-element
onScrollStop
- title: onScrollStop
- tags: browser,event,intermediate
Runs the callback whenever the user has stopped scrolling.
- Use
EventTarget.addEventListener()
to listen for the'scroll'
event. - Use
setTimeout()
to wait150
ms until calling the givencallback
. - Use
clearTimeout()
to clear the timeout if a new'scroll'
event is fired in under150
ms.
const onScrollStop = callback => {
let isScrolling;
window.addEventListener(
'scroll',
e => {
clearTimeout(isScrolling);
isScrolling = setTimeout(() => {
callback();
}, 150);
},
false
);
};
onScrollStop(() => {
console.log('The user has stopped scrolling');
});
onUserInputChange
- title: onUserInputChange
- tags: browser,event,advanced
Runs the callback whenever the user input type changes (mouse
or touch
).
- Use two event listeners.
- Assume
mouse
input initially and bind a'touchstart'
event listener to the document. - On
'touchstart'
, add a'mousemove'
event listener to listen for two consecutive'mousemove'
events firing within 20ms, usingperformance.now()
. - Run the callback with the input type as an argument in either of these situations.
const onUserInputChange = callback => {
let type = 'mouse',
lastTime = 0;
const mousemoveHandler = () => {
const now = performance.now();
if (now - lastTime < 20)
(type = 'mouse'),
callback(type),
document.removeEventListener('mousemove', mousemoveHandler);
lastTime = now;
};
document.addEventListener('touchstart', () => {
if (type === 'touch') return;
(type = 'touch'),
callback(type),
document.addEventListener('mousemove', mousemoveHandler);
});
};
onUserInputChange(type => {
console.log('The user is now using', type, 'as an input method.');
});
once
- title: once
- tags: function,intermediate
Ensures a function is called only once.
- Utilizing a closure, use a flag,
called
, and set it totrue
once the function is called for the first time, preventing it from being called again. - In order to allow the function to have its
this
context changed (such as in an event listener), thefunction
keyword must be used, and the supplied function must have the context applied. - Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (
...
) operator.
const once = fn => {
let called = false;
return function(...args) {
if (called) return;
called = true;
return fn.apply(this, args);
};
};
const startApp = function(event) {
console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp));
// only runs `startApp` once upon click
or
- title: or
- tags: math,logic,beginner unlisted: true
Checks if at least one of the arguments is true
.
- Use the logical or (
||
) operator on the two given values.
const or = (a, b) => a || b;
or(true, true); // true
or(true, false); // true
or(false, false); // false
orderBy
- title: orderBy
- tags: object,array,advanced
Sorts an array of objects, ordered by properties and orders.
- Uses
Array.prototype.sort()
,Array.prototype.reduce()
on theprops
array with a default value of0
. - Use array destructuring to swap the properties position depending on the order supplied.
- If no
orders
array is supplied, sort by'asc'
by default.
const orderBy = (arr, props, orders) =>
[...arr].sort((a, b) =>
props.reduce((acc, prop, i) => {
if (acc === 0) {
const [p1, p2] =
orders && orders[i] === 'desc'
? [b[prop], a[prop]]
: [a[prop], b[prop]];
acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;
}
return acc;
}, 0)
);
const users = [
{ name: 'fred', age: 48 },
{ name: 'barney', age: 36 },
{ name: 'fred', age: 40 },
];
orderBy(users, ['name', 'age'], ['asc', 'desc']);
// [{name: 'barney', age: 36}, {name: 'fred', age: 48}, {name: 'fred', age: 40}]
orderBy(users, ['name', 'age']);
// [{name: 'barney', age: 36}, {name: 'fred', age: 40}, {name: 'fred', age: 48}]
orderWith
- title: orderWith
- tags: array,object,intermediate
Sorts an array of objects, ordered by a property, based on the array of orders provided.
- Use
Array.prototype.reduce()
to create an object from theorder
array with the values as keys and their original index as the value. - Use
Array.prototype.sort()
to sort the given array, skipping elements for whichprop
is empty or not in theorder
array.
const orderWith = (arr, prop, order) => {
const orderValues = order.reduce((acc, v, i) => {
acc[v] = i;
return acc;
}, {});
return [...arr].sort((a, b) => {
if (orderValues[a[prop]] === undefined) return 1;
if (orderValues[b[prop]] === undefined) return -1;
return orderValues[a[prop]] - orderValues[b[prop]];
});
};
const users = [
{ name: 'fred', language: 'Javascript' },
{ name: 'barney', language: 'TypeScript' },
{ name: 'frannie', language: 'Javascript' },
{ name: 'anna', language: 'Java' },
{ name: 'jimmy' },
{ name: 'nicky', language: 'Python' },
];
orderWith(users, 'language', ['Javascript', 'TypeScript', 'Java']);
/*
[
{ name: 'fred', language: 'Javascript' },
{ name: 'frannie', language: 'Javascript' },
{ name: 'barney', language: 'TypeScript' },
{ name: 'anna', language: 'Java' },
{ name: 'jimmy' },
{ name: 'nicky', language: 'Python' }
]
*/
over
- title: over
- tags: function,intermediate
Creates a function that invokes each provided function with the arguments it receives and returns the results.
- Use
Array.prototype.map()
andFunction.prototype.apply()
to apply each function to the given arguments.
const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));
const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1, 5]
overArgs
- title: overArgs
- tags: function,intermediate
Creates a function that invokes the provided function with its arguments transformed.
- Use
Array.prototype.map()
to applytransforms
toargs
in combination with the spread operator (...
) to pass the transformed arguments tofn
.
const overArgs = (fn, transforms) =>
(...args) => fn(...args.map((val, i) => transforms[i](val)));
const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]
pad
- title: pad
- tags: string,beginner
Pads a string on both sides with the specified character, if it's shorter than the specified length
.
- Use
String.prototype.padStart()
andString.prototype.padEnd()
to pad both sides of the given string. - Omit the third argument,
char
, to use the whitespace character as the default padding character.
const pad = (str, length, char = ' ') =>
str.padStart((str.length + length) / 2, char).padEnd(length, char);
pad('cat', 8); // ' cat '
pad(String(42), 6, '0'); // '004200'
pad('foobar', 3); // 'foobar'
padNumber
- title: padNumber
- tags: string,math,beginner
Pads a given number to the specified length.
- Use
String.prototype.padStart()
to pad the number to specified length, after converting it to a string.
const padNumber = (n, l) => `${n}`.padStart(l, '0');
padNumber(1234, 6); // '001234'
palindrome
- title: palindrome
- tags: string,intermediate
Checks if the given string is a palindrome.
- Normalize the string to
String.prototype.toLowerCase()
and useString.prototype.replace()
to remove non-alphanumeric characters from it. - Use the spread operator (
...
) to split the normalized string into individual characters. - Use
Array.prototype.reverse()
,String.prototype.join('')
and compare the result to the normalized string.
const palindrome = str => {
const s = str.toLowerCase().replace(/[\W_]/g, '');
return s === [...s].reverse().join('');
};
palindrome('taco cat'); // true
parseCookie
- title: parseCookie
- tags: browser,string,intermediate
Parses an HTTP Cookie header string, returning an object of all cookie name-value pairs.
- Use
String.prototype.split(';')
to separate key-value pairs from each other. - Use
Array.prototype.map()
andString.prototype.split('=')
to separate keys from values in each pair. - Use
Array.prototype.reduce()
anddecodeURIComponent()
to create an object with all key-value pairs.
const parseCookie = str =>
str
.split(';')
.map(v => v.split('='))
.reduce((acc, v) => {
acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
return acc;
}, {});
parseCookie('foo=bar; equation=E%3Dmc%5E2');
// { foo: 'bar', equation: 'E=mc^2' }
partial
- title: partial
- tags: function,intermediate
Creates a function that invokes fn
with partials
prepended to the arguments it receives.
- Use the spread operator (
...
) to prependpartials
to the list of arguments offn
.
const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'
partialRight
- title: partialRight
- tags: function,intermediate
Creates a function that invokes fn
with partials
appended to the arguments it receives.
- Use the spread operator (
...
) to appendpartials
to the list of arguments offn
.
const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'
partition
- title: partition
- tags: array,object,intermediate
Groups the elements into two arrays, depending on the provided function's truthiness for each element.
- Use
Array.prototype.reduce()
to create an array of two arrays. - Use
Array.prototype.push()
to add elements for whichfn
returnstrue
to the first array and elements for whichfn
returnsfalse
to the second one.
const partition = (arr, fn) =>
arr.reduce(
(acc, val, i, arr) => {
acc[fn(val, i, arr) ? 0 : 1].push(val);
return acc;
},
[[], []]
);
const users = [
{ user: 'barney', age: 36, active: false },
{ user: 'fred', age: 40, active: true },
];
partition(users, o => o.active);
// [
// [{ user: 'fred', age: 40, active: true }],
// [{ user: 'barney', age: 36, active: false }]
// ]
partitionBy
- title: partitionBy
- tags: array,object,advanced
Applies fn
to each value in arr
, splitting it each time the provided function returns a new value.
- Use
Array.prototype.reduce()
with an accumulator object that will hold the resulting array and the last value returned fromfn
. - Use
Array.prototype.push()
to add each value inarr
to the appropriate partition in the accumulator array.
const partitionBy = (arr, fn) =>
arr.reduce(
({ res, last }, v, i, a) => {
const next = fn(v, i, a);
if (next !== last) res.push([v]);
else res[res.length - 1].push(v);
return { res, last: next };
},
{ res: [] }
).res;
const numbers = [1, 1, 3, 3, 4, 5, 5, 5];
partitionBy(numbers, n => n % 2 === 0); // [[1, 1, 3, 3], [4], [5, 5, 5]]
partitionBy(numbers, n => n); // [[1, 1], [3, 3], [4], [5, 5, 5]]
percentile
- title: percentile
- tags: math,intermediate
Calculates the percentage of numbers in the given array that are less or equal to the given value.
- Use
Array.prototype.reduce()
to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.
const percentile = (arr, val) =>
(100 *
arr.reduce(
(acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0),
0
)) /
arr.length;
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55
permutations
- title: permutations
- tags: array,algorithm,recursion,advanced
Generates all permutations of an array's elements (contains duplicates).
- Use recursion.
- For each element in the given array, create all the partial permutations for the rest of its elements.
- Use
Array.prototype.map()
to combine the element with each partial permutation, thenArray.prototype.reduce()
to combine all permutations in one array. - Base cases are for
Array.prototype.length
equal to2
or1
. - ⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries may cause your browser to hang as it tries to solve all the different combinations.
const permutations = arr => {
if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
return arr.reduce(
(acc, item, i) =>
acc.concat(
permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
item,
...val,
])
),
[]
);
};
permutations([1, 33, 5]);
// [ [1, 33, 5], [1, 5, 33], [33, 1, 5], [33, 5, 1], [5, 1, 33], [5, 33, 1] ]
pick
- title: pick
- tags: object,intermediate
Picks the key-value pairs corresponding to the given keys from an object.
- Use
Array.prototype.reduce()
to convert the filtered/picked keys back to an object with the corresponding key-value pairs if the key exists in the object.
const pick = (obj, arr) =>
arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
pick({ a: 1, b: '2', c: 3 }, ['a', 'c']); // { 'a': 1, 'c': 3 }
pickBy
- title: pickBy
- tags: object,intermediate
Creates an object composed of the properties the given function returns truthy for.
- Use
Object.keys(obj)
andArray.prototype.filter()
to remove the keys for whichfn
returns a falsy value. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs. - The callback function is invoked with two arguments: (value, key).
const pickBy = (obj, fn) =>
Object.keys(obj)
.filter(k => fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number');
// { 'a': 1, 'c': 3 }
pipeAsyncFunctions
- title: pipeAsyncFunctions
- tags: function,promise,intermediate
Performs left-to-right function composition for asynchronous functions.
- Use
Array.prototype.reduce()
and the spread operator (...
) to perform function composition usingPromise.prototype.then()
. - The functions can return a combination of normal values,
Promise
s or beasync
, returning throughawait
. - All functions must accept a single argument.
const pipeAsyncFunctions = (...fns) =>
arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));
const sum = pipeAsyncFunctions(
x => x + 1,
x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
x => x + 3,
async x => (await x) + 4
);
(async() => {
console.log(await sum(5)); // 15 (after one second)
})();
pipeFunctions
- title: pipeFunctions
- tags: function,intermediate
Performs left-to-right function composition.
- Use
Array.prototype.reduce()
with the spread operator (...
) to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const pipeFunctions = (...fns) =>
fns.reduce((f, g) => (...args) => g(f(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15
pluck
- title: pluck
- tags: array,object,beginner
Converts an array of objects into an array of values corresponding to the specified key
.
- Use
Array.prototype.map()
to map the array of objects to the value ofkey
for each one.
const pluck = (arr, key) => arr.map(i => i[key]);
const simpsons = [
{ name: 'lisa', age: 8 },
{ name: 'homer', age: 36 },
{ name: 'marge', age: 34 },
{ name: 'bart', age: 10 }
];
pluck(simpsons, 'age'); // [8, 36, 34, 10]
pluralize
- title: pluralize
- tags: string,advanced
Returns the singular or plural form of the word based on the input number, using an optional dictionary if supplied.
- Use a closure to define a function that pluralizes the given
word
based on the value ofnum
. - If
num
is either-1
or1
, return the singular form of the word. - If
num
is any other number, return theplural
form. - Omit the third argument,
plural
, to use the default of the singular word +s
, or supply a custom pluralizedword
when necessary. - If the first argument is an
object
, return a function which can use the supplied dictionary to resolve the correct plural form of the word.
const pluralize = (val, word, plural = word + 's') => {
const _pluralize = (num, word, plural = word + 's') =>
[1, -1].includes(Number(num)) ? word : plural;
if (typeof val === 'object')
return (num, word) => _pluralize(num, word, val[word]);
return _pluralize(val, word, plural);
};
pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'
const PLURALS = {
person: 'people',
radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'
powerset
- title: powerset
- tags: math,algorithm,beginner
Returns the powerset of a given array of numbers.
- Use
Array.prototype.reduce()
combined withArray.prototype.map()
to iterate over elements and combine into an array containing all combinations.
const powerset = arr =>
arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
powerset([1, 2]); // [[], [1], [2], [2, 1]]
prefersDarkColorScheme
- title: prefersDarkColorScheme
- tags: browser,intermediate
Checks if the user color scheme preference is dark
.
- Use
Window.matchMedia()
with the appropriate media query to check the user color scheme preference.
const prefersDarkColorScheme = () =>
window &&
window.matchMedia &&
window.matchMedia('(prefers-color-scheme: dark)').matches;
prefersDarkColorScheme(); // true
prefersLightColorScheme
- title: prefersLightColorScheme
- tags: browser,intermediate
Checks if the user color scheme preference is light
.
- Use
Window.matchMedia()
with the appropriate media query to check the user color scheme preference.
const prefersLightColorScheme = () =>
window &&
window.matchMedia &&
window.matchMedia('(prefers-color-scheme: light)').matches;
prefersLightColorScheme(); // true
prefix
- title: prefix
- tags: browser,intermediate
Prefixes a CSS property based on the current browser.
- Use
Array.prototype.findIndex()
on an array of vendor prefix strings to test ifDocument.body
has one of them defined in itsCSSStyleDeclaration
object, otherwise returnnull
. - Use
String.prototype.charAt()
andString.prototype.toUpperCase()
to capitalize the property, which will be appended to the vendor prefix string.
const prefix = prop => {
const capitalizedProp = prop.charAt(0).toUpperCase() + prop.slice(1);
const prefixes = ['', 'webkit', 'moz', 'ms', 'o'];
const i = prefixes.findIndex(
prefix =>
typeof document.body.style[prefix ? prefix + capitalizedProp : prop] !==
'undefined'
);
return i !== -1 ? (i === 0 ? prop : prefixes[i] + capitalizedProp) : null;
};
prefix('appearance');
// 'appearance' on a supported browser, otherwise 'webkitAppearance', 'mozAppearance', 'msAppearance' or 'oAppearance'
prettyBytes
- title: prettyBytes
- tags: string,math,advanced
Converts a number in bytes to a human-readable string.
- Use an array dictionary of units to be accessed based on the exponent.
- Use
Number.prototype.toPrecision()
to truncate the number to a certain number of digits. - Return the prettified string by building it up, taking into account the supplied options and whether it is negative or not.
- Omit the second argument,
precision
, to use a default precision of3
digits. - Omit the third argument,
addSpace
, to add space between the number and unit by default.
const prettyBytes = (num, precision = 3, addSpace = true) => {
const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
const exponent = Math.min(
Math.floor(Math.log10(num < 0 ? -num : num) / 3),
UNITS.length - 1
);
const n = Number(
((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision)
);
return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};
prettyBytes(1000); // '1 KB'
prettyBytes(-27145424323.5821, 5); // '-27.145 GB'
prettyBytes(123456789, 3, false); // '123MB'
primeFactors
- title: primeFactors
- tags: math,algorithm,beginner
Finds the prime factors of a given number using the trial division algorithm.
- Use a
while
loop to iterate over all possible prime factors, starting with2
. - If the current factor,
f
, exactly dividesn
, addf
to the factors array and dividen
byf
. Otherwise, incrementf
by one.
const primeFactors = n => {
let a = [],
f = 2;
while (n > 1) {
if (n % f === 0) {
a.push(f);
n /= f;
} else {
f++;
}
}
return a;
};
primeFactors(147); // [3, 7, 7]
primes
- title: primes
- tags: math,algorithm,intermediate
Generates primes up to a given number, using the Sieve of Eratosthenes.
- Generate an array from
2
to the given number. - Use
Array.prototype.filter()
to filter out the values divisible by any number from2
to the square root of the provided number.
const primes = num => {
let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
sqroot = Math.floor(Math.sqrt(num)),
numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
return arr;
};
primes(10); // [2, 3, 5, 7]
prod
- title: prod
- tags: math,array,intermediate
Calculates the product of two or more numbers/arrays.
- Use
Array.prototype.reduce()
to multiply each value with an accumulator, initialized with a value of1
.
const prod = (...arr) => [...arr].reduce((acc, val) => acc * val, 1);
prod(1, 2, 3, 4); // 24
prod(...[1, 2, 3, 4]); // 24
promisify
- title: promisify
- tags: function,promise,intermediate
Converts an asynchronous function to return a promise.
- Use currying to return a function returning a
Promise
that calls the original function. - Use the rest operator (
...
) to pass in all the parameters. - Note: In Node 8+, you can use
util.promisify
.
const promisify = func => (...args) =>
new Promise((resolve, reject) =>
func(...args, (err, result) => (err ? reject(err) : resolve(result)))
);
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // Promise resolves after 2s
pull
- title: pull
- tags: array,intermediate
Mutates the original array to filter out the values specified.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values.
const pull = (arr, ...args) => {
let argState = Array.isArray(args[0]) ? args[0] : args;
let pulled = arr.filter(v => !argState.includes(v));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]
pullAtIndex
- title: pullAtIndex
- tags: array,advanced
Mutates the original array to filter out the values at the specified indexes. Returns the removed elements.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values. - Use
Array.prototype.push()
to keep track of pulled values.
const pullAtIndex = (arr, pullArr) => {
let removed = [];
let pulled = arr
.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
.filter((v, i) => !pullArr.includes(i));
arr.length = 0;
pulled.forEach(v => arr.push(v));
return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]);
// myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
pullAtValue
- title: pullAtValue
- tags: array,advanced
Mutates the original array to filter out the values specified. Returns the removed elements.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values. - Use
Array.prototype.push()
to keep track of pulled values.
const pullAtValue = (arr, pullArr) => {
let removed = [],
pushToRemove = arr.forEach((v, i) =>
pullArr.includes(v) ? removed.push(v) : v
),
mutateTo = arr.filter((v, i) => !pullArr.includes(v));
arr.length = 0;
mutateTo.forEach(v => arr.push(v));
return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']);
// myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
pullBy
- title: pullBy
- tags: array,advanced
Mutates the original array to filter out the values specified, based on a given iterator function.
- Check if the last argument provided is a function.
- Use
Array.prototype.map()
to apply the iterator functionfn
to all array elements. - Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values.
const pullBy = (arr, ...args) => {
const length = args.length;
let fn = length > 1 ? args[length - 1] : undefined;
fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]
quarterOfYear
- title: quarterOfYear
- tags: date,beginner
Returns the quarter and year to which the supplied date belongs to.
- Use
Date.prototype.getMonth()
to get the current month in the range (0, 11), add1
to map it to the range (1, 12). - Use
Math.ceil()
and divide the month by3
to get the current quarter. - Use
Date.prototype.getFullYear()
to get the year from the givendate
. - Omit the argument,
date
, to use the current date by default.
const quarterOfYear = (date = new Date()) => [
Math.ceil((date.getMonth() + 1) / 3),
date.getFullYear()
];
quarterOfYear(new Date('07/10/2018')); // [ 3, 2018 ]
quarterOfYear(); // [ 4, 2020 ]
queryStringToObject
- title: queryStringToObject
- tags: object,intermediate
Generates an object from the given query string or URL.
- Use
String.prototype.split()
to get the params from the givenurl
. - Use
new URLSearchParams()
to create an appropriate object and convert it to an array of key-value pairs using the spread operator (...
). - Use
Array.prototype.reduce()
to convert the array of key-value pairs into an object.
const queryStringToObject = url =>
[...new URLSearchParams(url.split('?')[1])].reduce(
(a, [k, v]) => ((a[k] = v), a),
{}
);
queryStringToObject('https://google.com?page=1&count=10');
// {page: '1', count: '10'}
quickSort
- title: quickSort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the quicksort algorithm.
- Use recursion.
- Use the spread operator (
...
) to clone the original array,arr
. - If the
length
of the array is less than2
, return the cloned array. - Use
Math.floor()
to calculate the index of the pivot element. - Use
Array.prototype.reduce()
andArray.prototype.push()
to split the array into two subarrays (elements smaller or equal to thepivot
and elements greater than it), destructuring the result into two arrays. - Recursively call
quickSort()
on the created subarrays.
const quickSort = arr => {
const a = [...arr];
if (a.length < 2) return a;
const pivotIndex = Math.floor(arr.length / 2);
const pivot = a[pivotIndex];
const [lo, hi] = a.reduce(
(acc, val, i) => {
if (val < pivot || (val === pivot && i != pivotIndex)) {
acc[0].push(val);
} else if (val > pivot) {
acc[1].push(val);
}
return acc;
},
[[], []]
);
return [...quickSort(lo), pivot, ...quickSort(hi)];
};
quickSort([1, 6, 1, 5, 3, 2, 1, 4]); // [1, 1, 1, 2, 3, 4, 5, 6]
radsToDegrees
- title: radsToDegrees
- tags: math,beginner
Converts an angle from radians to degrees.
- Use
Math.PI
and the radian to degree formula to convert the angle from radians to degrees.
const radsToDegrees = rad => (rad * 180.0) / Math.PI;
radsToDegrees(Math.PI / 2); // 90
randomAlphaNumeric
- title: randomAlphaNumeric
- tags: string,random,advanced
Generates a random string with the specified length.
- Use
Array.from()
to create a new array with the specifiedlength
. - Use
Math.random()
generate a random floating-point number,Number.prototype.toString(36)
to convert it to an alphanumeric string. - Use
String.prototype.slice(2)
to remove the integral part and decimal point from each generated number. - Use
Array.prototype.some()
to repeat this process as many times as required, up tolength
, as it produces a variable-length string each time. - Finally, use
String.prototype.slice()
to trim down the generated string if it's longer than the givenlength
.
const randomAlphaNumeric = length => {
let s = '';
Array.from({ length }).some(() => {
s += Math.random().toString(36).slice(2);
return s.length >= length;
});
return s.slice(0, length);
};
randomAlphaNumeric(5); // '0afad'
randomBoolean
- title: randomBoolean
- tags: math,random,beginner
Generates a random boolean value.
- Use
Math.random()
to generate a random number and check if it is greater than or equal to0.5
.
const randomBoolean = () => Math.random() >= 0.5;
randomBoolean(); // true
randomHexColorCode
- title: randomHexColorCode
- tags: math,random,beginner
Generates a random hexadecimal color code.
- Use
Math.random()
to generate a random 24-bit (6 * 4bits) hexadecimal number. - Use bit shifting and then convert it to an hexadecimal string using
Number.prototype.toString(16)
.
const randomHexColorCode = () => {
let n = (Math.random() * 0xfffff * 1000000).toString(16);
return '##' + n.slice(0, 6);
};
randomHexColorCode(); // '##e34155'
randomIntArrayInRange
- title: randomIntArrayInRange
- tags: math,random,intermediate
Generates an array of n
random integers in the specified range.
- Use
Array.from()
to create an empty array of the specific length. - Use
Math.random()
to generate random numbers and map them to the desired range, usingMath.floor()
to make them integers.
const randomIntArrayInRange = (min, max, n = 1) =>
Array.from(
{ length: n },
() => Math.floor(Math.random() * (max - min + 1)) + min
);
randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]
randomIntegerInRange
- title: randomIntegerInRange
- tags: math,random,beginner
Generates a random integer in the specified range.
- Use
Math.random()
to generate a random number and map it to the desired range. - Use
Math.floor()
to make it an integer.
const randomIntegerInRange = (min, max) =>
Math.floor(Math.random() * (max - min + 1)) + min;
randomIntegerInRange(0, 5); // 2
randomNumberInRange
- title: randomNumberInRange
- tags: math,random,beginner
Generates a random number in the specified range.
- Use
Math.random()
to generate a random value, map it to the desired range using multiplication.
const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;
randomNumberInRange(2, 10); // 6.0211363285087005
rangeGenerator
- title: rangeGenerator
- tags: function,generator,advanced
Creates a generator, that generates all values in the given range using the given step.
- Use a
while
loop to iterate fromstart
toend
, usingyield
to return each value and then incrementing bystep
. - Omit the third argument,
step
, to use a default value of1
.
const rangeGenerator = function* (start, end, step = 1) {
let i = start;
while (i < end) {
yield i;
i += step;
}
};
for (let i of rangeGenerator(6, 10)) console.log(i);
// Logs 6, 7, 8, 9
readFileLines
- title: readFileLines
- tags: node,array,beginner
Returns an array of lines from the specified file.
- Use
fs.readFileSync()
to create aBuffer
from a file. - Convert buffer to string using
buf.toString(encoding)
function. - Use
String.prototype.split(\n)
to create an array of lines from the contents of the file.
const fs = require('fs');
const readFileLines = filename =>
fs
.readFileSync(filename)
.toString('UTF8')
.split('\n');
/*
contents of test.txt :
line1
line2
line3
___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']
rearg
- title: rearg
- tags: function,intermediate
Creates a function that invokes the provided function with its arguments arranged according to the specified indexes.
- Use
Array.prototype.map()
to reorder arguments based onindexes
. - Use the spread operator (
...
) to pass the transformed arguments tofn
.
const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));
var rearged = rearg(
function(a, b, c) {
return [a, b, c];
},
[2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']
recordAnimationFrames
- title: recordAnimationFrames
- tags: browser,recursion,intermediate
Invokes the provided callback on each animation frame.
- Use recursion.
- Provided that
running
istrue
, continue invokingWindow.requestAnimationFrame()
which invokes the provided callback. - Return an object with two methods
start
andstop
to allow manual control of the recording. - Omit the second argument,
autoStart
, to implicitly callstart
when the function is invoked.
const recordAnimationFrames = (callback, autoStart = true) => {
let running = false,
raf;
const stop = () => {
if (!running) return;
running = false;
cancelAnimationFrame(raf);
};
const start = () => {
if (running) return;
running = true;
run();
};
const run = () => {
raf = requestAnimationFrame(() => {
callback();
if (running) run();
});
};
if (autoStart) start();
return { start, stop };
};
const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb);
// logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false);
// `start` needs to be explicitly called to begin recording frames
redirect
- title: redirect
- tags: browser,beginner
Redirects to a specified URL.
- Use
Window.location.href
orWindow.location.replace()
to redirect tourl
. - Pass a second argument to simulate a link click (
true
- default) or an HTTP redirect (false
).
const redirect = (url, asLink = true) =>
asLink ? (window.location.href = url) : window.location.replace(url);
redirect('https://google.com');
reduceSuccessive
- title: reduceSuccessive
- tags: array,intermediate
Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.
- Use
Array.prototype.reduce()
to apply the given function to the given array, storing each new result.
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce(
(res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res),
[acc]
);
reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0);
// [0, 1, 3, 6, 10, 15, 21]
reduceWhich
- title: reduceWhich
- tags: array,intermediate
Returns the minimum/maximum value of an array, after applying the provided function to set the comparing rule.
- Use
Array.prototype.reduce()
in combination with thecomparator
function to get the appropriate element in the array. - Omit the second argument,
comparator
, to use the default one that returns the minimum element in the array.
const reduceWhich = (arr, comparator = (a, b) => a - b) =>
arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));
reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
[
{ name: 'Tom', age: 12 },
{ name: 'Jack', age: 18 },
{ name: 'Lucy', age: 9 }
],
(a, b) => a.age - b.age
); // {name: 'Lucy', age: 9}
reducedFilter
- title: reducedFilter
- tags: array,intermediate
Filters an array of objects based on a condition while also filtering out unspecified keys.
- Use
Array.prototype.filter()
to filter the array based on the predicatefn
so that it returns the objects for which the condition returned a truthy value. - On the filtered array, use
Array.prototype.map()
to return the new object. - Use
Array.prototype.reduce()
to filter out the keys which were not supplied as thekeys
argument.
const reducedFilter = (data, keys, fn) =>
data.filter(fn).map(el =>
keys.reduce((acc, key) => {
acc[key] = el[key];
return acc;
}, {})
);
const data = [
{
id: 1,
name: 'john',
age: 24
},
{
id: 2,
name: 'mike',
age: 50
}
];
reducedFilter(data, ['id', 'name'], item => item.age > 24);
// [{ id: 2, name: 'mike'}]
reject
- title: reject
- tags: array,beginner
Filters an array's values based on a predicate function, returning only values for which the predicate function returns false
.
- Use
Array.prototype.filter()
in combination with the predicate function,pred
, to return only the values for which it returnsfalse
.
const reject = (pred, array) => array.filter((...args) => !pred(...args));
reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']);
// ['Pear', 'Kiwi']
remove
- title: remove
- tags: array,intermediate
Mutates an array by removing elements for which the given function returns false
.
- Use
Array.prototype.filter()
to find array elements that return truthy values. - Use
Array.prototype.reduce()
to remove elements usingArray.prototype.splice()
. - The callback function is invoked with three arguments (value, index, array).
const remove = (arr, func) =>
Array.isArray(arr)
? arr.filter(func).reduce((acc, val) => {
arr.splice(arr.indexOf(val), 1);
return acc.concat(val);
}, [])
: [];
remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]
removeAccents
- title: removeAccents
- tags: string,beginner
Removes accents from strings.
- Use
String.prototype.normalize()
to convert the string to a normalized Unicode format. - Use
String.prototype.replace()
to replace diacritical marks in the given Unicode range by empty strings.
const removeAccents = str =>
str.normalize('NFD').replace(/[\u0300-\u036f]/g, '');
removeAccents('Antoine de Saint-Exupéry'); // 'Antoine de Saint-Exupery'
removeClass
- title: removeClass
- tags: browser,beginner
Removes a class from an HTML element.
- Use
Element.classList
andDOMTokenList.remove()
to remove the specified class from the element.
const removeClass = (el, className) => el.classList.remove(className);
removeClass(document.querySelector('p.special'), 'special');
// The paragraph will not have the 'special' class anymore
removeElement
- title: removeElement
- tags: browser,beginner
Removes an element from the DOM.
- Use
Element.parentNode
to get the given element's parent node. - Use
Element.removeChild()
to remove the given element from its parent node.
const removeElement = el => el.parentNode.removeChild(el);
removeElement(document.querySelector('##my-element'));
// Removes ##my-element from the DOM
removeNonASCII
- title: removeNonASCII
- tags: string,regexp,intermediate
Removes non-printable ASCII characters.
- Use
String.prototype.replace()
with a regular expression to remove non-printable ASCII characters.
const removeNonASCII = str => str.replace(/[^\x20-\x7E]/g, '');
removeNonASCII('äÄçÇéÉêlorem-ipsumöÖÐþúÚ'); // 'lorem-ipsum'
removeWhitespace
- title: removeWhitespace
- tags: string,regexp,beginner
Returns a string with whitespaces removed.
- Use
String.prototype.replace()
with a regular expression to replace all occurrences of whitespace characters with an empty string.
const removeWhitespace = str => str.replace(/\s+/g, '');
removeWhitespace('Lorem ipsum.\n Dolor sit amet. ');
// 'Loremipsum.Dolorsitamet.'
renameKeys
- title: renameKeys
- tags: object,intermediate
Replaces the names of multiple object keys with the values provided.
- Use
Object.keys()
in combination withArray.prototype.reduce()
and the spread operator (...
) to get the object's keys and rename them according tokeysMap
.
const renameKeys = (keysMap, obj) =>
Object.keys(obj).reduce(
(acc, key) => ({
...acc,
...{ [keysMap[key] || key]: obj[key] }
}),
{}
);
const obj = { name: 'Bobo', job: 'Front-End Master', shoeSize: 100 };
renameKeys({ name: 'firstName', job: 'passion' }, obj);
// { firstName: 'Bobo', passion: 'Front-End Master', shoeSize: 100 }
renderElement
- title: renderElement
- tags: browser,recursion,advanced
Renders the given DOM tree in the specified DOM element.
- Destructure the first argument into
type
andprops
, usingtype
to determine if the given element is a text element. - Based on the element's
type
, use eitherDocument.createTextNode()
orDocument.createElement()
to create the DOM element. - Use
Object.keys()
to add attributes to the DOM element and setting event listeners, as necessary. - Use recursion to render
props.children
, if any. - Finally, use
Node.appendChild()
to append the DOM element to the specifiedcontainer
.
const renderElement = ({ type, props = {} }, container) => {
const isTextElement = !type;
const element = isTextElement
? document.createTextNode('')
: document.createElement(type);
const isListener = p => p.startsWith('on');
const isAttribute = p => !isListener(p) && p !== 'children';
Object.keys(props).forEach(p => {
if (isAttribute(p)) element[p] = props[p];
if (!isTextElement && isListener(p))
element.addEventListener(p.toLowerCase().slice(2), props[p]);
});
if (!isTextElement && props.children && props.children.length)
props.children.forEach(childElement =>
renderElement(childElement, element)
);
container.appendChild(element);
};
const myElement = {
type: 'button',
props: {
type: 'button',
className: 'btn',
onClick: () => alert('Clicked'),
children: [{ props: { nodeValue: 'Click me' } }]
}
};
renderElement(myElement, document.body);
repeatGenerator
- title: repeatGenerator
- tags: function,generator,advanced
Creates a generator, repeating the given value indefinitely.
- Use a non-terminating
while
loop, that willyield
a value every timeGenerator.prototype.next()
is called. - Use the return value of the
yield
statement to update the returned value if the passed value is notundefined
.
const repeatGenerator = function* (val) {
let v = val;
while (true) {
let newV = yield v;
if (newV !== undefined) v = newV;
}
};
const repeater = repeatGenerator(5);
repeater.next(); // { value: 5, done: false }
repeater.next(); // { value: 5, done: false }
repeater.next(4); // { value: 4, done: false }
repeater.next(); // { value: 4, done: false }
requireUncached
- title: requireUncached
- tags: node,advanced
Loads a module after removing it from the cache (if exists).
- Use
delete
to remove the module from the cache (if exists). - Use
require()
to load the module again.
const requireUncached = module => {
delete require.cache[require.resolve(module)];
return require(module);
};
const fs = requireUncached('fs'); // 'fs' will be loaded fresh every time
reverseNumber
- title: reverseNumber
- tags: math,string,beginner
Reverses a number.
- Use
Object.prototype.toString()
to convertn
to a string. - Use
String.prototype.split('')
,Array.prototype.reverse()
andString.prototype.join('')
to get the reversed value ofn
as a string. - Use
parseFloat()
to convert the string to a number andMath.sign()
to preserve its sign.
const reverseNumber = n =>
parseFloat(`${n}`.split('').reverse().join('')) * Math.sign(n);
reverseNumber(981); // 189
reverseNumber(-500); // -5
reverseNumber(73.6); // 6.37
reverseNumber(-5.23); // -32.5
reverseString
- title: reverseString
- tags: string,beginner
Reverses a string.
- Use the spread operator (
...
) andArray.prototype.reverse()
to reverse the order of the characters in the string. - Combine characters to get a string using
String.prototype.join('')
.
const reverseString = str => [...str].reverse().join('');
reverseString('foobar'); // 'raboof'
round
- title: round
- tags: math,intermediate
Rounds a number to a specified amount of digits.
- Use
Math.round()
and template literals to round the number to the specified number of digits. - Omit the second argument,
decimals
, to round to an integer.
const round = (n, decimals = 0) =>
Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
round(1.005, 2); // 1.01
runAsync
- title: runAsync
- tags: browser,function,promise,advanced
Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.
- Create a
new Worker()
using aBlob
object URL, the contents of which should be the stringified version of the supplied function. - Immediately post the return value of calling the function back.
- Return a
new Promise()
, listening foronmessage
andonerror
events and resolving the data posted back from the worker, or throwing an error.
const runAsync = fn => {
const worker = new Worker(
URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
type: 'application/javascript; charset=utf-8'
})
);
return new Promise((res, rej) => {
worker.onmessage = ({ data }) => {
res(data), worker.terminate();
};
worker.onerror = err => {
rej(err), worker.terminate();
};
});
};
const longRunningFunction = () => {
let result = 0;
for (let i = 0; i < 1000; i++)
for (let j = 0; j < 700; j++)
for (let k = 0; k < 300; k++) result = result + i + j + k;
return result;
};
/*
NOTE: Since the function is running in a different context, closures are not supported.
The function supplied to `runAsync` gets stringified, so everything becomes literal.
All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'
runPromisesInSeries
- title: runPromisesInSeries
- tags: function,promise,intermediate
Runs an array of promises in series.
- Use
Array.prototype.reduce()
to create a promise chain, where each promise returns the next promise when resolved.
const runPromisesInSeries = ps =>
ps.reduce((p, next) => p.then(next), Promise.resolve());
const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]);
// Executes each promise sequentially, taking a total of 3 seconds to complete
sample
- title: sample
- tags: array,string,random,beginner
Gets a random element from an array.
- Use
Math.random()
to generate a random number. - Multiply it by
Array.prototype.length
and round it off to the nearest whole number usingMath.floor()
. - This method also works with strings.
const sample = arr => arr[Math.floor(Math.random() * arr.length)];
sample([3, 7, 9, 11]); // 9
sampleSize
- title: sampleSize
- tags: array,random,intermediate
Gets n
random elements at unique keys from an array up to the size of the array.
- Shuffle the array using the Fisher-Yates algorithm.
- Use
Array.prototype.slice()
to get the firstn
elements. - Omit the second argument,
n
, to get only one element at random from the array.
const sampleSize = ([...arr], n = 1) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr.slice(0, n);
};
sampleSize([1, 2, 3], 2); // [3, 1]
sampleSize([1, 2, 3], 4); // [2, 3, 1]
scrollToTop
- title: scrollToTop
- tags: browser,intermediate
Smooth-scrolls to the top of the page.
- Get distance from top using
Document.documentElement
orDocument.body
andElement.scrollTop
. - Scroll by a fraction of the distance from the top.
- Use
Window.requestAnimationFrame()
to animate the scrolling.
const scrollToTop = () => {
const c = document.documentElement.scrollTop || document.body.scrollTop;
if (c > 0) {
window.requestAnimationFrame(scrollToTop);
window.scrollTo(0, c - c / 8);
}
};
scrollToTop(); // Smooth-scrolls to the top of the page
sdbm
- title: sdbm
- tags: math,intermediate
Hashes the input string into a whole number.
- Use
String.prototype.split('')
andArray.prototype.reduce()
to create a hash of the input string, utilizing bit shifting.
const sdbm = str => {
let arr = str.split('');
return arr.reduce(
(hashCode, currentVal) =>
(hashCode =
currentVal.charCodeAt(0) +
(hashCode << 6) +
(hashCode << 16) -
hashCode),
0
);
};
sdbm('name'); // -3521204949
selectionSort
- title: selectionSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the selection sort algorithm.
- Use the spread operator (
...
) to clone the original array,arr
. - Use a
for
loop to iterate over elements in the array. - Use
Array.prototype.slice()
andArray.prototype.reduce()
to find the index of the minimum element in the subarray to the right of the current index and perform a swap, if necessary.
const selectionSort = arr => {
const a = [...arr];
for (let i = 0; i < a.length; i++) {
const min = a
.slice(i + 1)
.reduce((acc, val, j) => (val < a[acc] ? j + i + 1 : acc), i);
if (min !== i) [a[i], a[min]] = [a[min], a[i]];
}
return a;
};
selectionSort([5, 1, 4, 2, 3]); // [1, 2, 3, 4, 5]
serializeCookie
- title: serializeCookie
- tags: browser,string,intermediate
Serializes a cookie name-value pair into a Set-Cookie header string.
- Use template literals and
encodeURIComponent()
to create the appropriate string.
const serializeCookie = (name, val) =>
`${encodeURIComponent(name)}=${encodeURIComponent(val)}`;
serializeCookie('foo', 'bar'); // 'foo=bar'
serializeForm
- title: serializeForm
- tags: browser,string,intermediate
Encodes a set of form elements as a query string.
- Use the
Foata
constructor to convert the HTMLform
toFoata
. - Use
Array.from()
to convert to an array, passing a map function as the second argument. - Use
Array.prototype.map()
andencodeURIComponent()
to encode each field's value. - Use
Array.prototype.join()
with appropriate arguments to produce an appropriate query string.
const serializeForm = form =>
Array.from(new Foata(form), field =>
field.map(encodeURIComponent).join('=')
).join('&');
serializeForm(document.querySelector('##form'));
// email=test%40email.com&name=Test%20Name
setStyle
- title: setStyle
- tags: browser,beginner
Sets the value of a CSS rule for the specified HTML element.
- Use
ElementCSSInlineStyle.style
to set the value of the CSSrule
for the specified element toval
.
const setStyle = (el, rule, val) => (el.style[rule] = val);
setStyle(document.querySelector('p'), 'font-size', '20px');
// The first <p> element on the page will have a font-size of 20px
shallowClone
- title: shallowClone
- tags: object,beginner
Creates a shallow clone of an object.
- Use
Object.assign()
and an empty object ({}
) to create a shallow clone of the original.
const shallowClone = obj => Object.assign({}, obj);
const a = { x: true, y: 1 };
const b = shallowClone(a); // a !== b
shank
- title: shank
- tags: array,intermediate
Has the same functionality as Array.prototype.splice()
, but returning a new array instead of mutating the original array.
- Use
Array.prototype.slice()
andArray.prototype.concat()
to get an array with the new contents after removing existing elements and/or adding new elements. - Omit the second argument,
index
, to start at0
. - Omit the third argument,
delCount
, to remove0
elements. - Omit the fourth argument,
elements
, in order to not add any new elements.
const shank = (arr, index = 0, delCount = 0, ...elements) =>
arr
.slice(0, index)
.concat(elements)
.concat(arr.slice(index + delCount));
const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta');
// [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']
show
- title: show
- tags: browser,css,beginner
Shows all the elements specified.
- Use the spread operator (
...
) andArray.prototype.forEach()
to clear thedisplay
property for each element specified.
const show = (...el) => [...el].forEach(e => (e.style.display = ''));
show(...document.querySelectorAll('img'));
// Shows all <img> elements on the page
shuffle
- title: shuffle
- tags: array,random,intermediate
Randomizes the order of the values of an array, returning a new array.
- Use the Fisher-Yates algorithm to reorder the elements of the array.
const shuffle = ([...arr]) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr;
};
const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]
similarity
- title: similarity
- tags: array,math,beginner
Returns an array of elements that appear in both arrays.
- Use
Array.prototype.includes()
to determine values that are not part ofvalues
. - Use
Array.prototype.filter()
to remove them.
const similarity = (arr, values) => arr.filter(v => values.includes(v));
similarity([1, 2, 3], [1, 2, 4]); // [1, 2]
size
- title: size
- tags: object,array,string,intermediate
Gets the size of an array, object or string.
- Get type of
val
(array
,object
orstring
). - Use
Array.prototype.length
property for arrays. - Use
length
orsize
value if available or number of keys for objects. - Use
size
of aBlob
object created fromval
for strings. - Split strings into array of characters with
split('')
and return its length.
const size = val =>
Array.isArray(val)
? val.length
: val && typeof val === 'object'
? val.size || val.length || Object.keys(val).length
: typeof val === 'string'
? new Blob([val]).size
: 0;
size([1, 2, 3, 4, 5]); // 5
size('size'); // 4
size({ one: 1, two: 2, three: 3 }); // 3
sleep
- title: sleep
- tags: function,promise,intermediate
Delays the execution of an asynchronous function.
- Delay executing part of an
async
function, by putting it to sleep, returning anew Promise()
.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
async function sleepyWork() {
console.log("I'm going to sleep for 1 second.");
await sleep(1000);
console.log('I woke up after 1 second.');
}
slugify
- title: slugify
- tags: string,regexp,intermediate
Converts a string to a URL-friendly slug.
- Use
String.prototype.toLowerCase()
andString.prototype.trim()
to normalize the string. - Use
String.prototype.replace()
to replace spaces, dashes and underscores with-
and remove special characters.
const slugify = str =>
str
.toLowerCase()
.trim()
.replace(/[^\w\s-]/g, '')
.replace(/[\s_-]+/g, '-')
.replace(/^-+|-+$/g, '');
slugify('Hello World!'); // 'hello-world'
smoothScroll
- title: smoothScroll
- tags: browser,css,intermediate
Smoothly scrolls the element on which it's called into the visible area of the browser window.
- Use
Element.scrollIntoView()
to scroll the element. - Use
{ behavior: 'smooth' }
to scroll smoothly.
const smoothScroll = element =>
document.querySelector(element).scrollIntoView({
behavior: 'smooth'
});
smoothScroll('##fooBar'); // scrolls smoothly to the element with the id fooBar
smoothScroll('.fooBar');
// scrolls smoothly to the first element with a class of fooBar
sortCharactersInString
- title: sortCharactersInString
- tags: string,beginner
Alphabetically sorts the characters in a string.
- Use the spread operator (
...
),Array.prototype.sort()
andString.prototype.localeCompare()
to sort the characters instr
. - Recombine using
String.prototype.join('')
.
const sortCharactersInString = str =>
[...str].sort((a, b) => a.localeCompare(b)).join('');
sortCharactersInString('cabbage'); // 'aabbceg'
sortedIndex
- title: sortedIndex
- tags: array,math,intermediate
Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.findIndex()
to find the appropriate index where the element should be inserted.
const sortedIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
return index === -1 ? arr.length : index;
};
sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1
sortedIndexBy
- title: sortedIndexBy
- tags: array,math,intermediate
Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order, based on the provided iterator function.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.findIndex()
to find the appropriate index where the element should be inserted, based on the iterator functionfn
.
const sortedIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr.findIndex(el =>
isDescending ? val >= fn(el) : val <= fn(el)
);
return index === -1 ? arr.length : index;
};
sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0
sortedLastIndex
- title: sortedLastIndex
- tags: array,intermediate
Finds the highest index at which a value should be inserted into an array in order to maintain its sort order.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.reverse()
andArray.prototype.findIndex()
to find the appropriate last index where the element should be inserted.
const sortedLastIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr
.reverse()
.findIndex(el => (isDescending ? n <= el : n >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndex([10, 20, 30, 30, 40], 30); // 4
sortedLastIndexBy
- title: sortedLastIndexBy
- tags: array,intermediate
Finds the highest index at which a value should be inserted into an array in order to maintain its sort order, based on a provided iterator function.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.map()
to apply the iterator function to all elements of the array. - Use
Array.prototype.reverse()
andArray.prototype.findIndex()
to find the appropriate last index where the element should be inserted, based on the provided iterator function.
const sortedLastIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr
.map(fn)
.reverse()
.findIndex(el => (isDescending ? val <= el : val >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1
splitLines
- title: splitLines
- tags: string,regexp,beginner
Splits a multiline string into an array of lines.
- Use
String.prototype.split()
and a regular expression to match line breaks and create an array.
const splitLines = str => str.split(/\r?\n/);
splitLines('This\nis a\nmultiline\nstring.\n');
// ['This', 'is a', 'multiline', 'string.' , '']
spreadOver
- title: spreadOver
- tags: function,intermediate
Takes a variadic function and returns a function that accepts an array of arguments.
- Use a closure and the spread operator (
...
) to map the array of arguments to the inputs of the function.
const spreadOver = fn => argsArr => fn(...argsArr);
const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3
stableSort
- title: stableSort
- tags: array,advanced
Performs stable sorting of an array, preserving the initial indexes of items when their values are the same.
- Use
Array.prototype.map()
to pair each element of the input array with its corresponding index. - Use
Array.prototype.sort()
and acompare
function to sort the list, preserving their initial order if the items compared are equal. - Use
Array.prototype.map()
to convert back to the initial array items. - Does not mutate the original array, but returns a new array instead.
const stableSort = (arr, compare) =>
arr
.map((item, index) => ({ item, index }))
.sort((a, b) => compare(a.item, b.item) || a.index - b.index)
.map(({ item }) => item);
const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
standardDeviation
- title: standardDeviation
- tags: math,array,intermediate
Calculates the standard deviation of an array of numbers.
- Use
Array.prototype.reduce()
to calculate the mean, variance and the sum of the variance of the values and determine the standard deviation. - Omit the second argument,
usePopulation
, to get the sample standard deviation or set it totrue
to get the population standard deviation.
const standardDeviation = (arr, usePopulation = false) => {
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
return Math.sqrt(
arr
.reduce((acc, val) => acc.concat((val - mean) ** 2), [])
.reduce((acc, val) => acc + val, 0) /
(arr.length - (usePopulation ? 0 : 1))
);
};
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true);
// 12.29899614287479 (population)
stringPermutations
- title: stringPermutations
- tags: string,recursion,advanced
Generates all permutations of a string (contains duplicates).
- Use recursion.
- For each letter in the given string, create all the partial permutations for the rest of its letters.
- Use
Array.prototype.map()
to combine the letter with each partial permutation. - Use
Array.prototype.reduce()
to combine all permutations in one array. - Base cases are for
String.prototype.length
equal to2
or1
. - ⚠️ WARNING: The execution time increases exponentially with each character. Anything more than 8 to 10 characters will cause your environment to hang as it tries to solve all the different combinations.
const stringPermutations = str => {
if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
return str
.split('')
.reduce(
(acc, letter, i) =>
acc.concat(
stringPermutations(str.slice(0, i) + str.slice(i + 1)).map(
val => letter + val
)
),
[]
);
};
stringPermutations('abc'); // ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
stringifyCircularJSON
- title: stringifyCircularJSON
- tags: object,advanced
Serializes a JSON object containing circular references into a JSON format.
- Create a
new WeakSet()
to store and check seen values, usingWeakSet.prototype.add()
andWeakSet.prototype.has()
. - Use
JSON.stringify()
with a custom replacer function that omits values already inseen
, adding new values as necessary. - ⚠️ NOTICE: This function finds and removes circular references, which causes circular data loss in the serialized JSON.
const stringifyCircularJSON = obj => {
const seen = new WeakSet();
return JSON.stringify(obj, (k, v) => {
if (v !== null && typeof v === 'object') {
if (seen.has(v)) return;
seen.add(v);
}
return v;
});
};
const obj = { n: 42 };
obj.obj = obj;
stringifyCircularJSON(obj); // '{"n": 42}'
stripHTMLTags
- title: stripHTMLTags
- tags: string,regexp,beginner
Removes HTML/XML tags from string.
- Use a regular expression to remove HTML/XML tags from a string.
const stripHTMLTags = str => str.replace(/<[^>]*>/g, '');
stripHTMLTags('<p><em>lorem</em> <strong>ipsum</strong></p>'); // 'lorem ipsum'
subSet
- title: subSet
- tags: array,intermediate
Checks if the first iterable is a subset of the second one, excluding duplicate values.
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that each value in the first iterable is contained in the second one.
const subSet = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sA].every(v => sB.has(v));
};
subSet(new Set([1, 2]), new Set([1, 2, 3, 4])); // true
subSet(new Set([1, 5]), new Set([1, 2, 3, 4])); // false
sum
- title: sum
- tags: math,array,beginner
Calculates the sum of two or more numbers/arrays.
- Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
.
const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);
sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10
sumBy
- title: sumBy
- tags: math,array,intermediate
Calculates the sum of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
.
const sumBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val) => acc + val, 0);
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
sumN
- title: sumN
- tags: math,beginner
Sums all the numbers between 1
and n
.
- Use the formula
(n * (n + 1)) / 2
to get the sum of all the numbers between 1 andn
.
const sumN = n => (n * (n + 1)) / 2;
sumN(100); // 5050
sumPower
- title: sumPower
- tags: math,intermediate
Calculates the sum of the powers of all the numbers from start
to end
(both inclusive).
- Use
Array.prototype.fill()
to create an array of all the numbers in the target range. - Use
Array.prototype.map()
and the exponent operator (**
) to raise them topower
andArray.prototype.reduce()
to add them together. - Omit the second argument,
power
, to use a default power of2
. - Omit the third argument,
start
, to use a default starting value of1
.
const sumPower = (end, power = 2, start = 1) =>
Array(end + 1 - start)
.fill(0)
.map((x, i) => (i + start) ** power)
.reduce((a, b) => a + b, 0);
sumPower(10); // 385
sumPower(10, 3); // 3025
sumPower(10, 3, 5); // 2925
superSet
- title: superSet
- tags: array,intermediate
Checks if the first iterable is a superset of the second one, excluding duplicate values.
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that each value in the second iterable is contained in the first one.
const superSet = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sB].every(v => sA.has(v));
};
superSet(new Set([1, 2, 3, 4]), new Set([1, 2])); // true
superSet(new Set([1, 2, 3, 4]), new Set([1, 5])); // false
supportsTouchEvents
- title: supportsTouchEvents
- tags: browser,beginner
Checks if touch events are supported.
- Check if
'ontouchstart'
exists inwindow
.
const supportsTouchEvents = () =>
window && 'ontouchstart' in window;
supportsTouchEvents(); // true
swapCase
- title: swapCase
- tags: string,beginner
Creates a string with uppercase characters converted to lowercase and vice versa.
- Use the spread operator (
...
) to convertstr
into an array of characters. - Use
String.prototype.toLowerCase()
andString.prototype.toUpperCase()
to convert lowercase characters to uppercase and vice versa. - Use
Array.prototype.map()
to apply the transformation to each character,Array.prototype.join()
to combine back into a string. - Note that it is not necessarily true that
swapCase(swapCase(str)) === str
.
const swapCase = str =>
[...str]
.map(c => (c === c.toLowerCase() ? c.toUpperCase() : c.toLowerCase()))
.join('');
swapCase('Hello world!'); // 'hELLO WORLD!'
symmetricDifference
- title: symmetricDifference
- tags: array,math,intermediate
Returns the symmetric difference between two arrays, without filtering out duplicate values.
- Create a
new Set()
from each array to get the unique values of each one. - Use
Array.prototype.filter()
on each of them to only keep values not contained in the other.
const symmetricDifference = (a, b) => {
const sA = new Set(a),
sB = new Set(b);
return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};
symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]
symmetricDifferenceBy
- title: symmetricDifferenceBy
- tags: array,intermediate
Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.
- Create a
new Set()
from each array to get the unique values of each one after applyingfn
to them. - Use
Array.prototype.filter()
on each of them to only keep values not contained in the other.
const symmetricDifferenceBy = (a, b, fn) => {
const sA = new Set(a.map(v => fn(v))),
sB = new Set(b.map(v => fn(v)));
return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};
symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]
symmetricDifferenceBy(
[{ id: 1 }, { id: 2 }, { id: 3 }],
[{ id: 1 }, { id: 2 }, { id: 4 }],
i => i.id
);
// [{ id: 3 }, { id: 4 }]
symmetricDifferenceWith
- title: symmetricDifferenceWith
- tags: array,intermediate
Returns the symmetric difference between two arrays, using a provided function as a comparator.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
to find the appropriate values.
const symmetricDifferenceWith = (arr, val, comp) => [
...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
];
symmetricDifferenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 3.9]
tail
- title: tail
- tags: array,beginner
Returns all elements in an array except for the first one.
- Return
Array.prototype.slice(1)
ifArray.prototype.length
is more than1
, otherwise, return the whole array.
const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);
tail([1, 2, 3]); // [2, 3]
tail([1]); // [1]
take
- title: take
- tags: array,beginner
Creates an array with n
elements removed from the beginning.
- Use
Array.prototype.slice()
to create a slice of the array withn
elements taken from the beginning.
const take = (arr, n = 1) => arr.slice(0, n);
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []
takeRight
- title: takeRight
- tags: array,intermediate
Creates an array with n
elements removed from the end.
- Use
Array.prototype.slice()
to create a slice of the array withn
elements taken from the end.
const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);
takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]
takeRightUntil
- title: takeRightUntil
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns true
.
Returns the removed elements.
- Create a reversed copy of the array, using the spread operator (
...
) andArray.prototype.reverse()
. - Loop through the reversed copy, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is truthy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeRightUntil = (arr, fn) => {
for (const [i, val] of [...arr].reverse().entries())
if (fn(val)) return i === 0 ? [] : arr.slice(-i);
return arr;
};
takeRightUntil([1, 2, 3, 4], n => n < 3); // [3, 4]
takeRightWhile
- title: takeRightWhile
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns false
.
Returns the removed elements.
- Create a reversed copy of the array, using the spread operator (
...
) andArray.prototype.reverse()
. - Loop through the reversed copy, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is falsy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeRightWhile = (arr, fn) => {
for (const [i, val] of [...arr].reverse().entries())
if (!fn(val)) return i === 0 ? [] : arr.slice(-i);
return arr;
};
takeRightWhile([1, 2, 3, 4], n => n >= 3); // [3, 4]
takeUntil
- title: takeUntil
- tags: array,intermediate
Removes elements in an array until the passed function returns true
.
Returns the removed elements.
- Loop through the array, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is truthy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeUntil = (arr, fn) => {
for (const [i, val] of arr.entries()) if (fn(val)) return arr.slice(0, i);
return arr;
};
takeUntil([1, 2, 3, 4], n => n >= 3); // [1, 2]
takeWhile
- title: takeWhile
- tags: array,intermediate
Removes elements in an array until the passed function returns false
.
Returns the removed elements.
- Loop through the array, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is falsy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeWhile = (arr, fn) => {
for (const [i, val] of arr.entries()) if (!fn(val)) return arr.slice(0, i);
return arr;
};
takeWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
throttle
- title: throttle
- tags: function,advanced
Creates a throttled function that only invokes the provided function at most once per every wait
milliseconds
- Use
setTimeout()
andclearTimeout()
to throttle the given method,fn
. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessaryarguments
. - Use
Date.now()
to keep track of the last time the throttled function was invoked. - Use a variable,
inThrottle
, to prevent a race condition between the first execution offn
and the next loop. - Omit the second argument,
wait
, to set the timeout at a default of 0 ms.
const throttle = (fn, wait) => {
let inThrottle, lastFn, lastTime;
return function() {
const context = this,
args = arguments;
if (!inThrottle) {
fn.apply(context, args);
lastTime = Date.now();
inThrottle = true;
} else {
clearTimeout(lastFn);
lastFn = setTimeout(function() {
if (Date.now() - lastTime >= wait) {
fn.apply(context, args);
lastTime = Date.now();
}
}, Math.max(wait - (Date.now() - lastTime), 0));
}
};
};
window.addEventListener(
'resize',
throttle(function(evt) {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250ms
timeTaken
- title: timeTaken
- tags: function,beginner
Measures the time it takes for a function to execute.
- Use
Console.time()
andConsole.timeEnd()
to measure the difference between the start and end times to determine how long the callback took to execute.
const timeTaken = callback => {
console.time('timeTaken');
const r = callback();
console.timeEnd('timeTaken');
return r;
};
timeTaken(() => Math.pow(2, 10)); // 1024, (logged): timeTaken: 0.02099609375ms
times
- title: times
- tags: function,intermediate
Iterates over a callback n
times
- Use
Function.prototype.call()
to callfn
n
times or until it returnsfalse
. - Omit the last argument,
context
, to use anundefined
object (or the global object in non-strict mode).
const times = (n, fn, context = undefined) => {
let i = 0;
while (fn.call(context, i) !== false && ++i < n) {}
};
var output = '';
times(5, i => (output += i));
console.log(output); // 01234
toCamelCase
- title: toCamelCase
- tags: string,regexp,intermediate
Converts a string to camelcase.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
,String.prototype.toLowerCase()
andString.prototype.toUpperCase()
to combine them, capitalizing the first letter of each one.
const toCamelCase = str => {
let s =
str &&
str
.match(
/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g
)
.map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
.join('');
return s.slice(0, 1).toLowerCase() + s.slice(1);
};
toCamelCase('some_database_field_name'); // 'someDatabaseFieldName'
toCamelCase('Some label that needs to be camelized');
// 'someLabelThatNeedsToBeCamelized'
toCamelCase('some-javascript-property'); // 'someJavascriptProperty'
toCamelCase('some-mixed_string with spaces_underscores-and-hyphens');
// 'someMixedStringWithSpacesUnderscoresAndHyphens'
toCharArray
- title: toCharArray
- tags: string,beginner
Converts a string to an array of characters.
- Use the spread operator (
...
) to convert the string into an array of characters.
const toCharArray = s => [...s];
toCharArray('hello'); // ['h', 'e', 'l', 'l', 'o']
toCurrency
- title: toCurrency
- tags: math,string,intermediate
Takes a number and returns it in the specified currency formatting.
- Use
Intl.NumberFormat
to enable country / currency sensitive formatting.
const toCurrency = (n, curr, LanguageFormat = undefined) =>
Intl.NumberFormat(LanguageFormat, {
style: 'currency',
currency: curr,
}).format(n);
toCurrency(123456.789, 'EUR');
// €123,456.79 | currency: Euro | currencyLangFormat: Local
toCurrency(123456.789, 'USD', 'en-us');
// $123,456.79 | currency: US Dollar | currencyLangFormat: English (United States)
toCurrency(123456.789, 'USD', 'fa');
// ۱۲۳٬۴۵۶٫۷۹ $ | currency: US Dollar | currencyLangFormat: Farsi
toCurrency(322342436423.2435, 'JPY');
// ¥322,342,436,423 | currency: Japanese Yen | currencyLangFormat: Local
toCurrency(322342436423.2435, 'JPY', 'fi');
// 322 342 436 423 ¥ | currency: Japanese Yen | currencyLangFormat: Finnish
toDecimalMark
- title: toDecimalMark
- tags: math,beginner
Converts a number to a decimal mark formatted string.
- Use
Number.prototype.toLocaleString()
to convert the number to decimal mark format.
const toDecimalMark = num => num.toLocaleString('en-US');
toDecimalMark(12305030388.9087); // '12,305,030,388.909'
toHSLArray
- title: toHSLArray
- tags: string,browser,regexp,beginner
Converts an hsl()
color string to an array of values.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values.
const toHSLArray = hslStr => hslStr.match(/\d+/g).map(Number);
toHSLArray('hsl(50, 10%, 10%)'); // [50, 10, 10]
toHSLObject
- title: toHSLObject
- tags: string,browser,regexp,intermediate
Converts an hsl()
color string to an object with the values of each color.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Use array destructuring to store the values into named variables and create an appropriate object from them.
const toHSLObject = hslStr => {
const [hue, saturation, lightness] = hslStr.match(/\d+/g).map(Number);
return { hue, saturation, lightness };
};
toHSLObject('hsl(50, 10%, 10%)'); // { hue: 50, saturation: 10, lightness: 10 }
toHash
- title: toHash
- tags: array,intermediate
Reduces a given array-like into a value hash (keyed data store).
- Given an iterable object or array-like structure, call
Array.prototype.reduce.call()
on the provided object to step over it and return anObject
, keyed by the reference value.
const toHash = (object, key) =>
Array.prototype.reduce.call(
object,
(acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
{}
);
toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 3: 1 }
toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
// A more in depth example:
let users = [
{ id: 1, first: 'Jon' },
{ id: 2, first: 'Joe' },
{ id: 3, first: 'Moe' },
];
let managers = [{ manager: 1, employees: [2, 3] }];
// We use function here because we want a bindable reference,
// but a closure referencing the hash would work, too.
managers.forEach(
manager =>
(manager.employees = manager.employees.map(function(id) {
return this[id];
}, toHash(users, 'id')))
);
managers;
// [ {manager:1, employees: [ {id: 2, first: 'Joe'}, {id: 3, first: 'Moe'} ] } ]
toISOStringWithTimezone
- title: toISOStringWithTimezone
- tags: date,intermediate
Converts a date to extended ISO format (ISO 8601), including timezone offset.
- Use
Date.prototype.getTimezoneOffset()
to get the timezone offset and reverse it, storing its sign indiff
. - Define a helper function,
pad
, that normalizes any passed number to an integer usingMath.floor()
andMath.abs()
and pads it to2
digits, usingString.prototype.padStart()
. - Use
pad()
and the built-in methods in theDate
prototype to build the ISO 8601 string with timezone offset.
const toISOStringWithTimezone = date => {
const tzOffset = -date.getTimezoneOffset();
const diff = tzOffset >= 0 ? '+' : '-';
const pad = n => `${Math.floor(Math.abs(n))}`.padStart(2, '0');
return date.getFullYear() +
'-' + pad(date.getMonth() + 1) +
'-' + pad(date.getDate()) +
'T' + pad(date.getHours()) +
':' + pad(date.getMinutes()) +
':' + pad(date.getSeconds()) +
diff + pad(tzOffset / 60) +
':' + pad(tzOffset % 60);
};
toISOStringWithTimezone(new Date()); // '2020-10-06T20:43:33-04:00'
toKebabCase
- title: toKebabCase
- tags: string,regexp,intermediate
Converts a string to kebab case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.join()
andString.prototype.toLowerCase()
to combine them, adding-
as a separator.
const toKebabCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('-');
toKebabCase('camelCase'); // 'camel-case'
toKebabCase('some text'); // 'some-text'
toKebabCase('some-mixed_string With spaces_underscores-and-hyphens');
// 'some-mixed-string-with-spaces-underscores-and-hyphens'
toKebabCase('AllThe-small Things'); // 'all-the-small-things'
toKebabCase('IAmEditingSomeXMLAndHTML');
// 'i-am-editing-some-xml-and-html'
toOrdinalSuffix
- title: toOrdinalSuffix
- tags: math,intermediate
Takes a number and returns it as a string with the correct ordinal indicator suffix.
- Use the modulo operator (
%
) to find values of single and tens digits. - Find which ordinal pattern digits match.
- If digit is found in teens pattern, use teens ordinal.
const toOrdinalSuffix = num => {
const int = parseInt(num),
digits = [int % 10, int % 100],
ordinals = ['st', 'nd', 'rd', 'th'],
oPattern = [1, 2, 3, 4],
tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
? int + ordinals[digits[0] - 1]
: int + ordinals[3];
};
toOrdinalSuffix('123'); // '123rd'
toPairs
- title: toPairs
- tags: object,array,intermediate
Creates an array of key-value pair arrays from an object or other iterable.
- Check if
Symbol.iterator
is defined and, if so, useArray.prototype.entries()
to get an iterator for the given iterable. - Use
Array.from()
to convert the result to an array of key-value pair arrays. - If
Symbol.iterator
is not defined forobj
, useObject.entries()
instead.
const toPairs = obj =>
obj[Symbol.iterator] instanceof Function && obj.entries instanceof Function
? Array.from(obj.entries())
: Object.entries(obj);
toPairs({ a: 1, b: 2 }); // [['a', 1], ['b', 2]]
toPairs([2, 4, 8]); // [[0, 2], [1, 4], [2, 8]]
toPairs('shy'); // [['0', 's'], ['1', 'h'], ['2', 'y']]
toPairs(new Set(['a', 'b', 'c', 'a'])); // [['a', 'a'], ['b', 'b'], ['c', 'c']]
toRGBArray
- title: toRGBArray
- tags: string,browser,regexp,beginner
Converts an rgb()
color string to an array of values.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values.
const toRGBArray = rgbStr => rgbStr.match(/\d+/g).map(Number);
toRGBArray('rgb(255, 12, 0)'); // [255, 12, 0]
toRGBObject
- title: toRGBObject
- tags: string,browser,regexp,intermediate
Converts an rgb()
color string to an object with the values of each color.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Use array destructuring to store the values into named variables and create an appropriate object from them.
const toRGBObject = rgbStr => {
const [red, green, blue] = rgbStr.match(/\d+/g).map(Number);
return { red, green, blue };
};
toRGBObject('rgb(255, 12, 0)'); // {red: 255, green: 12, blue: 0}
toRomanNumeral
- title: toRomanNumeral
- tags: math,string,intermediate
Converts an integer to its roman numeral representation.
Accepts value between 1
and 3999
(both inclusive).
- Create a lookup table containing 2-value arrays in the form of (roman value, integer).
- Use
Array.prototype.reduce()
to loop over the values inlookup
and repeatedly dividenum
by the value. - Use
String.prototype.repeat()
to add the roman numeral representation to the accumulator.
const toRomanNumeral = num => {
const lookup = [
['M', 1000],
['CM', 900],
['D', 500],
['CD', 400],
['C', 100],
['XC', 90],
['L', 50],
['XL', 40],
['X', 10],
['IX', 9],
['V', 5],
['IV', 4],
['I', 1],
];
return lookup.reduce((acc, [k, v]) => {
acc += k.repeat(Math.floor(num / v));
num = num % v;
return acc;
}, '');
};
toRomanNumeral(3); // 'III'
toRomanNumeral(11); // 'XI'
toRomanNumeral(1998); // 'MCMXCVIII'
toSafeInteger
- title: toSafeInteger
- tags: math,beginner
Converts a value to a safe integer.
- Use
Math.max()
andMath.min()
to find the closest safe value. - Use
Math.round()
to convert to an integer.
const toSafeInteger = num =>
Math.round(
Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER)
);
toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991
toSnakeCase
- title: toSnakeCase
- tags: string,regexp,intermediate
Converts a string to snake case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
andString.prototype.toLowerCase()
to combine them, adding_
as a separator.
const toSnakeCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('_');
toSnakeCase('camelCase'); // 'camel_case'
toSnakeCase('some text'); // 'some_text'
toSnakeCase('some-mixed_string With spaces_underscores-and-hyphens');
// 'some_mixed_string_with_spaces_underscores_and_hyphens'
toSnakeCase('AllThe-small Things'); // 'all_the_small_things'
toKebabCase('IAmEditingSomeXMLAndHTML');
// 'i_am_editing_some_xml_and_html'
toTitleCase
- title: toTitleCase
- tags: string,regexp,intermediate
Converts a string to title case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
andString.prototype.toUpperCase()
to combine them, capitalizing the first letter of each word and adding a whitespace between them.
const toTitleCase = str =>
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.charAt(0).toUpperCase() + x.slice(1))
.join(' ');
toTitleCase('some_database_field_name'); // 'Some Database Field Name'
toTitleCase('Some label that needs to be title-cased');
// 'Some Label That Needs To Be Title Cased'
toTitleCase('some-package-name'); // 'Some Package Name'
toTitleCase('some-mixed_string with spaces_underscores-and-hyphens');
// 'Some Mixed String With Spaces Underscores And Hyphens'
toggleClass
- title: toggleClass
- tags: browser,beginner
Toggles a class for an HTML element.
- Use
Element.classList
andDOMTokenList.toggle()
to toggle the specified class for the element.
const toggleClass = (el, className) => el.classList.toggle(className);
toggleClass(document.querySelector('p.special'), 'special');
// The paragraph will not have the 'special' class anymore
tomorrow
- title: tomorrow
- tags: date,intermediate
Results in a string representation of tomorrow's date.
- Use
new Date()
to get the current date. - Increment it by one using
Date.prototype.getDate()
and set the value to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const tomorrow = () => {
let d = new Date();
d.setDate(d.getDate() + 1);
return d.toISOString().split('T')[0];
};
tomorrow(); // 2018-10-19 (if current date is 2018-10-18)
transform
- title: transform
- tags: object,intermediate
Applies a function against an accumulator and each key in the object (from left to right).
- Use
Object.keys()
to iterate over each key in the object. - Use
Array.prototype.reduce()
to apply the specified function against the given accumulator.
const transform = (obj, fn, acc) =>
Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);
transform(
{ a: 1, b: 2, c: 1 },
(r, v, k) => {
(r[v] || (r[v] = [])).push(k);
return r;
},
{}
); // { '1': ['a', 'c'], '2': ['b'] }
triggerEvent
- title: triggerEvent
- tags: browser,event,intermediate
Triggers a specific event on a given element, optionally passing custom data.
- Use
new CustomEvent()
to create an event from the specifiedeventType
and details. - Use
EventTarget.dispatchEvent()
to trigger the newly created event on the given element. - Omit the third argument,
detail
, if you do not want to pass custom data to the triggered event.
const triggerEvent = (el, eventType, detail) =>
el.dispatchEvent(new CustomEvent(eventType, { detail }));
triggerEvent(document.getElementById('myId'), 'click');
triggerEvent(document.getElementById('myId'), 'click', { username: 'bob' });
truncateString
- title: truncateString
- tags: string,beginner
Truncates a string up to a specified length.
- Determine if
String.prototype.length
is greater thannum
. - Return the string truncated to the desired length, with
'...'
appended to the end or the original string.
const truncateString = (str, num) =>
str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
truncateString('boomerang', 7); // 'boom...'
truncateStringAtWhitespace
- title: truncateStringAtWhitespace
- tags: string,intermediate
Truncates a string up to specified length, respecting whitespace when possible.
- Determine if
String.prototype.length
is greater or equal tolim
. If not, return it as-is. - Use
String.prototype.slice()
andString.prototype.lastIndexOf()
to find the index of the last space below the desiredlim
. - Use
String.prototype.slice()
to appropriately truncatestr
based onlastSpace
, respecting whitespace if possible and appendingending
at the end. - Omit the third argument,
ending
, to use the default ending of'...'
.
const truncateStringAtWhitespace = (str, lim, ending = '...') => {
if (str.length <= lim) return str;
const lastSpace = str.slice(0, lim - ending.length + 1).lastIndexOf(' ');
return str.slice(0, lastSpace > 0 ? lastSpace : lim - ending.length) + ending;
};
truncateStringAtWhitespace('short', 10); // 'short'
truncateStringAtWhitespace('not so short', 10); // 'not so...'
truncateStringAtWhitespace('trying a thing', 10); // 'trying...'
truncateStringAtWhitespace('javascripting', 10); // 'javascr...'
truthCheckCollection
- title: truthCheckCollection
- tags: object,logic,array,intermediate
Checks if the predicate function is truthy for all elements of a collection.
- Use
Array.prototype.every()
to check if each passed object has the specified property and if it returns a truthy value.
const truthCheckCollection = (collection, pre) =>
collection.every(obj => obj[pre]);
truthCheckCollection(
[
{ user: 'Tinky-Winky', sex: 'male' },
{ user: 'Dipsy', sex: 'male' },
],
'sex'
); // true
unary
- title: unary
- tags: function,beginner
Creates a function that accepts up to one argument, ignoring any additional arguments.
- Call the provided function,
fn
, with just the first argument supplied.
const unary = fn => val => fn(val);
['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]
uncurry
- title: uncurry
- tags: function,advanced
Uncurries a function up to depth n
.
- Return a variadic function.
- Use
Array.prototype.reduce()
on the provided arguments to call each subsequent curry level of the function. - If the
length
of the provided arguments is less thann
throw an error. - Otherwise, call
fn
with the proper amount of arguments, usingArray.prototype.slice(0, n)
. - Omit the second argument,
n
, to uncurry up to depth1
.
const uncurry = (fn, n = 1) => (...args) => {
const next = acc => args => args.reduce((x, y) => x(y), acc);
if (n > args.length) throw new RangeError('Arguments too few!');
return next(fn)(args.slice(0, n));
};
const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6
unescapeHTML
- title: unescapeHTML
- tags: string,browser,regexp,beginner
Unescapes escaped HTML characters.
- Use
String.prototype.replace()
with a regexp that matches the characters that need to be unescaped. - Use the function's callback to replace each escaped character instance with its associated unescaped character using a dictionary (object).
const unescapeHTML = str =>
str.replace(
/&|<|>|&##39;|"/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
'&##39;': "'",
'"': '"'
}[tag] || tag)
);
unescapeHTML('<a href="##">Me & you</a>');
// '<a href="##">Me & you</a>'
unflattenObject
- title: unflattenObject
- tags: object,advanced
Unflatten an object with the paths for keys.
- Use nested
Array.prototype.reduce()
to convert the flat path to a leaf node. - Use
String.prototype.split('.')
to split each key with a dot delimiter andArray.prototype.reduce()
to add objects against the keys. - If the current accumulator already contains a value against a particular key, return its value as the next accumulator.
- Otherwise, add the appropriate key-value pair to the accumulator object and return the value as the accumulator.
const unflattenObject = obj =>
Object.keys(obj).reduce((res, k) => {
k.split('.').reduce(
(acc, e, i, keys) =>
acc[e] ||
(acc[e] = isNaN(Number(keys[i + 1]))
? keys.length - 1 === i
? obj[k]
: {}
: []),
res
);
return res;
}, {});
unflattenObject({ 'a.b.c': 1, d: 1 }); // { a: { b: { c: 1 } }, d: 1 }
unflattenObject({ 'a.b': 1, 'a.c': 2, d: 3 }); // { a: { b: 1, c: 2 }, d: 3 }
unflattenObject({ 'a.b.0': 8, d: 3 }); // { a: { b: [ 8 ] }, d: 3 }
unfold
- title: unfold
- tags: function,array,intermediate
Builds an array, using an iterator function and an initial seed value.
- Use a
while
loop andArray.prototype.push()
to call the function repeatedly until it returnsfalse
. - The iterator function accepts one argument (
seed
) and must always return an array with two elements ([value
,nextSeed
]) orfalse
to terminate.
const unfold = (fn, seed) => {
let result = [],
val = [null, seed];
while ((val = fn(val[1]))) result.push(val[0]);
return result;
};
var f = n => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]
union
- title: union
- tags: array,beginner
Returns every element that exists in any of the two arrays at least once.
- Create a
new Set()
with all values ofa
andb
and convert it to an array.
const union = (a, b) => Array.from(new Set([...a, ...b]));
union([1, 2, 3], [4, 3, 2]); // [1, 2, 3, 4]
unionBy
- title: unionBy
- tags: array,intermediate
Returns every element that exists in any of the two arrays at least once, after applying the provided function to each array element of both.
- Create a
new Set()
by applying allfn
to all values ofa
. - Create a
new Set()
froma
and all elements inb
whose value, after applyingfn
does not match a value in the previously created set. - Return the last set converted to an array.
const unionBy = (a, b, fn) => {
const s = new Set(a.map(fn));
return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
};
unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]
unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], x => x.id)
// [{ id: 1 }, { id: 2 }, { id: 3 }]
unionWith
- title: unionWith
- tags: array,intermediate
Returns every element that exists in any of the two arrays at least once, using a provided comparator function.
- Create a
new Set()
with all values ofa
and values inb
for which the comparator finds no matches ina
, usingArray.prototype.findIndex()
.
const unionWith = (a, b, comp) =>
Array.from(
new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])
);
unionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
);
// [1, 1.2, 1.5, 3, 0, 3.9]
uniqueElements
- title: uniqueElements
- tags: array,beginner
Finds all unique values in an array.
- Create a
new Set()
from the given array to discard duplicated values. - Use the spread operator (
...
) to convert it back to an array.
const uniqueElements = arr => [...new Set(arr)];
uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]
uniqueElementsBy
- title: uniqueElementsBy
- tags: array,intermediate
Finds all unique values of an array, based on a provided comparator function.
- Use
Array.prototype.reduce()
andArray.prototype.some()
for an array containing only the first unique occurrence of each value, based on the comparator function,fn
. - The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsBy = (arr, fn) =>
arr.reduce((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);
uniqueElementsBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]
uniqueElementsByRight
- title: uniqueElementsByRight
- tags: array,intermediate
Finds all unique values of an array, based on a provided comparator function, starting from the right.
- Use
Array.prototype.reduceRight()
andArray.prototype.some()
for an array containing only the last unique occurrence of each value, based on the comparator function,fn
. - The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsByRight = (arr, fn) =>
arr.reduceRight((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);
uniqueElementsByRight(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]
uniqueSymmetricDifference
- title: uniqueSymmetricDifference
- tags: array,math,intermediate
Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.
- Use
Array.prototype.filter()
andArray.prototype.includes()
on each array to remove values contained in the other. - Create a
new Set()
from the results, removing duplicate values.
const uniqueSymmetricDifference = (a, b) => [
...new Set([
...a.filter(v => !b.includes(v)),
...b.filter(v => !a.includes(v)),
]),
];
uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]
untildify
- title: untildify
- tags: node,string,beginner
Converts a tilde path to an absolute path.
- Use
String.prototype.replace()
with a regular expression andos.homedir()
to replace the~
in the start of the path with the home directory.
const untildify = str =>
str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);
untildify('~/node'); // '/Users/aUser/node'
unzip
- title: unzip
- tags: array,intermediate
Creates an array of arrays, ungrouping the elements in an array produced by zip.
- Use
Math.max()
,Function.prototype.apply()
to get the longest subarray in the array,Array.prototype.map()
to make each element an array. - Use
Array.prototype.reduce()
andArray.prototype.forEach()
to map grouped values to individual arrays.
const unzip = arr =>
arr.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
);
unzip([['a', 1, true], ['b', 2, false]]); // [['a', 'b'], [1, 2], [true, false]]
unzip([['a', 1, true], ['b', 2]]); // [['a', 'b'], [1, 2], [true]]
unzipWith
- title: unzipWith
- tags: array,advanced
Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.
- Use
Math.max()
,Function.prototype.apply()
to get the longest subarray in the array,Array.prototype.map()
to make each element an array. - Use
Array.prototype.reduce()
andArray.prototype.forEach()
to map grouped values to individual arrays. - Use
Array.prototype.map()
and the spread operator (...
) to applyfn
to each individual group of elements.
const unzipWith = (arr, fn) =>
arr
.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
)
.map(val => fn(...val));
unzipWith(
[
[1, 10, 100],
[2, 20, 200],
],
(...args) => args.reduce((acc, v) => acc + v, 0)
);
// [3, 30, 300]
validateNumber
- title: validateNumber
- tags: math,intermediate
Checks if the given value is a number.
- Use
parseFloat()
to try to convertn
to a number. - Use
!Number.isNaN()
to check ifnum
is a number. - Use
Number.isFinite()
to check ifnum
is finite. - Use
Number()
and the loose equality operator (==
) to check if the coercion holds.
const validateNumber = n => {
const num = parseFloat(n);
return !Number.isNaN(num) && Number.isFinite(num) && Number(n) == n;
}
validateNumber('10'); // true
validateNumber('a'); // false
vectorAngle
- title: vectorAngle
- tags: math,beginner
Calculates the angle (theta) between two vectors.
- Use
Array.prototype.reduce()
,Math.pow()
andMath.sqrt()
to calculate the magnitude of each vector and the scalar product of the two vectors. - Use
Math.acos()
to calculate the arccosine and get the theta value.
const vectorAngle = (x, y) => {
let mX = Math.sqrt(x.reduce((acc, n) => acc + Math.pow(n, 2), 0));
let mY = Math.sqrt(y.reduce((acc, n) => acc + Math.pow(n, 2), 0));
return Math.acos(x.reduce((acc, n, i) => acc + n * y[i], 0) / (mX * mY));
};
vectorAngle([3, 4], [4, 3]); // 0.283794109208328
vectorDistance
- title: vectorDistance
- tags: math,algorithm,beginner
Calculates the distance between two vectors.
- Use
Array.prototype.reduce()
,Math.pow()
andMath.sqrt()
to calculate the Euclidean distance between two vectors.
const vectorDistance = (x, y) =>
Math.sqrt(x.reduce((acc, val, i) => acc + Math.pow(val - y[i], 2), 0));
vectorDistance([10, 0, 5], [20, 0, 10]); // 11.180339887498949
walkThrough
- title: walkThrough
- tags: object,recursion,generator,advanced
Creates a generator, that walks through all the keys of a given object.
- Use recursion.
- Define a generator function,
walk
, that takes an object and an array of keys. - Use a
for...of
loop andObject.keys()
to iterate over the keys of the object. - Use
typeof
to check if each value in the given object is itself an object. - If so, use the
yield*
expression to recursively delegate to the same generator function,walk
, appending the currentkey
to the array of keys. Otherwise,yield
the an array of keys representing the current path and the value of the givenkey
. - Use the
yield*
expression to delegate to thewalk
generator function.
const walkThrough = function* (obj) {
const walk = function* (x, previous = []) {
for (let key of Object.keys(x)) {
if (typeof x[key] === 'object') yield* walk(x[key], [...previous, key]);
else yield [[...previous, key], x[key]];
}
};
yield* walk(obj);
};
const obj = {
a: 10,
b: 20,
c: {
d: 10,
e: 20,
f: [30, 40]
},
g: [
{
h: 10,
i: 20
},
{
j: 30
},
40
]
};
[...walkThrough(obj)];
/*
[
[['a'], 10],
[['b'], 20],
[['c', 'd'], 10],
[['c', 'e'], 20],
[['c', 'f', '0'], 30],
[['c', 'f', '1'], 40],
[['g', '0', 'h'], 10],
[['g', '0', 'i'], 20],
[['g', '1', 'j'], 30],
[['g', '2'], 40]
]
*/
weightedAverage
- title: weightedAverage
- tags: math,intermediate
Calculates the weighted average of two or more numbers.
- Use
Array.prototype.reduce()
to create the weighted sum of the values and the sum of the weights. - Divide them with each other to get the weighted average.
const weightedAverage = (nums, weights) => {
const [sum, weightSum] = weights.reduce(
(acc, w, i) => {
acc[0] = acc[0] + nums[i] * w;
acc[1] = acc[1] + w;
return acc;
},
[0, 0]
);
return sum / weightSum;
};
weightedAverage([1, 2, 3], [0.6, 0.2, 0.3]); // 1.72727
weightedSample
- title: weightedSample
- tags: array,random,advanced
Gets a random element from an array, using the provided weights
as the probabilities for each element.
- Use
Array.prototype.reduce()
to create an array of partial sums for each value inweights
. - Use
Math.random()
to generate a random number andArray.prototype.findIndex()
to find the correct index based on the array previously produced. - Finally, return the element of
arr
with the produced index.
const weightedSample = (arr, weights) => {
let roll = Math.random();
return arr[
weights
.reduce(
(acc, w, i) => (i === 0 ? [w] : [...acc, acc[acc.length - 1] + w]),
[]
)
.findIndex((v, i, s) => roll >= (i === 0 ? 0 : s[i - 1]) && roll < v)
];
};
weightedSample([3, 7, 9, 11], [0.1, 0.2, 0.6, 0.1]); // 9
when
- title: when
- tags: function,logic,beginner
Returns a function that takes one argument and runs a callback if it's truthy or returns it if falsy.
- Return a function expecting a single value,
x
, that returns the appropriate value based onpred
.
const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);
const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1
without
- title: without
- tags: array,beginner
Filters out the elements of an array that have one of the specified values.
- Use
Array.prototype.includes()
to find values to exclude. - Use
Array.prototype.filter()
to create an array excluding them.
const without = (arr, ...args) => arr.filter(v => !args.includes(v));
without([2, 1, 2, 3], 1, 2); // [3]
wordWrap
- title: wordWrap
- tags: string,regexp,intermediate
Wraps a string to a given number of characters using a string break character.
- Use
String.prototype.replace()
and a regular expression to insert a given break character at the nearest whitespace ofmax
characters. - Omit the third argument,
br
, to use the default value of'\n'
.
const wordWrap = (str, max, br = '\n') => str.replace(
new RegExp(`(?![^\\n]{1,${max}}$)([^\\n]{1,${max}})\\s`, 'g'), '$1' + br
);
wordWrap(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce tempus.',
32
);
// 'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit.\nFusce tempus.'
wordWrap(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce tempus.',
32,
'\r\n'
);
// 'Lorem ipsum dolor sit amet,\r\nconsectetur adipiscing elit.\r\nFusce tempus.'
words
- title: words
- tags: string,regexp,intermediate
Converts a given string into an array of words.
- Use
String.prototype.split()
with a suppliedpattern
(defaults to non-alpha as a regexp) to convert to an array of strings. - Use
Array.prototype.filter()
to remove any empty strings. - Omit the second argument,
pattern
, to use the default regexp.
const words = (str, pattern = /[^a-zA-Z-]+/) =>
str.split(pattern).filter(Boolean);
words('I love javaScript!!'); // ['I', 'love', 'javaScript']
words('python, javaScript & coffee'); // ['python', 'javaScript', 'coffee']
xProd
- title: xProd
- tags: array,math,intermediate
Creates a new array out of the two supplied by creating each possible pair from the arrays.
- Use
Array.prototype.reduce()
,Array.prototype.map()
andArray.prototype.concat()
to produce every possible pair from the elements of the two arrays.
const xProd = (a, b) =>
a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);
xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]
xor
- title: xor
- tags: math,logic,beginner unlisted: true
Checks if only one of the arguments is true
.
- Use the logical or (
||
), and (&&
) and not (!
) operators on the two given values to create the logical xor.
const xor = (a, b) => (( a || b ) && !( a && b ));
xor(true, true); // false
xor(true, false); // true
xor(false, true); // true
xor(false, false); // false
yesNo
- title: yesNo
- tags: string,regexp,intermediate unlisted: true
Returns true
if the string is y
/yes
or false
if the string is n
/no
.
- Use
RegExp.prototype.test()
to check if the string evaluates toy/yes
orn/no
. - Omit the second argument,
def
to set the default answer asno
.
const yesNo = (val, def = false) =>
/^(y|yes)$/i.test(val) ? true : /^(n|no)$/i.test(val) ? false : def;
yesNo('Y'); // true
yesNo('yes'); // true
yesNo('No'); // false
yesNo('Foo', true); // true
yesterday
- title: yesterday
- tags: date,intermediate
Results in a string representation of yesterday's date.
- Use
new Date()
to get the current date. - Decrement it by one using
Date.prototype.getDate()
and set the value to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const yesterday = () => {
let d = new Date();
d.setDate(d.getDate() - 1);
return d.toISOString().split('T')[0];
};
yesterday(); // 2018-10-17 (if current date is 2018-10-18)
zip
- title: zip
- tags: array,intermediate
Creates an array of elements, grouped based on their position in the original arrays.
- Use
Math.max()
,Function.prototype.apply()
to get the longest array in the arguments. - Create an array with that length as return value and use
Array.from()
with a mapping function to create an array of grouped elements. - If lengths of the argument arrays vary,
undefined
is used where no value could be found.
const zip = (...arrays) => {
const maxLength = Math.max(...arrays.map(x => x.length));
return Array.from({ length: maxLength }).map((_, i) => {
return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
});
};
zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]
zipObject
- title: zipObject
- tags: array,object,intermediate
Associates properties to values, given array of valid property identifiers and an array of values.
- Use
Array.prototype.reduce()
to build an object from the two arrays. - If the length of
props
is longer thanvalues
, remaining keys will beundefined
. - If the length of
values
is longer thanprops
, remaining values will be ignored.
const zipObject = (props, values) =>
props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});
zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}
zipWith
- title: zipWith
- tags: array,advanced
Creates an array of elements, grouped based on the position in the original arrays and using a function to specify how grouped values should be combined.
- Check if the last argument provided is a function.
- Use
Math.max()
to get the longest array in the arguments. - Use
Array.from()
to create an array with appropriate length and a mapping function to create array of grouped elements. - If lengths of the argument arrays vary,
undefined
is used where no value could be found. - The function is invoked with the elements of each group.
const zipWith = (...array) => {
const fn =
typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
return Array.from({ length: Math.max(...array.map(a => a.length)) }, (_, i) =>
fn ? fn(...array.map(a => a[i])) : array.map(a => a[i])
);
};
zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111, 222]
zipWith(
[1, 2, 3],
[10, 20],
[100, 200],
(a, b, c) =>
(a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']
midpoint
- title: midpoint
- tags: math,beginner
Calculates the midpoint between two pairs of (x,y) points.
- Destructure the array to get
x1
,y1
,x2
andy2
. - Calculate the midpoint for each dimension by dividing the sum of the two endpoints by
2
.
const midpoint = ([x1, y1], [x2, y2]) => [(x1 + x2) / 2, (y1 + y2) / 2];
midpoint([2, 2], [4, 4]); // [3, 3]
midpoint([4, 4], [6, 6]); // [5, 5]
midpoint([1, 3], [2, 4]); // [1.5, 3.5]
milesToKm
- title: milesToKm
- tags: math,beginner unlisted: true
Converts miles to kilometers.
- Follow the conversion formula
km = mi * 1.609344
.
const milesToKm = miles => miles * 1.609344;
milesToKm(5); // ~8.04672
minBy
- title: minBy
- tags: math,array,beginner
Returns the minimum value of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Math.min()
to get the minimum value.
const minBy = (arr, fn) =>
Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2
minDate
- title: minDate
- tags: date,intermediate
Returns the minimum of the given dates.
- Use the ES6 spread syntax with
Math.min()
to find the minimum date value. - Use
new Date()
to convert it to aDate
object.
const minDate = (...dates) => new Date(Math.min(...dates));
const dates = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
minDate(...dates); // 2016-01-08T22:00:00.000Z
minN
- title: minN
- tags: array,math,intermediate
Returns the n
minimum elements from the provided array.
- Use
Array.prototype.sort()
combined with the spread operator (...
) to create a shallow clone of the array and sort it in ascending order. - Use
Array.prototype.slice()
to get the specified number of elements. - Omit the second argument,
n
, to get a one-element array. - If
n
is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).
const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1, 2]
mostFrequent
- title: mostFrequent
- tags: array,intermediate
Returns the most frequent element in an array.
- Use
Array.prototype.reduce()
to map unique values to an object's keys, adding to existing keys every time the same value is encountered. - Use
Object.entries()
on the result in combination withArray.prototype.reduce()
to get the most frequent value in the array.
const mostFrequent = arr =>
Object.entries(
arr.reduce((a, v) => {
a[v] = a[v] ? a[v] + 1 : 1;
return a;
}, {})
).reduce((a, v) => (v[1] >= a[1] ? v : a), [null, 0])[0];
mostFrequent(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // 'a'
mostPerformant
- title: mostPerformant
- tags: function,advanced
Returns the index of the function in an array of functions which executed the fastest.
- Use
Array.prototype.map()
to generate an array where each value is the total time taken to execute the function afteriterations
times. - Use the difference in
performance.now()
values before and after to get the total time in milliseconds to a high degree of accuracy. - Use
Math.min()
to find the minimum execution time, and return the index of that shortest time which corresponds to the index of the most performant function. - Omit the second argument,
iterations
, to use a default of10000
iterations. - The more iterations, the more reliable the result but the longer it will take.
const mostPerformant = (fns, iterations = 10000) => {
const times = fns.map(fn => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return performance.now() - before;
});
return times.indexOf(Math.min(...times));
};
mostPerformant([
() => {
// Loops through the entire array before returning `false`
[1, 2, 3, 4, 5, 6, 7, 8, 9, '10'].every(el => typeof el === 'number');
},
() => {
// Only needs to reach index `1` before returning `false`
[1, '2', 3, 4, 5, 6, 7, 8, 9, 10].every(el => typeof el === 'number');
}
]); // 1
negate
- title: negate
- tags: function,beginner
Negates a predicate function.
- Take a predicate function and apply the not operator (
!
) to it with its arguments.
const negate = func => (...args) => !func(...args);
[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 === 0)); // [ 1, 3, 5 ]
nest
- title: nest
- tags: object,recursion,intermediate
Nests recursively objects linked to one another in a flat array.
- Use recursion.
- Use
Array.prototype.filter()
to filter the items where theid
matches thelink
. - Use
Array.prototype.map()
to map each item to a new object that has achildren
property which recursively nests the items based on which ones are children of the current item. - Omit the second argument,
id
, to default tonull
which indicates the object is not linked to another one (i.e. it is a top level object). - Omit the third argument,
link
, to use'parent_id'
as the default property which links the object to another one by itsid
.
const nest = (items, id = null, link = 'parent_id') =>
items
.filter(item => item[link] === id)
.map(item => ({ ...item, children: nest(items, item.id, link) }));
const comments = [
{ id: 1, parent_id: null },
{ id: 2, parent_id: 1 },
{ id: 3, parent_id: 1 },
{ id: 4, parent_id: 2 },
{ id: 5, parent_id: 4 }
];
const nestedComments = nest(comments);
// [{ id: 1, parent_id: null, children: [...] }]
nodeListToArray
- title: nodeListToArray
- tags: browser,array,beginner
Converts a NodeList
to an array.
- Use spread operator (
...
) inside new array to convert aNodeList
to an array.
const nodeListToArray = nodeList => [...nodeList];
nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]
none
- title: none
- tags: array,beginner
Checks if the provided predicate function returns false
for all elements in a collection.
- Use
Array.prototype.some()
to test if any elements in the collection returntrue
based onfn
. - Omit the second argument,
fn
, to useBoolean
as a default.
const none = (arr, fn = Boolean) => !arr.some(fn);
none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // true
normalizeLineEndings
- title: normalizeLineEndings
- tags: string,regexp,intermediate
Normalizes line endings in a string.
- Use
String.prototype.replace()
and a regular expression to match and replace line endings with thenormalized
version. - Omit the second argument,
normalized
, to use the default value of'\r\n'
.
const normalizeLineEndings = (str, normalized = '\r\n') =>
str.replace(/\r?\n/g, normalized);
normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n');
// 'This\r\nis a\r\nmultiline\r\nstring.\r\n'
normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n', '\n');
// 'This\nis a\nmultiline\nstring.\n'
not
- title: not
- tags: math,logic,beginner unlisted: true
Returns the logical inverse of the given value.
- Use the logical not (
!
) operator to return the inverse of the given value.
const not = a => !a;
not(true); // false
not(false); // true
nthArg
- title: nthArg
- tags: function,beginner
Creates a function that gets the argument at index n
.
- Use
Array.prototype.slice()
to get the desired argument at indexn
. - If
n
is negative, the nth argument from the end is returned.
const nthArg = n => (...args) => args.slice(n)[0];
const third = nthArg(2);
third(1, 2, 3); // 3
third(1, 2); // undefined
const last = nthArg(-1);
last(1, 2, 3, 4, 5); // 5
nthElement
- title: nthElement
- tags: array,beginner
Returns the nth element of an array.
- Use
Array.prototype.slice()
to get an array containing the nth element at the first place. - If the index is out of bounds, return
undefined
. - Omit the second argument,
n
, to get the first element of the array.
const nthElement = (arr, n = 0) =>
(n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'
nthRoot
- title: nthRoot
- tags: math,beginner
Calculates the nth root of a given number.
- Use
Math.pow()
to calculatex
to the power of1/n
which is equal to the nth root ofx
.
const nthRoot = (x, n) => Math.pow(x, 1 / n);
nthRoot(32, 5); // 2
objectFromPairs
- title: objectFromPairs
- tags: object,array,beginner
Creates an object from the given key-value pairs.
- Use
Array.prototype.reduce()
to create and combine key-value pairs.
const objectFromPairs = arr =>
arr.reduce((a, [key, val]) => ((a[key] = val), a), {});
objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}
objectToEntries
- title: objectToEntries
- tags: object,array,beginner
Creates an array of key-value pair arrays from an object.
- Use
Object.keys()
andArray.prototype.map()
to iterate over the object's keys and produce an array with key-value pairs.
const objectToEntries = obj => Object.keys(obj).map(k => [k, obj[k]]);
objectToEntries({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]
objectToPairs
- title: objectToPairs
- tags: object,array,beginner
Creates an array of key-value pair arrays from an object.
- Use
Object.entries()
to get an array of key-value pair arrays from the given object.
const objectToPairs = obj => Object.entries(obj);
objectToPairs({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]
objectToQueryString
- title: objectToQueryString
- tags: object,advanced
Generates a query string from the key-value pairs of the given object.
- Use
Array.prototype.reduce()
onObject.entries(queryParameters)
to create the query string. - Determine the
symbol
to be either?
or&
based on the length ofqueryString
. - Concatenate
val
toqueryString
only if it's a string. - Return the
queryString
or an empty string when thequeryParameters
are falsy.
const objectToQueryString = queryParameters => {
return queryParameters
? Object.entries(queryParameters).reduce(
(queryString, [key, val], index) => {
const symbol = queryString.length === 0 ? '?' : '&';
queryString +=
typeof val === 'string' ? `${symbol}${key}=${val}` : '';
return queryString;
},
''
)
: '';
};
objectToQueryString({ page: '1', size: '2kg', key: undefined });
// '?page=1&size=2kg'
observeMutations
- title: observeMutations
- tags: browser,event,advanced
Creates a new MutationObserver
and runs the provided callback for each mutation on the specified element.
- Use a
MutationObserver
to observe mutations on the given element. - Use
Array.prototype.forEach()
to run the callback for each mutation that is observed. - Omit the third argument,
options
, to use the default options (alltrue
).
const observeMutations = (element, callback, options) => {
const observer = new MutationObserver(mutations =>
mutations.forEach(m => callback(m))
);
observer.observe(
element,
Object.assign(
{
childList: true,
attributes: true,
attributeOldValue: true,
characterData: true,
characterDataOldValue: true,
subtree: true,
},
options
)
);
return observer;
};
const obs = observeMutations(document, console.log);
// Logs all mutations that happen on the page
obs.disconnect();
// Disconnects the observer and stops logging mutations on the page
off
- title: off
- tags: browser,event,intermediate
Removes an event listener from an element.
- Use
EventTarget.removeEventListener()
to remove an event listener from an element. - Omit the fourth argument
opts
to usefalse
or specify it based on the options used when the event listener was added.
const off = (el, evt, fn, opts = false) =>
el.removeEventListener(evt, fn, opts);
const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the page
offset
- title: offset
- tags: array,beginner
Moves the specified amount of elements to the end of the array.
- Use
Array.prototype.slice()
twice to get the elements after the specified index and the elements before that. - Use the spread operator (
...
) to combine the two into one array. - If
offset
is negative, the elements will be moved from end to start.
const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];
offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]
omit
- title: omit
- tags: object,intermediate
Omits the key-value pairs corresponding to the given keys from an object.
- Use
Object.keys()
,Array.prototype.filter()
andArray.prototype.includes()
to remove the provided keys. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs.
const omit = (obj, arr) =>
Object.keys(obj)
.filter(k => !arr.includes(k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omit({ a: 1, b: '2', c: 3 }, ['b']); // { 'a': 1, 'c': 3 }
omitBy
- title: omitBy
- tags: object,intermediate
Omits the key-value pairs corresponding to the keys of the object for which the given function returns falsy.
- Use
Object.keys()
andArray.prototype.filter()
to remove the keys for whichfn
returns a truthy value. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs. - The callback function is invoked with two arguments: (value, key).
const omitBy = (obj, fn) =>
Object.keys(obj)
.filter(k => !fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }
on
- title: on
- tags: browser,event,intermediate
Adds an event listener to an element with the ability to use event delegation.
- Use
EventTarget.addEventListener()
to add an event listener to an element. - If there is a
target
property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correctthis
context. - Omit
opts
to default to non-delegation behavior and event bubbling. - Returns a reference to the custom delegator function, in order to be possible to use with
off
.
const on = (el, evt, fn, opts = {}) => {
const delegatorFn = e =>
e.target.matches(opts.target) && fn.call(e.target, e);
el.addEventListener(
evt,
opts.target ? delegatorFn : fn,
opts.options || false
);
if (opts.target) return delegatorFn;
};
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' });
// logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true });
// use capturing instead of bubbling
onClickOutside
- title: onClickOutside
- tags: browser,event,intermediate
Runs the callback whenever the user clicks outside of the specified element.
- Use
EventTarget.addEventListener()
to listen for'click'
events. - Use
Node.contains()
to check ifEvent.target
is a descendant ofelement
and runcallback
if not.
const onClickOutside = (element, callback) => {
document.addEventListener('click', e => {
if (!element.contains(e.target)) callback();
});
};
onClickOutside('##my-element', () => console.log('Hello'));
// Will log 'Hello' whenever the user clicks outside of ##my-element
onScrollStop
- title: onScrollStop
- tags: browser,event,intermediate
Runs the callback whenever the user has stopped scrolling.
- Use
EventTarget.addEventListener()
to listen for the'scroll'
event. - Use
setTimeout()
to wait150
ms until calling the givencallback
. - Use
clearTimeout()
to clear the timeout if a new'scroll'
event is fired in under150
ms.
const onScrollStop = callback => {
let isScrolling;
window.addEventListener(
'scroll',
e => {
clearTimeout(isScrolling);
isScrolling = setTimeout(() => {
callback();
}, 150);
},
false
);
};
onScrollStop(() => {
console.log('The user has stopped scrolling');
});
onUserInputChange
- title: onUserInputChange
- tags: browser,event,advanced
Runs the callback whenever the user input type changes (mouse
or touch
).
- Use two event listeners.
- Assume
mouse
input initially and bind a'touchstart'
event listener to the document. - On
'touchstart'
, add a'mousemove'
event listener to listen for two consecutive'mousemove'
events firing within 20ms, usingperformance.now()
. - Run the callback with the input type as an argument in either of these situations.
const onUserInputChange = callback => {
let type = 'mouse',
lastTime = 0;
const mousemoveHandler = () => {
const now = performance.now();
if (now - lastTime < 20)
(type = 'mouse'),
callback(type),
document.removeEventListener('mousemove', mousemoveHandler);
lastTime = now;
};
document.addEventListener('touchstart', () => {
if (type === 'touch') return;
(type = 'touch'),
callback(type),
document.addEventListener('mousemove', mousemoveHandler);
});
};
onUserInputChange(type => {
console.log('The user is now using', type, 'as an input method.');
});
once
- title: once
- tags: function,intermediate
Ensures a function is called only once.
- Utilizing a closure, use a flag,
called
, and set it totrue
once the function is called for the first time, preventing it from being called again. - In order to allow the function to have its
this
context changed (such as in an event listener), thefunction
keyword must be used, and the supplied function must have the context applied. - Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (
...
) operator.
const once = fn => {
let called = false;
return function(...args) {
if (called) return;
called = true;
return fn.apply(this, args);
};
};
const startApp = function(event) {
console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp));
// only runs `startApp` once upon click
or
- title: or
- tags: math,logic,beginner unlisted: true
Checks if at least one of the arguments is true
.
- Use the logical or (
||
) operator on the two given values.
const or = (a, b) => a || b;
or(true, true); // true
or(true, false); // true
or(false, false); // false
orderBy
- title: orderBy
- tags: object,array,advanced
Sorts an array of objects, ordered by properties and orders.
- Uses
Array.prototype.sort()
,Array.prototype.reduce()
on theprops
array with a default value of0
. - Use array destructuring to swap the properties position depending on the order supplied.
- If no
orders
array is supplied, sort by'asc'
by default.
const orderBy = (arr, props, orders) =>
[...arr].sort((a, b) =>
props.reduce((acc, prop, i) => {
if (acc === 0) {
const [p1, p2] =
orders && orders[i] === 'desc'
? [b[prop], a[prop]]
: [a[prop], b[prop]];
acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;
}
return acc;
}, 0)
);
const users = [
{ name: 'fred', age: 48 },
{ name: 'barney', age: 36 },
{ name: 'fred', age: 40 },
];
orderBy(users, ['name', 'age'], ['asc', 'desc']);
// [{name: 'barney', age: 36}, {name: 'fred', age: 48}, {name: 'fred', age: 40}]
orderBy(users, ['name', 'age']);
// [{name: 'barney', age: 36}, {name: 'fred', age: 40}, {name: 'fred', age: 48}]
orderWith
- title: orderWith
- tags: array,object,intermediate
Sorts an array of objects, ordered by a property, based on the array of orders provided.
- Use
Array.prototype.reduce()
to create an object from theorder
array with the values as keys and their original index as the value. - Use
Array.prototype.sort()
to sort the given array, skipping elements for whichprop
is empty or not in theorder
array.
const orderWith = (arr, prop, order) => {
const orderValues = order.reduce((acc, v, i) => {
acc[v] = i;
return acc;
}, {});
return [...arr].sort((a, b) => {
if (orderValues[a[prop]] === undefined) return 1;
if (orderValues[b[prop]] === undefined) return -1;
return orderValues[a[prop]] - orderValues[b[prop]];
});
};
const users = [
{ name: 'fred', language: 'Javascript' },
{ name: 'barney', language: 'TypeScript' },
{ name: 'frannie', language: 'Javascript' },
{ name: 'anna', language: 'Java' },
{ name: 'jimmy' },
{ name: 'nicky', language: 'Python' },
];
orderWith(users, 'language', ['Javascript', 'TypeScript', 'Java']);
/*
[
{ name: 'fred', language: 'Javascript' },
{ name: 'frannie', language: 'Javascript' },
{ name: 'barney', language: 'TypeScript' },
{ name: 'anna', language: 'Java' },
{ name: 'jimmy' },
{ name: 'nicky', language: 'Python' }
]
*/
over
- title: over
- tags: function,intermediate
Creates a function that invokes each provided function with the arguments it receives and returns the results.
- Use
Array.prototype.map()
andFunction.prototype.apply()
to apply each function to the given arguments.
const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));
const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1, 5]
overArgs
- title: overArgs
- tags: function,intermediate
Creates a function that invokes the provided function with its arguments transformed.
- Use
Array.prototype.map()
to applytransforms
toargs
in combination with the spread operator (...
) to pass the transformed arguments tofn
.
const overArgs = (fn, transforms) =>
(...args) => fn(...args.map((val, i) => transforms[i](val)));
const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]
pad
- title: pad
- tags: string,beginner
Pads a string on both sides with the specified character, if it's shorter than the specified length
.
- Use
String.prototype.padStart()
andString.prototype.padEnd()
to pad both sides of the given string. - Omit the third argument,
char
, to use the whitespace character as the default padding character.
const pad = (str, length, char = ' ') =>
str.padStart((str.length + length) / 2, char).padEnd(length, char);
pad('cat', 8); // ' cat '
pad(String(42), 6, '0'); // '004200'
pad('foobar', 3); // 'foobar'
padNumber
- title: padNumber
- tags: string,math,beginner
Pads a given number to the specified length.
- Use
String.prototype.padStart()
to pad the number to specified length, after converting it to a string.
const padNumber = (n, l) => `${n}`.padStart(l, '0');
padNumber(1234, 6); // '001234'
palindrome
- title: palindrome
- tags: string,intermediate
Checks if the given string is a palindrome.
- Normalize the string to
String.prototype.toLowerCase()
and useString.prototype.replace()
to remove non-alphanumeric characters from it. - Use the spread operator (
...
) to split the normalized string into individual characters. - Use
Array.prototype.reverse()
,String.prototype.join('')
and compare the result to the normalized string.
const palindrome = str => {
const s = str.toLowerCase().replace(/[\W_]/g, '');
return s === [...s].reverse().join('');
};
palindrome('taco cat'); // true
parseCookie
- title: parseCookie
- tags: browser,string,intermediate
Parses an HTTP Cookie header string, returning an object of all cookie name-value pairs.
- Use
String.prototype.split(';')
to separate key-value pairs from each other. - Use
Array.prototype.map()
andString.prototype.split('=')
to separate keys from values in each pair. - Use
Array.prototype.reduce()
anddecodeURIComponent()
to create an object with all key-value pairs.
const parseCookie = str =>
str
.split(';')
.map(v => v.split('='))
.reduce((acc, v) => {
acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
return acc;
}, {});
parseCookie('foo=bar; equation=E%3Dmc%5E2');
// { foo: 'bar', equation: 'E=mc^2' }
partial
- title: partial
- tags: function,intermediate
Creates a function that invokes fn
with partials
prepended to the arguments it receives.
- Use the spread operator (
...
) to prependpartials
to the list of arguments offn
.
const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'
partialRight
- title: partialRight
- tags: function,intermediate
Creates a function that invokes fn
with partials
appended to the arguments it receives.
- Use the spread operator (
...
) to appendpartials
to the list of arguments offn
.
const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'
partition
- title: partition
- tags: array,object,intermediate
Groups the elements into two arrays, depending on the provided function's truthiness for each element.
- Use
Array.prototype.reduce()
to create an array of two arrays. - Use
Array.prototype.push()
to add elements for whichfn
returnstrue
to the first array and elements for whichfn
returnsfalse
to the second one.
const partition = (arr, fn) =>
arr.reduce(
(acc, val, i, arr) => {
acc[fn(val, i, arr) ? 0 : 1].push(val);
return acc;
},
[[], []]
);
const users = [
{ user: 'barney', age: 36, active: false },
{ user: 'fred', age: 40, active: true },
];
partition(users, o => o.active);
// [
// [{ user: 'fred', age: 40, active: true }],
// [{ user: 'barney', age: 36, active: false }]
// ]
partitionBy
- title: partitionBy
- tags: array,object,advanced
Applies fn
to each value in arr
, splitting it each time the provided function returns a new value.
- Use
Array.prototype.reduce()
with an accumulator object that will hold the resulting array and the last value returned fromfn
. - Use
Array.prototype.push()
to add each value inarr
to the appropriate partition in the accumulator array.
const partitionBy = (arr, fn) =>
arr.reduce(
({ res, last }, v, i, a) => {
const next = fn(v, i, a);
if (next !== last) res.push([v]);
else res[res.length - 1].push(v);
return { res, last: next };
},
{ res: [] }
).res;
const numbers = [1, 1, 3, 3, 4, 5, 5, 5];
partitionBy(numbers, n => n % 2 === 0); // [[1, 1, 3, 3], [4], [5, 5, 5]]
partitionBy(numbers, n => n); // [[1, 1], [3, 3], [4], [5, 5, 5]]
percentile
- title: percentile
- tags: math,intermediate
Calculates the percentage of numbers in the given array that are less or equal to the given value.
- Use
Array.prototype.reduce()
to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.
const percentile = (arr, val) =>
(100 *
arr.reduce(
(acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0),
0
)) /
arr.length;
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55
permutations
- title: permutations
- tags: array,algorithm,recursion,advanced
Generates all permutations of an array's elements (contains duplicates).
- Use recursion.
- For each element in the given array, create all the partial permutations for the rest of its elements.
- Use
Array.prototype.map()
to combine the element with each partial permutation, thenArray.prototype.reduce()
to combine all permutations in one array. - Base cases are for
Array.prototype.length
equal to2
or1
. - ⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries may cause your browser to hang as it tries to solve all the different combinations.
const permutations = arr => {
if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
return arr.reduce(
(acc, item, i) =>
acc.concat(
permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
item,
...val,
])
),
[]
);
};
permutations([1, 33, 5]);
// [ [1, 33, 5], [1, 5, 33], [33, 1, 5], [33, 5, 1], [5, 1, 33], [5, 33, 1] ]
pick
- title: pick
- tags: object,intermediate
Picks the key-value pairs corresponding to the given keys from an object.
- Use
Array.prototype.reduce()
to convert the filtered/picked keys back to an object with the corresponding key-value pairs if the key exists in the object.
const pick = (obj, arr) =>
arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
pick({ a: 1, b: '2', c: 3 }, ['a', 'c']); // { 'a': 1, 'c': 3 }
pickBy
- title: pickBy
- tags: object,intermediate
Creates an object composed of the properties the given function returns truthy for.
- Use
Object.keys(obj)
andArray.prototype.filter()
to remove the keys for whichfn
returns a falsy value. - Use
Array.prototype.reduce()
to convert the filtered keys back to an object with the corresponding key-value pairs. - The callback function is invoked with two arguments: (value, key).
const pickBy = (obj, fn) =>
Object.keys(obj)
.filter(k => fn(obj[k], k))
.reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number');
// { 'a': 1, 'c': 3 }
pipeAsyncFunctions
- title: pipeAsyncFunctions
- tags: function,promise,intermediate
Performs left-to-right function composition for asynchronous functions.
- Use
Array.prototype.reduce()
and the spread operator (...
) to perform function composition usingPromise.prototype.then()
. - The functions can return a combination of normal values,
Promise
s or beasync
, returning throughawait
. - All functions must accept a single argument.
const pipeAsyncFunctions = (...fns) =>
arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));
const sum = pipeAsyncFunctions(
x => x + 1,
x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
x => x + 3,
async x => (await x) + 4
);
(async() => {
console.log(await sum(5)); // 15 (after one second)
})();
pipeFunctions
- title: pipeFunctions
- tags: function,intermediate
Performs left-to-right function composition.
- Use
Array.prototype.reduce()
with the spread operator (...
) to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const pipeFunctions = (...fns) =>
fns.reduce((f, g) => (...args) => g(f(...args)));
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15
pluck
- title: pluck
- tags: array,object,beginner
Converts an array of objects into an array of values corresponding to the specified key
.
- Use
Array.prototype.map()
to map the array of objects to the value ofkey
for each one.
const pluck = (arr, key) => arr.map(i => i[key]);
const simpsons = [
{ name: 'lisa', age: 8 },
{ name: 'homer', age: 36 },
{ name: 'marge', age: 34 },
{ name: 'bart', age: 10 }
];
pluck(simpsons, 'age'); // [8, 36, 34, 10]
pluralize
- title: pluralize
- tags: string,advanced
Returns the singular or plural form of the word based on the input number, using an optional dictionary if supplied.
- Use a closure to define a function that pluralizes the given
word
based on the value ofnum
. - If
num
is either-1
or1
, return the singular form of the word. - If
num
is any other number, return theplural
form. - Omit the third argument,
plural
, to use the default of the singular word +s
, or supply a custom pluralizedword
when necessary. - If the first argument is an
object
, return a function which can use the supplied dictionary to resolve the correct plural form of the word.
const pluralize = (val, word, plural = word + 's') => {
const _pluralize = (num, word, plural = word + 's') =>
[1, -1].includes(Number(num)) ? word : plural;
if (typeof val === 'object')
return (num, word) => _pluralize(num, word, val[word]);
return _pluralize(val, word, plural);
};
pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'
const PLURALS = {
person: 'people',
radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'
powerset
- title: powerset
- tags: math,algorithm,beginner
Returns the powerset of a given array of numbers.
- Use
Array.prototype.reduce()
combined withArray.prototype.map()
to iterate over elements and combine into an array containing all combinations.
const powerset = arr =>
arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
powerset([1, 2]); // [[], [1], [2], [2, 1]]
prefersDarkColorScheme
- title: prefersDarkColorScheme
- tags: browser,intermediate
Checks if the user color scheme preference is dark
.
- Use
Window.matchMedia()
with the appropriate media query to check the user color scheme preference.
const prefersDarkColorScheme = () =>
window &&
window.matchMedia &&
window.matchMedia('(prefers-color-scheme: dark)').matches;
prefersDarkColorScheme(); // true
prefersLightColorScheme
- title: prefersLightColorScheme
- tags: browser,intermediate
Checks if the user color scheme preference is light
.
- Use
Window.matchMedia()
with the appropriate media query to check the user color scheme preference.
const prefersLightColorScheme = () =>
window &&
window.matchMedia &&
window.matchMedia('(prefers-color-scheme: light)').matches;
prefersLightColorScheme(); // true
prefix
- title: prefix
- tags: browser,intermediate
Prefixes a CSS property based on the current browser.
- Use
Array.prototype.findIndex()
on an array of vendor prefix strings to test ifDocument.body
has one of them defined in itsCSSStyleDeclaration
object, otherwise returnnull
. - Use
String.prototype.charAt()
andString.prototype.toUpperCase()
to capitalize the property, which will be appended to the vendor prefix string.
const prefix = prop => {
const capitalizedProp = prop.charAt(0).toUpperCase() + prop.slice(1);
const prefixes = ['', 'webkit', 'moz', 'ms', 'o'];
const i = prefixes.findIndex(
prefix =>
typeof document.body.style[prefix ? prefix + capitalizedProp : prop] !==
'undefined'
);
return i !== -1 ? (i === 0 ? prop : prefixes[i] + capitalizedProp) : null;
};
prefix('appearance');
// 'appearance' on a supported browser, otherwise 'webkitAppearance', 'mozAppearance', 'msAppearance' or 'oAppearance'
prettyBytes
- title: prettyBytes
- tags: string,math,advanced
Converts a number in bytes to a human-readable string.
- Use an array dictionary of units to be accessed based on the exponent.
- Use
Number.prototype.toPrecision()
to truncate the number to a certain number of digits. - Return the prettified string by building it up, taking into account the supplied options and whether it is negative or not.
- Omit the second argument,
precision
, to use a default precision of3
digits. - Omit the third argument,
addSpace
, to add space between the number and unit by default.
const prettyBytes = (num, precision = 3, addSpace = true) => {
const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
const exponent = Math.min(
Math.floor(Math.log10(num < 0 ? -num : num) / 3),
UNITS.length - 1
);
const n = Number(
((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision)
);
return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};
prettyBytes(1000); // '1 KB'
prettyBytes(-27145424323.5821, 5); // '-27.145 GB'
prettyBytes(123456789, 3, false); // '123MB'
primeFactors
- title: primeFactors
- tags: math,algorithm,beginner
Finds the prime factors of a given number using the trial division algorithm.
- Use a
while
loop to iterate over all possible prime factors, starting with2
. - If the current factor,
f
, exactly dividesn
, addf
to the factors array and dividen
byf
. Otherwise, incrementf
by one.
const primeFactors = n => {
let a = [],
f = 2;
while (n > 1) {
if (n % f === 0) {
a.push(f);
n /= f;
} else {
f++;
}
}
return a;
};
primeFactors(147); // [3, 7, 7]
primes
- title: primes
- tags: math,algorithm,intermediate
Generates primes up to a given number, using the Sieve of Eratosthenes.
- Generate an array from
2
to the given number. - Use
Array.prototype.filter()
to filter out the values divisible by any number from2
to the square root of the provided number.
const primes = num => {
let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
sqroot = Math.floor(Math.sqrt(num)),
numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
return arr;
};
primes(10); // [2, 3, 5, 7]
prod
- title: prod
- tags: math,array,intermediate
Calculates the product of two or more numbers/arrays.
- Use
Array.prototype.reduce()
to multiply each value with an accumulator, initialized with a value of1
.
const prod = (...arr) => [...arr].reduce((acc, val) => acc * val, 1);
prod(1, 2, 3, 4); // 24
prod(...[1, 2, 3, 4]); // 24
promisify
- title: promisify
- tags: function,promise,intermediate
Converts an asynchronous function to return a promise.
- Use currying to return a function returning a
Promise
that calls the original function. - Use the rest operator (
...
) to pass in all the parameters. - Note: In Node 8+, you can use
util.promisify
.
const promisify = func => (...args) =>
new Promise((resolve, reject) =>
func(...args, (err, result) => (err ? reject(err) : resolve(result)))
);
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // Promise resolves after 2s
pull
- title: pull
- tags: array,intermediate
Mutates the original array to filter out the values specified.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values.
const pull = (arr, ...args) => {
let argState = Array.isArray(args[0]) ? args[0] : args;
let pulled = arr.filter(v => !argState.includes(v));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]
pullAtIndex
- title: pullAtIndex
- tags: array,advanced
Mutates the original array to filter out the values at the specified indexes. Returns the removed elements.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values. - Use
Array.prototype.push()
to keep track of pulled values.
const pullAtIndex = (arr, pullArr) => {
let removed = [];
let pulled = arr
.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
.filter((v, i) => !pullArr.includes(i));
arr.length = 0;
pulled.forEach(v => arr.push(v));
return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]);
// myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
pullAtValue
- title: pullAtValue
- tags: array,advanced
Mutates the original array to filter out the values specified. Returns the removed elements.
- Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values. - Use
Array.prototype.push()
to keep track of pulled values.
const pullAtValue = (arr, pullArr) => {
let removed = [],
pushToRemove = arr.forEach((v, i) =>
pullArr.includes(v) ? removed.push(v) : v
),
mutateTo = arr.filter((v, i) => !pullArr.includes(v));
arr.length = 0;
mutateTo.forEach(v => arr.push(v));
return removed;
};
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']);
// myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]
pullBy
- title: pullBy
- tags: array,advanced
Mutates the original array to filter out the values specified, based on a given iterator function.
- Check if the last argument provided is a function.
- Use
Array.prototype.map()
to apply the iterator functionfn
to all array elements. - Use
Array.prototype.filter()
andArray.prototype.includes()
to pull out the values that are not needed. - Set
Array.prototype.length
to mutate the passed in an array by resetting its length to0
. - Use
Array.prototype.push()
to re-populate it with only the pulled values.
const pullBy = (arr, ...args) => {
const length = args.length;
let fn = length > 1 ? args[length - 1] : undefined;
fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};
var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]
quarterOfYear
- title: quarterOfYear
- tags: date,beginner
Returns the quarter and year to which the supplied date belongs to.
- Use
Date.prototype.getMonth()
to get the current month in the range (0, 11), add1
to map it to the range (1, 12). - Use
Math.ceil()
and divide the month by3
to get the current quarter. - Use
Date.prototype.getFullYear()
to get the year from the givendate
. - Omit the argument,
date
, to use the current date by default.
const quarterOfYear = (date = new Date()) => [
Math.ceil((date.getMonth() + 1) / 3),
date.getFullYear()
];
quarterOfYear(new Date('07/10/2018')); // [ 3, 2018 ]
quarterOfYear(); // [ 4, 2020 ]
queryStringToObject
- title: queryStringToObject
- tags: object,intermediate
Generates an object from the given query string or URL.
- Use
String.prototype.split()
to get the params from the givenurl
. - Use
new URLSearchParams()
to create an appropriate object and convert it to an array of key-value pairs using the spread operator (...
). - Use
Array.prototype.reduce()
to convert the array of key-value pairs into an object.
const queryStringToObject = url =>
[...new URLSearchParams(url.split('?')[1])].reduce(
(a, [k, v]) => ((a[k] = v), a),
{}
);
queryStringToObject('https://google.com?page=1&count=10');
// {page: '1', count: '10'}
quickSort
- title: quickSort
- tags: algorithm,array,recursion,advanced
Sorts an array of numbers, using the quicksort algorithm.
- Use recursion.
- Use the spread operator (
...
) to clone the original array,arr
. - If the
length
of the array is less than2
, return the cloned array. - Use
Math.floor()
to calculate the index of the pivot element. - Use
Array.prototype.reduce()
andArray.prototype.push()
to split the array into two subarrays (elements smaller or equal to thepivot
and elements greater than it), destructuring the result into two arrays. - Recursively call
quickSort()
on the created subarrays.
const quickSort = arr => {
const a = [...arr];
if (a.length < 2) return a;
const pivotIndex = Math.floor(arr.length / 2);
const pivot = a[pivotIndex];
const [lo, hi] = a.reduce(
(acc, val, i) => {
if (val < pivot || (val === pivot && i != pivotIndex)) {
acc[0].push(val);
} else if (val > pivot) {
acc[1].push(val);
}
return acc;
},
[[], []]
);
return [...quickSort(lo), pivot, ...quickSort(hi)];
};
quickSort([1, 6, 1, 5, 3, 2, 1, 4]); // [1, 1, 1, 2, 3, 4, 5, 6]
radsToDegrees
- title: radsToDegrees
- tags: math,beginner
Converts an angle from radians to degrees.
- Use
Math.PI
and the radian to degree formula to convert the angle from radians to degrees.
const radsToDegrees = rad => (rad * 180.0) / Math.PI;
radsToDegrees(Math.PI / 2); // 90
randomAlphaNumeric
- title: randomAlphaNumeric
- tags: string,random,advanced
Generates a random string with the specified length.
- Use
Array.from()
to create a new array with the specifiedlength
. - Use
Math.random()
generate a random floating-point number,Number.prototype.toString(36)
to convert it to an alphanumeric string. - Use
String.prototype.slice(2)
to remove the integral part and decimal point from each generated number. - Use
Array.prototype.some()
to repeat this process as many times as required, up tolength
, as it produces a variable-length string each time. - Finally, use
String.prototype.slice()
to trim down the generated string if it's longer than the givenlength
.
const randomAlphaNumeric = length => {
let s = '';
Array.from({ length }).some(() => {
s += Math.random().toString(36).slice(2);
return s.length >= length;
});
return s.slice(0, length);
};
randomAlphaNumeric(5); // '0afad'
randomBoolean
- title: randomBoolean
- tags: math,random,beginner
Generates a random boolean value.
- Use
Math.random()
to generate a random number and check if it is greater than or equal to0.5
.
const randomBoolean = () => Math.random() >= 0.5;
randomBoolean(); // true
randomHexColorCode
- title: randomHexColorCode
- tags: math,random,beginner
Generates a random hexadecimal color code.
- Use
Math.random()
to generate a random 24-bit (6 * 4bits) hexadecimal number. - Use bit shifting and then convert it to an hexadecimal string using
Number.prototype.toString(16)
.
const randomHexColorCode = () => {
let n = (Math.random() * 0xfffff * 1000000).toString(16);
return '##' + n.slice(0, 6);
};
randomHexColorCode(); // '##e34155'
randomIntArrayInRange
- title: randomIntArrayInRange
- tags: math,random,intermediate
Generates an array of n
random integers in the specified range.
- Use
Array.from()
to create an empty array of the specific length. - Use
Math.random()
to generate random numbers and map them to the desired range, usingMath.floor()
to make them integers.
const randomIntArrayInRange = (min, max, n = 1) =>
Array.from(
{ length: n },
() => Math.floor(Math.random() * (max - min + 1)) + min
);
randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]
randomIntegerInRange
- title: randomIntegerInRange
- tags: math,random,beginner
Generates a random integer in the specified range.
- Use
Math.random()
to generate a random number and map it to the desired range. - Use
Math.floor()
to make it an integer.
const randomIntegerInRange = (min, max) =>
Math.floor(Math.random() * (max - min + 1)) + min;
randomIntegerInRange(0, 5); // 2
randomNumberInRange
- title: randomNumberInRange
- tags: math,random,beginner
Generates a random number in the specified range.
- Use
Math.random()
to generate a random value, map it to the desired range using multiplication.
const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;
randomNumberInRange(2, 10); // 6.0211363285087005
rangeGenerator
- title: rangeGenerator
- tags: function,generator,advanced
Creates a generator, that generates all values in the given range using the given step.
- Use a
while
loop to iterate fromstart
toend
, usingyield
to return each value and then incrementing bystep
. - Omit the third argument,
step
, to use a default value of1
.
const rangeGenerator = function* (start, end, step = 1) {
let i = start;
while (i < end) {
yield i;
i += step;
}
};
for (let i of rangeGenerator(6, 10)) console.log(i);
// Logs 6, 7, 8, 9
readFileLines
- title: readFileLines
- tags: node,array,beginner
Returns an array of lines from the specified file.
- Use
fs.readFileSync()
to create aBuffer
from a file. - Convert buffer to string using
buf.toString(encoding)
function. - Use
String.prototype.split(\n)
to create an array of lines from the contents of the file.
const fs = require('fs');
const readFileLines = filename =>
fs
.readFileSync(filename)
.toString('UTF8')
.split('\n');
/*
contents of test.txt :
line1
line2
line3
___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']
rearg
- title: rearg
- tags: function,intermediate
Creates a function that invokes the provided function with its arguments arranged according to the specified indexes.
- Use
Array.prototype.map()
to reorder arguments based onindexes
. - Use the spread operator (
...
) to pass the transformed arguments tofn
.
const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));
var rearged = rearg(
function(a, b, c) {
return [a, b, c];
},
[2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']
recordAnimationFrames
- title: recordAnimationFrames
- tags: browser,recursion,intermediate
Invokes the provided callback on each animation frame.
- Use recursion.
- Provided that
running
istrue
, continue invokingWindow.requestAnimationFrame()
which invokes the provided callback. - Return an object with two methods
start
andstop
to allow manual control of the recording. - Omit the second argument,
autoStart
, to implicitly callstart
when the function is invoked.
const recordAnimationFrames = (callback, autoStart = true) => {
let running = false,
raf;
const stop = () => {
if (!running) return;
running = false;
cancelAnimationFrame(raf);
};
const start = () => {
if (running) return;
running = true;
run();
};
const run = () => {
raf = requestAnimationFrame(() => {
callback();
if (running) run();
});
};
if (autoStart) start();
return { start, stop };
};
const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb);
// logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false);
// `start` needs to be explicitly called to begin recording frames
redirect
- title: redirect
- tags: browser,beginner
Redirects to a specified URL.
- Use
Window.location.href
orWindow.location.replace()
to redirect tourl
. - Pass a second argument to simulate a link click (
true
- default) or an HTTP redirect (false
).
const redirect = (url, asLink = true) =>
asLink ? (window.location.href = url) : window.location.replace(url);
redirect('https://google.com');
reduceSuccessive
- title: reduceSuccessive
- tags: array,intermediate
Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.
- Use
Array.prototype.reduce()
to apply the given function to the given array, storing each new result.
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce(
(res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res),
[acc]
);
reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0);
// [0, 1, 3, 6, 10, 15, 21]
reduceWhich
- title: reduceWhich
- tags: array,intermediate
Returns the minimum/maximum value of an array, after applying the provided function to set the comparing rule.
- Use
Array.prototype.reduce()
in combination with thecomparator
function to get the appropriate element in the array. - Omit the second argument,
comparator
, to use the default one that returns the minimum element in the array.
const reduceWhich = (arr, comparator = (a, b) => a - b) =>
arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));
reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
[
{ name: 'Tom', age: 12 },
{ name: 'Jack', age: 18 },
{ name: 'Lucy', age: 9 }
],
(a, b) => a.age - b.age
); // {name: 'Lucy', age: 9}
reducedFilter
- title: reducedFilter
- tags: array,intermediate
Filters an array of objects based on a condition while also filtering out unspecified keys.
- Use
Array.prototype.filter()
to filter the array based on the predicatefn
so that it returns the objects for which the condition returned a truthy value. - On the filtered array, use
Array.prototype.map()
to return the new object. - Use
Array.prototype.reduce()
to filter out the keys which were not supplied as thekeys
argument.
const reducedFilter = (data, keys, fn) =>
data.filter(fn).map(el =>
keys.reduce((acc, key) => {
acc[key] = el[key];
return acc;
}, {})
);
const data = [
{
id: 1,
name: 'john',
age: 24
},
{
id: 2,
name: 'mike',
age: 50
}
];
reducedFilter(data, ['id', 'name'], item => item.age > 24);
// [{ id: 2, name: 'mike'}]
reject
- title: reject
- tags: array,beginner
Filters an array's values based on a predicate function, returning only values for which the predicate function returns false
.
- Use
Array.prototype.filter()
in combination with the predicate function,pred
, to return only the values for which it returnsfalse
.
const reject = (pred, array) => array.filter((...args) => !pred(...args));
reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']);
// ['Pear', 'Kiwi']
remove
- title: remove
- tags: array,intermediate
Mutates an array by removing elements for which the given function returns false
.
- Use
Array.prototype.filter()
to find array elements that return truthy values. - Use
Array.prototype.reduce()
to remove elements usingArray.prototype.splice()
. - The callback function is invoked with three arguments (value, index, array).
const remove = (arr, func) =>
Array.isArray(arr)
? arr.filter(func).reduce((acc, val) => {
arr.splice(arr.indexOf(val), 1);
return acc.concat(val);
}, [])
: [];
remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]
removeAccents
- title: removeAccents
- tags: string,beginner
Removes accents from strings.
- Use
String.prototype.normalize()
to convert the string to a normalized Unicode format. - Use
String.prototype.replace()
to replace diacritical marks in the given Unicode range by empty strings.
const removeAccents = str =>
str.normalize('NFD').replace(/[\u0300-\u036f]/g, '');
removeAccents('Antoine de Saint-Exupéry'); // 'Antoine de Saint-Exupery'
removeClass
- title: removeClass
- tags: browser,beginner
Removes a class from an HTML element.
- Use
Element.classList
andDOMTokenList.remove()
to remove the specified class from the element.
const removeClass = (el, className) => el.classList.remove(className);
removeClass(document.querySelector('p.special'), 'special');
// The paragraph will not have the 'special' class anymore
removeElement
- title: removeElement
- tags: browser,beginner
Removes an element from the DOM.
- Use
Element.parentNode
to get the given element's parent node. - Use
Element.removeChild()
to remove the given element from its parent node.
const removeElement = el => el.parentNode.removeChild(el);
removeElement(document.querySelector('##my-element'));
// Removes ##my-element from the DOM
removeNonASCII
- title: removeNonASCII
- tags: string,regexp,intermediate
Removes non-printable ASCII characters.
- Use
String.prototype.replace()
with a regular expression to remove non-printable ASCII characters.
const removeNonASCII = str => str.replace(/[^\x20-\x7E]/g, '');
removeNonASCII('äÄçÇéÉêlorem-ipsumöÖÐþúÚ'); // 'lorem-ipsum'
removeWhitespace
- title: removeWhitespace
- tags: string,regexp,beginner
Returns a string with whitespaces removed.
- Use
String.prototype.replace()
with a regular expression to replace all occurrences of whitespace characters with an empty string.
const removeWhitespace = str => str.replace(/\s+/g, '');
removeWhitespace('Lorem ipsum.\n Dolor sit amet. ');
// 'Loremipsum.Dolorsitamet.'
renameKeys
- title: renameKeys
- tags: object,intermediate
Replaces the names of multiple object keys with the values provided.
- Use
Object.keys()
in combination withArray.prototype.reduce()
and the spread operator (...
) to get the object's keys and rename them according tokeysMap
.
const renameKeys = (keysMap, obj) =>
Object.keys(obj).reduce(
(acc, key) => ({
...acc,
...{ [keysMap[key] || key]: obj[key] }
}),
{}
);
const obj = { name: 'Bobo', job: 'Front-End Master', shoeSize: 100 };
renameKeys({ name: 'firstName', job: 'passion' }, obj);
// { firstName: 'Bobo', passion: 'Front-End Master', shoeSize: 100 }
renderElement
- title: renderElement
- tags: browser,recursion,advanced
Renders the given DOM tree in the specified DOM element.
- Destructure the first argument into
type
andprops
, usingtype
to determine if the given element is a text element. - Based on the element's
type
, use eitherDocument.createTextNode()
orDocument.createElement()
to create the DOM element. - Use
Object.keys()
to add attributes to the DOM element and setting event listeners, as necessary. - Use recursion to render
props.children
, if any. - Finally, use
Node.appendChild()
to append the DOM element to the specifiedcontainer
.
const renderElement = ({ type, props = {} }, container) => {
const isTextElement = !type;
const element = isTextElement
? document.createTextNode('')
: document.createElement(type);
const isListener = p => p.startsWith('on');
const isAttribute = p => !isListener(p) && p !== 'children';
Object.keys(props).forEach(p => {
if (isAttribute(p)) element[p] = props[p];
if (!isTextElement && isListener(p))
element.addEventListener(p.toLowerCase().slice(2), props[p]);
});
if (!isTextElement && props.children && props.children.length)
props.children.forEach(childElement =>
renderElement(childElement, element)
);
container.appendChild(element);
};
const myElement = {
type: 'button',
props: {
type: 'button',
className: 'btn',
onClick: () => alert('Clicked'),
children: [{ props: { nodeValue: 'Click me' } }]
}
};
renderElement(myElement, document.body);
repeatGenerator
- title: repeatGenerator
- tags: function,generator,advanced
Creates a generator, repeating the given value indefinitely.
- Use a non-terminating
while
loop, that willyield
a value every timeGenerator.prototype.next()
is called. - Use the return value of the
yield
statement to update the returned value if the passed value is notundefined
.
const repeatGenerator = function* (val) {
let v = val;
while (true) {
let newV = yield v;
if (newV !== undefined) v = newV;
}
};
const repeater = repeatGenerator(5);
repeater.next(); // { value: 5, done: false }
repeater.next(); // { value: 5, done: false }
repeater.next(4); // { value: 4, done: false }
repeater.next(); // { value: 4, done: false }
requireUncached
- title: requireUncached
- tags: node,advanced
Loads a module after removing it from the cache (if exists).
- Use
delete
to remove the module from the cache (if exists). - Use
require()
to load the module again.
const requireUncached = module => {
delete require.cache[require.resolve(module)];
return require(module);
};
const fs = requireUncached('fs'); // 'fs' will be loaded fresh every time
reverseNumber
- title: reverseNumber
- tags: math,string,beginner
Reverses a number.
- Use
Object.prototype.toString()
to convertn
to a string. - Use
String.prototype.split('')
,Array.prototype.reverse()
andString.prototype.join('')
to get the reversed value ofn
as a string. - Use
parseFloat()
to convert the string to a number andMath.sign()
to preserve its sign.
const reverseNumber = n =>
parseFloat(`${n}`.split('').reverse().join('')) * Math.sign(n);
reverseNumber(981); // 189
reverseNumber(-500); // -5
reverseNumber(73.6); // 6.37
reverseNumber(-5.23); // -32.5
reverseString
- title: reverseString
- tags: string,beginner
Reverses a string.
- Use the spread operator (
...
) andArray.prototype.reverse()
to reverse the order of the characters in the string. - Combine characters to get a string using
String.prototype.join('')
.
const reverseString = str => [...str].reverse().join('');
reverseString('foobar'); // 'raboof'
round
- title: round
- tags: math,intermediate
Rounds a number to a specified amount of digits.
- Use
Math.round()
and template literals to round the number to the specified number of digits. - Omit the second argument,
decimals
, to round to an integer.
const round = (n, decimals = 0) =>
Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
round(1.005, 2); // 1.01
runAsync
- title: runAsync
- tags: browser,function,promise,advanced
Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.
- Create a
new Worker()
using aBlob
object URL, the contents of which should be the stringified version of the supplied function. - Immediately post the return value of calling the function back.
- Return a
new Promise()
, listening foronmessage
andonerror
events and resolving the data posted back from the worker, or throwing an error.
const runAsync = fn => {
const worker = new Worker(
URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
type: 'application/javascript; charset=utf-8'
})
);
return new Promise((res, rej) => {
worker.onmessage = ({ data }) => {
res(data), worker.terminate();
};
worker.onerror = err => {
rej(err), worker.terminate();
};
});
};
const longRunningFunction = () => {
let result = 0;
for (let i = 0; i < 1000; i++)
for (let j = 0; j < 700; j++)
for (let k = 0; k < 300; k++) result = result + i + j + k;
return result;
};
/*
NOTE: Since the function is running in a different context, closures are not supported.
The function supplied to `runAsync` gets stringified, so everything becomes literal.
All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'
runPromisesInSeries
- title: runPromisesInSeries
- tags: function,promise,intermediate
Runs an array of promises in series.
- Use
Array.prototype.reduce()
to create a promise chain, where each promise returns the next promise when resolved.
const runPromisesInSeries = ps =>
ps.reduce((p, next) => p.then(next), Promise.resolve());
const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]);
// Executes each promise sequentially, taking a total of 3 seconds to complete
sample
- title: sample
- tags: array,string,random,beginner
Gets a random element from an array.
- Use
Math.random()
to generate a random number. - Multiply it by
Array.prototype.length
and round it off to the nearest whole number usingMath.floor()
. - This method also works with strings.
const sample = arr => arr[Math.floor(Math.random() * arr.length)];
sample([3, 7, 9, 11]); // 9
sampleSize
- title: sampleSize
- tags: array,random,intermediate
Gets n
random elements at unique keys from an array up to the size of the array.
- Shuffle the array using the Fisher-Yates algorithm.
- Use
Array.prototype.slice()
to get the firstn
elements. - Omit the second argument,
n
, to get only one element at random from the array.
const sampleSize = ([...arr], n = 1) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr.slice(0, n);
};
sampleSize([1, 2, 3], 2); // [3, 1]
sampleSize([1, 2, 3], 4); // [2, 3, 1]
scrollToTop
- title: scrollToTop
- tags: browser,intermediate
Smooth-scrolls to the top of the page.
- Get distance from top using
Document.documentElement
orDocument.body
andElement.scrollTop
. - Scroll by a fraction of the distance from the top.
- Use
Window.requestAnimationFrame()
to animate the scrolling.
const scrollToTop = () => {
const c = document.documentElement.scrollTop || document.body.scrollTop;
if (c > 0) {
window.requestAnimationFrame(scrollToTop);
window.scrollTo(0, c - c / 8);
}
};
scrollToTop(); // Smooth-scrolls to the top of the page
sdbm
- title: sdbm
- tags: math,intermediate
Hashes the input string into a whole number.
- Use
String.prototype.split('')
andArray.prototype.reduce()
to create a hash of the input string, utilizing bit shifting.
const sdbm = str => {
let arr = str.split('');
return arr.reduce(
(hashCode, currentVal) =>
(hashCode =
currentVal.charCodeAt(0) +
(hashCode << 6) +
(hashCode << 16) -
hashCode),
0
);
};
sdbm('name'); // -3521204949
selectionSort
- title: selectionSort
- tags: algorithm,array,intermediate
Sorts an array of numbers, using the selection sort algorithm.
- Use the spread operator (
...
) to clone the original array,arr
. - Use a
for
loop to iterate over elements in the array. - Use
Array.prototype.slice()
andArray.prototype.reduce()
to find the index of the minimum element in the subarray to the right of the current index and perform a swap, if necessary.
const selectionSort = arr => {
const a = [...arr];
for (let i = 0; i < a.length; i++) {
const min = a
.slice(i + 1)
.reduce((acc, val, j) => (val < a[acc] ? j + i + 1 : acc), i);
if (min !== i) [a[i], a[min]] = [a[min], a[i]];
}
return a;
};
selectionSort([5, 1, 4, 2, 3]); // [1, 2, 3, 4, 5]
serializeCookie
- title: serializeCookie
- tags: browser,string,intermediate
Serializes a cookie name-value pair into a Set-Cookie header string.
- Use template literals and
encodeURIComponent()
to create the appropriate string.
const serializeCookie = (name, val) =>
`${encodeURIComponent(name)}=${encodeURIComponent(val)}`;
serializeCookie('foo', 'bar'); // 'foo=bar'
serializeForm
- title: serializeForm
- tags: browser,string,intermediate
Encodes a set of form elements as a query string.
- Use the
Foata
constructor to convert the HTMLform
toFoata
. - Use
Array.from()
to convert to an array, passing a map function as the second argument. - Use
Array.prototype.map()
andencodeURIComponent()
to encode each field's value. - Use
Array.prototype.join()
with appropriate arguments to produce an appropriate query string.
const serializeForm = form =>
Array.from(new Foata(form), field =>
field.map(encodeURIComponent).join('=')
).join('&');
serializeForm(document.querySelector('##form'));
// email=test%40email.com&name=Test%20Name
setStyle
- title: setStyle
- tags: browser,beginner
Sets the value of a CSS rule for the specified HTML element.
- Use
ElementCSSInlineStyle.style
to set the value of the CSSrule
for the specified element toval
.
const setStyle = (el, rule, val) => (el.style[rule] = val);
setStyle(document.querySelector('p'), 'font-size', '20px');
// The first <p> element on the page will have a font-size of 20px
shallowClone
- title: shallowClone
- tags: object,beginner
Creates a shallow clone of an object.
- Use
Object.assign()
and an empty object ({}
) to create a shallow clone of the original.
const shallowClone = obj => Object.assign({}, obj);
const a = { x: true, y: 1 };
const b = shallowClone(a); // a !== b
shank
- title: shank
- tags: array,intermediate
Has the same functionality as Array.prototype.splice()
, but returning a new array instead of mutating the original array.
- Use
Array.prototype.slice()
andArray.prototype.concat()
to get an array with the new contents after removing existing elements and/or adding new elements. - Omit the second argument,
index
, to start at0
. - Omit the third argument,
delCount
, to remove0
elements. - Omit the fourth argument,
elements
, in order to not add any new elements.
const shank = (arr, index = 0, delCount = 0, ...elements) =>
arr
.slice(0, index)
.concat(elements)
.concat(arr.slice(index + delCount));
const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta');
// [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']
show
- title: show
- tags: browser,css,beginner
Shows all the elements specified.
- Use the spread operator (
...
) andArray.prototype.forEach()
to clear thedisplay
property for each element specified.
const show = (...el) => [...el].forEach(e => (e.style.display = ''));
show(...document.querySelectorAll('img'));
// Shows all <img> elements on the page
shuffle
- title: shuffle
- tags: array,random,intermediate
Randomizes the order of the values of an array, returning a new array.
- Use the Fisher-Yates algorithm to reorder the elements of the array.
const shuffle = ([...arr]) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr;
};
const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]
similarity
- title: similarity
- tags: array,math,beginner
Returns an array of elements that appear in both arrays.
- Use
Array.prototype.includes()
to determine values that are not part ofvalues
. - Use
Array.prototype.filter()
to remove them.
const similarity = (arr, values) => arr.filter(v => values.includes(v));
similarity([1, 2, 3], [1, 2, 4]); // [1, 2]
size
- title: size
- tags: object,array,string,intermediate
Gets the size of an array, object or string.
- Get type of
val
(array
,object
orstring
). - Use
Array.prototype.length
property for arrays. - Use
length
orsize
value if available or number of keys for objects. - Use
size
of aBlob
object created fromval
for strings. - Split strings into array of characters with
split('')
and return its length.
const size = val =>
Array.isArray(val)
? val.length
: val && typeof val === 'object'
? val.size || val.length || Object.keys(val).length
: typeof val === 'string'
? new Blob([val]).size
: 0;
size([1, 2, 3, 4, 5]); // 5
size('size'); // 4
size({ one: 1, two: 2, three: 3 }); // 3
sleep
- title: sleep
- tags: function,promise,intermediate
Delays the execution of an asynchronous function.
- Delay executing part of an
async
function, by putting it to sleep, returning anew Promise()
.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
async function sleepyWork() {
console.log("I'm going to sleep for 1 second.");
await sleep(1000);
console.log('I woke up after 1 second.');
}
slugify
- title: slugify
- tags: string,regexp,intermediate
Converts a string to a URL-friendly slug.
- Use
String.prototype.toLowerCase()
andString.prototype.trim()
to normalize the string. - Use
String.prototype.replace()
to replace spaces, dashes and underscores with-
and remove special characters.
const slugify = str =>
str
.toLowerCase()
.trim()
.replace(/[^\w\s-]/g, '')
.replace(/[\s_-]+/g, '-')
.replace(/^-+|-+$/g, '');
slugify('Hello World!'); // 'hello-world'
smoothScroll
- title: smoothScroll
- tags: browser,css,intermediate
Smoothly scrolls the element on which it's called into the visible area of the browser window.
- Use
Element.scrollIntoView()
to scroll the element. - Use
{ behavior: 'smooth' }
to scroll smoothly.
const smoothScroll = element =>
document.querySelector(element).scrollIntoView({
behavior: 'smooth'
});
smoothScroll('##fooBar'); // scrolls smoothly to the element with the id fooBar
smoothScroll('.fooBar');
// scrolls smoothly to the first element with a class of fooBar
sortCharactersInString
- title: sortCharactersInString
- tags: string,beginner
Alphabetically sorts the characters in a string.
- Use the spread operator (
...
),Array.prototype.sort()
andString.prototype.localeCompare()
to sort the characters instr
. - Recombine using
String.prototype.join('')
.
const sortCharactersInString = str =>
[...str].sort((a, b) => a.localeCompare(b)).join('');
sortCharactersInString('cabbage'); // 'aabbceg'
sortedIndex
- title: sortedIndex
- tags: array,math,intermediate
Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.findIndex()
to find the appropriate index where the element should be inserted.
const sortedIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
return index === -1 ? arr.length : index;
};
sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1
sortedIndexBy
- title: sortedIndexBy
- tags: array,math,intermediate
Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order, based on the provided iterator function.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.findIndex()
to find the appropriate index where the element should be inserted, based on the iterator functionfn
.
const sortedIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr.findIndex(el =>
isDescending ? val >= fn(el) : val <= fn(el)
);
return index === -1 ? arr.length : index;
};
sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0
sortedLastIndex
- title: sortedLastIndex
- tags: array,intermediate
Finds the highest index at which a value should be inserted into an array in order to maintain its sort order.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.reverse()
andArray.prototype.findIndex()
to find the appropriate last index where the element should be inserted.
const sortedLastIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr
.reverse()
.findIndex(el => (isDescending ? n <= el : n >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndex([10, 20, 30, 30, 40], 30); // 4
sortedLastIndexBy
- title: sortedLastIndexBy
- tags: array,intermediate
Finds the highest index at which a value should be inserted into an array in order to maintain its sort order, based on a provided iterator function.
- Loosely check if the array is sorted in descending order.
- Use
Array.prototype.map()
to apply the iterator function to all elements of the array. - Use
Array.prototype.reverse()
andArray.prototype.findIndex()
to find the appropriate last index where the element should be inserted, based on the provided iterator function.
const sortedLastIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr
.map(fn)
.reverse()
.findIndex(el => (isDescending ? val <= el : val >= el));
return index === -1 ? 0 : arr.length - index;
};
sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1
splitLines
- title: splitLines
- tags: string,regexp,beginner
Splits a multiline string into an array of lines.
- Use
String.prototype.split()
and a regular expression to match line breaks and create an array.
const splitLines = str => str.split(/\r?\n/);
splitLines('This\nis a\nmultiline\nstring.\n');
// ['This', 'is a', 'multiline', 'string.' , '']
spreadOver
- title: spreadOver
- tags: function,intermediate
Takes a variadic function and returns a function that accepts an array of arguments.
- Use a closure and the spread operator (
...
) to map the array of arguments to the inputs of the function.
const spreadOver = fn => argsArr => fn(...argsArr);
const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3
stableSort
- title: stableSort
- tags: array,advanced
Performs stable sorting of an array, preserving the initial indexes of items when their values are the same.
- Use
Array.prototype.map()
to pair each element of the input array with its corresponding index. - Use
Array.prototype.sort()
and acompare
function to sort the list, preserving their initial order if the items compared are equal. - Use
Array.prototype.map()
to convert back to the initial array items. - Does not mutate the original array, but returns a new array instead.
const stableSort = (arr, compare) =>
arr
.map((item, index) => ({ item, index }))
.sort((a, b) => compare(a.item, b.item) || a.index - b.index)
.map(({ item }) => item);
const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
standardDeviation
- title: standardDeviation
- tags: math,array,intermediate
Calculates the standard deviation of an array of numbers.
- Use
Array.prototype.reduce()
to calculate the mean, variance and the sum of the variance of the values and determine the standard deviation. - Omit the second argument,
usePopulation
, to get the sample standard deviation or set it totrue
to get the population standard deviation.
const standardDeviation = (arr, usePopulation = false) => {
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
return Math.sqrt(
arr
.reduce((acc, val) => acc.concat((val - mean) ** 2), [])
.reduce((acc, val) => acc + val, 0) /
(arr.length - (usePopulation ? 0 : 1))
);
};
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true);
// 12.29899614287479 (population)
stringPermutations
- title: stringPermutations
- tags: string,recursion,advanced
Generates all permutations of a string (contains duplicates).
- Use recursion.
- For each letter in the given string, create all the partial permutations for the rest of its letters.
- Use
Array.prototype.map()
to combine the letter with each partial permutation. - Use
Array.prototype.reduce()
to combine all permutations in one array. - Base cases are for
String.prototype.length
equal to2
or1
. - ⚠️ WARNING: The execution time increases exponentially with each character. Anything more than 8 to 10 characters will cause your environment to hang as it tries to solve all the different combinations.
const stringPermutations = str => {
if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
return str
.split('')
.reduce(
(acc, letter, i) =>
acc.concat(
stringPermutations(str.slice(0, i) + str.slice(i + 1)).map(
val => letter + val
)
),
[]
);
};
stringPermutations('abc'); // ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
stringifyCircularJSON
- title: stringifyCircularJSON
- tags: object,advanced
Serializes a JSON object containing circular references into a JSON format.
- Create a
new WeakSet()
to store and check seen values, usingWeakSet.prototype.add()
andWeakSet.prototype.has()
. - Use
JSON.stringify()
with a custom replacer function that omits values already inseen
, adding new values as necessary. - ⚠️ NOTICE: This function finds and removes circular references, which causes circular data loss in the serialized JSON.
const stringifyCircularJSON = obj => {
const seen = new WeakSet();
return JSON.stringify(obj, (k, v) => {
if (v !== null && typeof v === 'object') {
if (seen.has(v)) return;
seen.add(v);
}
return v;
});
};
const obj = { n: 42 };
obj.obj = obj;
stringifyCircularJSON(obj); // '{"n": 42}'
stripHTMLTags
- title: stripHTMLTags
- tags: string,regexp,beginner
Removes HTML/XML tags from string.
- Use a regular expression to remove HTML/XML tags from a string.
const stripHTMLTags = str => str.replace(/<[^>]*>/g, '');
stripHTMLTags('<p><em>lorem</em> <strong>ipsum</strong></p>'); // 'lorem ipsum'
subSet
- title: subSet
- tags: array,intermediate
Checks if the first iterable is a subset of the second one, excluding duplicate values.
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that each value in the first iterable is contained in the second one.
const subSet = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sA].every(v => sB.has(v));
};
subSet(new Set([1, 2]), new Set([1, 2, 3, 4])); // true
subSet(new Set([1, 5]), new Set([1, 2, 3, 4])); // false
sum
- title: sum
- tags: math,array,beginner
Calculates the sum of two or more numbers/arrays.
- Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
.
const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);
sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10
sumBy
- title: sumBy
- tags: math,array,intermediate
Calculates the sum of an array, after mapping each element to a value using the provided function.
- Use
Array.prototype.map()
to map each element to the value returned byfn
. - Use
Array.prototype.reduce()
to add each value to an accumulator, initialized with a value of0
.
const sumBy = (arr, fn) =>
arr
.map(typeof fn === 'function' ? fn : val => val[fn])
.reduce((acc, val) => acc + val, 0);
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
sumN
- title: sumN
- tags: math,beginner
Sums all the numbers between 1
and n
.
- Use the formula
(n * (n + 1)) / 2
to get the sum of all the numbers between 1 andn
.
const sumN = n => (n * (n + 1)) / 2;
sumN(100); // 5050
sumPower
- title: sumPower
- tags: math,intermediate
Calculates the sum of the powers of all the numbers from start
to end
(both inclusive).
- Use
Array.prototype.fill()
to create an array of all the numbers in the target range. - Use
Array.prototype.map()
and the exponent operator (**
) to raise them topower
andArray.prototype.reduce()
to add them together. - Omit the second argument,
power
, to use a default power of2
. - Omit the third argument,
start
, to use a default starting value of1
.
const sumPower = (end, power = 2, start = 1) =>
Array(end + 1 - start)
.fill(0)
.map((x, i) => (i + start) ** power)
.reduce((a, b) => a + b, 0);
sumPower(10); // 385
sumPower(10, 3); // 3025
sumPower(10, 3, 5); // 2925
superSet
- title: superSet
- tags: array,intermediate
Checks if the first iterable is a superset of the second one, excluding duplicate values.
- Use the
new Set()
constructor to create a newSet
object from each iterable. - Use
Array.prototype.every()
andSet.prototype.has()
to check that each value in the second iterable is contained in the first one.
const superSet = (a, b) => {
const sA = new Set(a), sB = new Set(b);
return [...sB].every(v => sA.has(v));
};
superSet(new Set([1, 2, 3, 4]), new Set([1, 2])); // true
superSet(new Set([1, 2, 3, 4]), new Set([1, 5])); // false
supportsTouchEvents
- title: supportsTouchEvents
- tags: browser,beginner
Checks if touch events are supported.
- Check if
'ontouchstart'
exists inwindow
.
const supportsTouchEvents = () =>
window && 'ontouchstart' in window;
supportsTouchEvents(); // true
swapCase
- title: swapCase
- tags: string,beginner
Creates a string with uppercase characters converted to lowercase and vice versa.
- Use the spread operator (
...
) to convertstr
into an array of characters. - Use
String.prototype.toLowerCase()
andString.prototype.toUpperCase()
to convert lowercase characters to uppercase and vice versa. - Use
Array.prototype.map()
to apply the transformation to each character,Array.prototype.join()
to combine back into a string. - Note that it is not necessarily true that
swapCase(swapCase(str)) === str
.
const swapCase = str =>
[...str]
.map(c => (c === c.toLowerCase() ? c.toUpperCase() : c.toLowerCase()))
.join('');
swapCase('Hello world!'); // 'hELLO WORLD!'
symmetricDifference
- title: symmetricDifference
- tags: array,math,intermediate
Returns the symmetric difference between two arrays, without filtering out duplicate values.
- Create a
new Set()
from each array to get the unique values of each one. - Use
Array.prototype.filter()
on each of them to only keep values not contained in the other.
const symmetricDifference = (a, b) => {
const sA = new Set(a),
sB = new Set(b);
return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};
symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]
symmetricDifferenceBy
- title: symmetricDifferenceBy
- tags: array,intermediate
Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.
- Create a
new Set()
from each array to get the unique values of each one after applyingfn
to them. - Use
Array.prototype.filter()
on each of them to only keep values not contained in the other.
const symmetricDifferenceBy = (a, b, fn) => {
const sA = new Set(a.map(v => fn(v))),
sB = new Set(b.map(v => fn(v)));
return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};
symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]
symmetricDifferenceBy(
[{ id: 1 }, { id: 2 }, { id: 3 }],
[{ id: 1 }, { id: 2 }, { id: 4 }],
i => i.id
);
// [{ id: 3 }, { id: 4 }]
symmetricDifferenceWith
- title: symmetricDifferenceWith
- tags: array,intermediate
Returns the symmetric difference between two arrays, using a provided function as a comparator.
- Use
Array.prototype.filter()
andArray.prototype.findIndex()
to find the appropriate values.
const symmetricDifferenceWith = (arr, val, comp) => [
...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
];
symmetricDifferenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 3.9]
tail
- title: tail
- tags: array,beginner
Returns all elements in an array except for the first one.
- Return
Array.prototype.slice(1)
ifArray.prototype.length
is more than1
, otherwise, return the whole array.
const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);
tail([1, 2, 3]); // [2, 3]
tail([1]); // [1]
take
- title: take
- tags: array,beginner
Creates an array with n
elements removed from the beginning.
- Use
Array.prototype.slice()
to create a slice of the array withn
elements taken from the beginning.
const take = (arr, n = 1) => arr.slice(0, n);
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []
takeRight
- title: takeRight
- tags: array,intermediate
Creates an array with n
elements removed from the end.
- Use
Array.prototype.slice()
to create a slice of the array withn
elements taken from the end.
const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);
takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]
takeRightUntil
- title: takeRightUntil
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns true
.
Returns the removed elements.
- Create a reversed copy of the array, using the spread operator (
...
) andArray.prototype.reverse()
. - Loop through the reversed copy, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is truthy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeRightUntil = (arr, fn) => {
for (const [i, val] of [...arr].reverse().entries())
if (fn(val)) return i === 0 ? [] : arr.slice(-i);
return arr;
};
takeRightUntil([1, 2, 3, 4], n => n < 3); // [3, 4]
takeRightWhile
- title: takeRightWhile
- tags: array,intermediate
Removes elements from the end of an array until the passed function returns false
.
Returns the removed elements.
- Create a reversed copy of the array, using the spread operator (
...
) andArray.prototype.reverse()
. - Loop through the reversed copy, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is falsy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeRightWhile = (arr, fn) => {
for (const [i, val] of [...arr].reverse().entries())
if (!fn(val)) return i === 0 ? [] : arr.slice(-i);
return arr;
};
takeRightWhile([1, 2, 3, 4], n => n >= 3); // [3, 4]
takeUntil
- title: takeUntil
- tags: array,intermediate
Removes elements in an array until the passed function returns true
.
Returns the removed elements.
- Loop through the array, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is truthy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeUntil = (arr, fn) => {
for (const [i, val] of arr.entries()) if (fn(val)) return arr.slice(0, i);
return arr;
};
takeUntil([1, 2, 3, 4], n => n >= 3); // [1, 2]
takeWhile
- title: takeWhile
- tags: array,intermediate
Removes elements in an array until the passed function returns false
.
Returns the removed elements.
- Loop through the array, using a
for...of
loop overArray.prototype.entries()
until the returned value from the function is falsy. - Return the removed elements, using
Array.prototype.slice()
. - The callback function,
fn
, accepts a single argument which is the value of the element.
const takeWhile = (arr, fn) => {
for (const [i, val] of arr.entries()) if (!fn(val)) return arr.slice(0, i);
return arr;
};
takeWhile([1, 2, 3, 4], n => n < 3); // [1, 2]
throttle
- title: throttle
- tags: function,advanced
Creates a throttled function that only invokes the provided function at most once per every wait
milliseconds
- Use
setTimeout()
andclearTimeout()
to throttle the given method,fn
. - Use
Function.prototype.apply()
to apply thethis
context to the function and provide the necessaryarguments
. - Use
Date.now()
to keep track of the last time the throttled function was invoked. - Use a variable,
inThrottle
, to prevent a race condition between the first execution offn
and the next loop. - Omit the second argument,
wait
, to set the timeout at a default of 0 ms.
const throttle = (fn, wait) => {
let inThrottle, lastFn, lastTime;
return function() {
const context = this,
args = arguments;
if (!inThrottle) {
fn.apply(context, args);
lastTime = Date.now();
inThrottle = true;
} else {
clearTimeout(lastFn);
lastFn = setTimeout(function() {
if (Date.now() - lastTime >= wait) {
fn.apply(context, args);
lastTime = Date.now();
}
}, Math.max(wait - (Date.now() - lastTime), 0));
}
};
};
window.addEventListener(
'resize',
throttle(function(evt) {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250ms
timeTaken
- title: timeTaken
- tags: function,beginner
Measures the time it takes for a function to execute.
- Use
Console.time()
andConsole.timeEnd()
to measure the difference between the start and end times to determine how long the callback took to execute.
const timeTaken = callback => {
console.time('timeTaken');
const r = callback();
console.timeEnd('timeTaken');
return r;
};
timeTaken(() => Math.pow(2, 10)); // 1024, (logged): timeTaken: 0.02099609375ms
times
- title: times
- tags: function,intermediate
Iterates over a callback n
times
- Use
Function.prototype.call()
to callfn
n
times or until it returnsfalse
. - Omit the last argument,
context
, to use anundefined
object (or the global object in non-strict mode).
const times = (n, fn, context = undefined) => {
let i = 0;
while (fn.call(context, i) !== false && ++i < n) {}
};
var output = '';
times(5, i => (output += i));
console.log(output); // 01234
toCamelCase
- title: toCamelCase
- tags: string,regexp,intermediate
Converts a string to camelcase.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
,String.prototype.toLowerCase()
andString.prototype.toUpperCase()
to combine them, capitalizing the first letter of each one.
const toCamelCase = str => {
let s =
str &&
str
.match(
/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g
)
.map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
.join('');
return s.slice(0, 1).toLowerCase() + s.slice(1);
};
toCamelCase('some_database_field_name'); // 'someDatabaseFieldName'
toCamelCase('Some label that needs to be camelized');
// 'someLabelThatNeedsToBeCamelized'
toCamelCase('some-javascript-property'); // 'someJavascriptProperty'
toCamelCase('some-mixed_string with spaces_underscores-and-hyphens');
// 'someMixedStringWithSpacesUnderscoresAndHyphens'
toCharArray
- title: toCharArray
- tags: string,beginner
Converts a string to an array of characters.
- Use the spread operator (
...
) to convert the string into an array of characters.
const toCharArray = s => [...s];
toCharArray('hello'); // ['h', 'e', 'l', 'l', 'o']
toCurrency
- title: toCurrency
- tags: math,string,intermediate
Takes a number and returns it in the specified currency formatting.
- Use
Intl.NumberFormat
to enable country / currency sensitive formatting.
const toCurrency = (n, curr, LanguageFormat = undefined) =>
Intl.NumberFormat(LanguageFormat, {
style: 'currency',
currency: curr,
}).format(n);
toCurrency(123456.789, 'EUR');
// €123,456.79 | currency: Euro | currencyLangFormat: Local
toCurrency(123456.789, 'USD', 'en-us');
// $123,456.79 | currency: US Dollar | currencyLangFormat: English (United States)
toCurrency(123456.789, 'USD', 'fa');
// ۱۲۳٬۴۵۶٫۷۹ $ | currency: US Dollar | currencyLangFormat: Farsi
toCurrency(322342436423.2435, 'JPY');
// ¥322,342,436,423 | currency: Japanese Yen | currencyLangFormat: Local
toCurrency(322342436423.2435, 'JPY', 'fi');
// 322 342 436 423 ¥ | currency: Japanese Yen | currencyLangFormat: Finnish
toDecimalMark
- title: toDecimalMark
- tags: math,beginner
Converts a number to a decimal mark formatted string.
- Use
Number.prototype.toLocaleString()
to convert the number to decimal mark format.
const toDecimalMark = num => num.toLocaleString('en-US');
toDecimalMark(12305030388.9087); // '12,305,030,388.909'
toHSLArray
- title: toHSLArray
- tags: string,browser,regexp,beginner
Converts an hsl()
color string to an array of values.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values.
const toHSLArray = hslStr => hslStr.match(/\d+/g).map(Number);
toHSLArray('hsl(50, 10%, 10%)'); // [50, 10, 10]
toHSLObject
- title: toHSLObject
- tags: string,browser,regexp,intermediate
Converts an hsl()
color string to an object with the values of each color.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Use array destructuring to store the values into named variables and create an appropriate object from them.
const toHSLObject = hslStr => {
const [hue, saturation, lightness] = hslStr.match(/\d+/g).map(Number);
return { hue, saturation, lightness };
};
toHSLObject('hsl(50, 10%, 10%)'); // { hue: 50, saturation: 10, lightness: 10 }
toHash
- title: toHash
- tags: array,intermediate
Reduces a given array-like into a value hash (keyed data store).
- Given an iterable object or array-like structure, call
Array.prototype.reduce.call()
on the provided object to step over it and return anObject
, keyed by the reference value.
const toHash = (object, key) =>
Array.prototype.reduce.call(
object,
(acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
{}
);
toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 3: 1 }
toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
// A more in depth example:
let users = [
{ id: 1, first: 'Jon' },
{ id: 2, first: 'Joe' },
{ id: 3, first: 'Moe' },
];
let managers = [{ manager: 1, employees: [2, 3] }];
// We use function here because we want a bindable reference,
// but a closure referencing the hash would work, too.
managers.forEach(
manager =>
(manager.employees = manager.employees.map(function(id) {
return this[id];
}, toHash(users, 'id')))
);
managers;
// [ {manager:1, employees: [ {id: 2, first: 'Joe'}, {id: 3, first: 'Moe'} ] } ]
toISOStringWithTimezone
- title: toISOStringWithTimezone
- tags: date,intermediate
Converts a date to extended ISO format (ISO 8601), including timezone offset.
- Use
Date.prototype.getTimezoneOffset()
to get the timezone offset and reverse it, storing its sign indiff
. - Define a helper function,
pad
, that normalizes any passed number to an integer usingMath.floor()
andMath.abs()
and pads it to2
digits, usingString.prototype.padStart()
. - Use
pad()
and the built-in methods in theDate
prototype to build the ISO 8601 string with timezone offset.
const toISOStringWithTimezone = date => {
const tzOffset = -date.getTimezoneOffset();
const diff = tzOffset >= 0 ? '+' : '-';
const pad = n => `${Math.floor(Math.abs(n))}`.padStart(2, '0');
return date.getFullYear() +
'-' + pad(date.getMonth() + 1) +
'-' + pad(date.getDate()) +
'T' + pad(date.getHours()) +
':' + pad(date.getMinutes()) +
':' + pad(date.getSeconds()) +
diff + pad(tzOffset / 60) +
':' + pad(tzOffset % 60);
};
toISOStringWithTimezone(new Date()); // '2020-10-06T20:43:33-04:00'
toKebabCase
- title: toKebabCase
- tags: string,regexp,intermediate
Converts a string to kebab case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.join()
andString.prototype.toLowerCase()
to combine them, adding-
as a separator.
const toKebabCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('-');
toKebabCase('camelCase'); // 'camel-case'
toKebabCase('some text'); // 'some-text'
toKebabCase('some-mixed_string With spaces_underscores-and-hyphens');
// 'some-mixed-string-with-spaces-underscores-and-hyphens'
toKebabCase('AllThe-small Things'); // 'all-the-small-things'
toKebabCase('IAmEditingSomeXMLAndHTML');
// 'i-am-editing-some-xml-and-html'
toOrdinalSuffix
- title: toOrdinalSuffix
- tags: math,intermediate
Takes a number and returns it as a string with the correct ordinal indicator suffix.
- Use the modulo operator (
%
) to find values of single and tens digits. - Find which ordinal pattern digits match.
- If digit is found in teens pattern, use teens ordinal.
const toOrdinalSuffix = num => {
const int = parseInt(num),
digits = [int % 10, int % 100],
ordinals = ['st', 'nd', 'rd', 'th'],
oPattern = [1, 2, 3, 4],
tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
? int + ordinals[digits[0] - 1]
: int + ordinals[3];
};
toOrdinalSuffix('123'); // '123rd'
toPairs
- title: toPairs
- tags: object,array,intermediate
Creates an array of key-value pair arrays from an object or other iterable.
- Check if
Symbol.iterator
is defined and, if so, useArray.prototype.entries()
to get an iterator for the given iterable. - Use
Array.from()
to convert the result to an array of key-value pair arrays. - If
Symbol.iterator
is not defined forobj
, useObject.entries()
instead.
const toPairs = obj =>
obj[Symbol.iterator] instanceof Function && obj.entries instanceof Function
? Array.from(obj.entries())
: Object.entries(obj);
toPairs({ a: 1, b: 2 }); // [['a', 1], ['b', 2]]
toPairs([2, 4, 8]); // [[0, 2], [1, 4], [2, 8]]
toPairs('shy'); // [['0', 's'], ['1', 'h'], ['2', 'y']]
toPairs(new Set(['a', 'b', 'c', 'a'])); // [['a', 'a'], ['b', 'b'], ['c', 'c']]
toRGBArray
- title: toRGBArray
- tags: string,browser,regexp,beginner
Converts an rgb()
color string to an array of values.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values.
const toRGBArray = rgbStr => rgbStr.match(/\d+/g).map(Number);
toRGBArray('rgb(255, 12, 0)'); // [255, 12, 0]
toRGBObject
- title: toRGBObject
- tags: string,browser,regexp,intermediate
Converts an rgb()
color string to an object with the values of each color.
- Use
String.prototype.match()
to get an array of 3 string with the numeric values. - Use
Array.prototype.map()
in combination withNumber
to convert them into an array of numeric values. - Use array destructuring to store the values into named variables and create an appropriate object from them.
const toRGBObject = rgbStr => {
const [red, green, blue] = rgbStr.match(/\d+/g).map(Number);
return { red, green, blue };
};
toRGBObject('rgb(255, 12, 0)'); // {red: 255, green: 12, blue: 0}
toRomanNumeral
- title: toRomanNumeral
- tags: math,string,intermediate
Converts an integer to its roman numeral representation.
Accepts value between 1
and 3999
(both inclusive).
- Create a lookup table containing 2-value arrays in the form of (roman value, integer).
- Use
Array.prototype.reduce()
to loop over the values inlookup
and repeatedly dividenum
by the value. - Use
String.prototype.repeat()
to add the roman numeral representation to the accumulator.
const toRomanNumeral = num => {
const lookup = [
['M', 1000],
['CM', 900],
['D', 500],
['CD', 400],
['C', 100],
['XC', 90],
['L', 50],
['XL', 40],
['X', 10],
['IX', 9],
['V', 5],
['IV', 4],
['I', 1],
];
return lookup.reduce((acc, [k, v]) => {
acc += k.repeat(Math.floor(num / v));
num = num % v;
return acc;
}, '');
};
toRomanNumeral(3); // 'III'
toRomanNumeral(11); // 'XI'
toRomanNumeral(1998); // 'MCMXCVIII'
toSafeInteger
- title: toSafeInteger
- tags: math,beginner
Converts a value to a safe integer.
- Use
Math.max()
andMath.min()
to find the closest safe value. - Use
Math.round()
to convert to an integer.
const toSafeInteger = num =>
Math.round(
Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER)
);
toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991
toSnakeCase
- title: toSnakeCase
- tags: string,regexp,intermediate
Converts a string to snake case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
andString.prototype.toLowerCase()
to combine them, adding_
as a separator.
const toSnakeCase = str =>
str &&
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.toLowerCase())
.join('_');
toSnakeCase('camelCase'); // 'camel_case'
toSnakeCase('some text'); // 'some_text'
toSnakeCase('some-mixed_string With spaces_underscores-and-hyphens');
// 'some_mixed_string_with_spaces_underscores_and_hyphens'
toSnakeCase('AllThe-small Things'); // 'all_the_small_things'
toKebabCase('IAmEditingSomeXMLAndHTML');
// 'i_am_editing_some_xml_and_html'
toTitleCase
- title: toTitleCase
- tags: string,regexp,intermediate
Converts a string to title case.
- Use
String.prototype.match()
to break the string into words using an appropriate regexp. - Use
Array.prototype.map()
,Array.prototype.slice()
,Array.prototype.join()
andString.prototype.toUpperCase()
to combine them, capitalizing the first letter of each word and adding a whitespace between them.
const toTitleCase = str =>
str
.match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
.map(x => x.charAt(0).toUpperCase() + x.slice(1))
.join(' ');
toTitleCase('some_database_field_name'); // 'Some Database Field Name'
toTitleCase('Some label that needs to be title-cased');
// 'Some Label That Needs To Be Title Cased'
toTitleCase('some-package-name'); // 'Some Package Name'
toTitleCase('some-mixed_string with spaces_underscores-and-hyphens');
// 'Some Mixed String With Spaces Underscores And Hyphens'
toggleClass
- title: toggleClass
- tags: browser,beginner
Toggles a class for an HTML element.
- Use
Element.classList
andDOMTokenList.toggle()
to toggle the specified class for the element.
const toggleClass = (el, className) => el.classList.toggle(className);
toggleClass(document.querySelector('p.special'), 'special');
// The paragraph will not have the 'special' class anymore
tomorrow
- title: tomorrow
- tags: date,intermediate
Results in a string representation of tomorrow's date.
- Use
new Date()
to get the current date. - Increment it by one using
Date.prototype.getDate()
and set the value to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const tomorrow = () => {
let d = new Date();
d.setDate(d.getDate() + 1);
return d.toISOString().split('T')[0];
};
tomorrow(); // 2018-10-19 (if current date is 2018-10-18)
transform
- title: transform
- tags: object,intermediate
Applies a function against an accumulator and each key in the object (from left to right).
- Use
Object.keys()
to iterate over each key in the object. - Use
Array.prototype.reduce()
to apply the specified function against the given accumulator.
const transform = (obj, fn, acc) =>
Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);
transform(
{ a: 1, b: 2, c: 1 },
(r, v, k) => {
(r[v] || (r[v] = [])).push(k);
return r;
},
{}
); // { '1': ['a', 'c'], '2': ['b'] }
triggerEvent
- title: triggerEvent
- tags: browser,event,intermediate
Triggers a specific event on a given element, optionally passing custom data.
- Use
new CustomEvent()
to create an event from the specifiedeventType
and details. - Use
EventTarget.dispatchEvent()
to trigger the newly created event on the given element. - Omit the third argument,
detail
, if you do not want to pass custom data to the triggered event.
const triggerEvent = (el, eventType, detail) =>
el.dispatchEvent(new CustomEvent(eventType, { detail }));
triggerEvent(document.getElementById('myId'), 'click');
triggerEvent(document.getElementById('myId'), 'click', { username: 'bob' });
truncateString
- title: truncateString
- tags: string,beginner
Truncates a string up to a specified length.
- Determine if
String.prototype.length
is greater thannum
. - Return the string truncated to the desired length, with
'...'
appended to the end or the original string.
const truncateString = (str, num) =>
str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
truncateString('boomerang', 7); // 'boom...'
truncateStringAtWhitespace
- title: truncateStringAtWhitespace
- tags: string,intermediate
Truncates a string up to specified length, respecting whitespace when possible.
- Determine if
String.prototype.length
is greater or equal tolim
. If not, return it as-is. - Use
String.prototype.slice()
andString.prototype.lastIndexOf()
to find the index of the last space below the desiredlim
. - Use
String.prototype.slice()
to appropriately truncatestr
based onlastSpace
, respecting whitespace if possible and appendingending
at the end. - Omit the third argument,
ending
, to use the default ending of'...'
.
const truncateStringAtWhitespace = (str, lim, ending = '...') => {
if (str.length <= lim) return str;
const lastSpace = str.slice(0, lim - ending.length + 1).lastIndexOf(' ');
return str.slice(0, lastSpace > 0 ? lastSpace : lim - ending.length) + ending;
};
truncateStringAtWhitespace('short', 10); // 'short'
truncateStringAtWhitespace('not so short', 10); // 'not so...'
truncateStringAtWhitespace('trying a thing', 10); // 'trying...'
truncateStringAtWhitespace('javascripting', 10); // 'javascr...'
truthCheckCollection
- title: truthCheckCollection
- tags: object,logic,array,intermediate
Checks if the predicate function is truthy for all elements of a collection.
- Use
Array.prototype.every()
to check if each passed object has the specified property and if it returns a truthy value.
const truthCheckCollection = (collection, pre) =>
collection.every(obj => obj[pre]);
truthCheckCollection(
[
{ user: 'Tinky-Winky', sex: 'male' },
{ user: 'Dipsy', sex: 'male' },
],
'sex'
); // true
unary
- title: unary
- tags: function,beginner
Creates a function that accepts up to one argument, ignoring any additional arguments.
- Call the provided function,
fn
, with just the first argument supplied.
const unary = fn => val => fn(val);
['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]
uncurry
- title: uncurry
- tags: function,advanced
Uncurries a function up to depth n
.
- Return a variadic function.
- Use
Array.prototype.reduce()
on the provided arguments to call each subsequent curry level of the function. - If the
length
of the provided arguments is less thann
throw an error. - Otherwise, call
fn
with the proper amount of arguments, usingArray.prototype.slice(0, n)
. - Omit the second argument,
n
, to uncurry up to depth1
.
const uncurry = (fn, n = 1) => (...args) => {
const next = acc => args => args.reduce((x, y) => x(y), acc);
if (n > args.length) throw new RangeError('Arguments too few!');
return next(fn)(args.slice(0, n));
};
const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6
unescapeHTML
- title: unescapeHTML
- tags: string,browser,regexp,beginner
Unescapes escaped HTML characters.
- Use
String.prototype.replace()
with a regexp that matches the characters that need to be unescaped. - Use the function's callback to replace each escaped character instance with its associated unescaped character using a dictionary (object).
const unescapeHTML = str =>
str.replace(
/&|<|>|&##39;|"/g,
tag =>
({
'&': '&',
'<': '<',
'>': '>',
'&##39;': "'",
'"': '"'
}[tag] || tag)
);
unescapeHTML('<a href="##">Me & you</a>');
// '<a href="##">Me & you</a>'
unflattenObject
- title: unflattenObject
- tags: object,advanced
Unflatten an object with the paths for keys.
- Use nested
Array.prototype.reduce()
to convert the flat path to a leaf node. - Use
String.prototype.split('.')
to split each key with a dot delimiter andArray.prototype.reduce()
to add objects against the keys. - If the current accumulator already contains a value against a particular key, return its value as the next accumulator.
- Otherwise, add the appropriate key-value pair to the accumulator object and return the value as the accumulator.
const unflattenObject = obj =>
Object.keys(obj).reduce((res, k) => {
k.split('.').reduce(
(acc, e, i, keys) =>
acc[e] ||
(acc[e] = isNaN(Number(keys[i + 1]))
? keys.length - 1 === i
? obj[k]
: {}
: []),
res
);
return res;
}, {});
unflattenObject({ 'a.b.c': 1, d: 1 }); // { a: { b: { c: 1 } }, d: 1 }
unflattenObject({ 'a.b': 1, 'a.c': 2, d: 3 }); // { a: { b: 1, c: 2 }, d: 3 }
unflattenObject({ 'a.b.0': 8, d: 3 }); // { a: { b: [ 8 ] }, d: 3 }
unfold
- title: unfold
- tags: function,array,intermediate
Builds an array, using an iterator function and an initial seed value.
- Use a
while
loop andArray.prototype.push()
to call the function repeatedly until it returnsfalse
. - The iterator function accepts one argument (
seed
) and must always return an array with two elements ([value
,nextSeed
]) orfalse
to terminate.
const unfold = (fn, seed) => {
let result = [],
val = [null, seed];
while ((val = fn(val[1]))) result.push(val[0]);
return result;
};
var f = n => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]
union
- title: union
- tags: array,beginner
Returns every element that exists in any of the two arrays at least once.
- Create a
new Set()
with all values ofa
andb
and convert it to an array.
const union = (a, b) => Array.from(new Set([...a, ...b]));
union([1, 2, 3], [4, 3, 2]); // [1, 2, 3, 4]
unionBy
- title: unionBy
- tags: array,intermediate
Returns every element that exists in any of the two arrays at least once, after applying the provided function to each array element of both.
- Create a
new Set()
by applying allfn
to all values ofa
. - Create a
new Set()
froma
and all elements inb
whose value, after applyingfn
does not match a value in the previously created set. - Return the last set converted to an array.
const unionBy = (a, b, fn) => {
const s = new Set(a.map(fn));
return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
};
unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]
unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], x => x.id)
// [{ id: 1 }, { id: 2 }, { id: 3 }]
unionWith
- title: unionWith
- tags: array,intermediate
Returns every element that exists in any of the two arrays at least once, using a provided comparator function.
- Create a
new Set()
with all values ofa
and values inb
for which the comparator finds no matches ina
, usingArray.prototype.findIndex()
.
const unionWith = (a, b, comp) =>
Array.from(
new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])
);
unionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
);
// [1, 1.2, 1.5, 3, 0, 3.9]
uniqueElements
- title: uniqueElements
- tags: array,beginner
Finds all unique values in an array.
- Create a
new Set()
from the given array to discard duplicated values. - Use the spread operator (
...
) to convert it back to an array.
const uniqueElements = arr => [...new Set(arr)];
uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]
uniqueElementsBy
- title: uniqueElementsBy
- tags: array,intermediate
Finds all unique values of an array, based on a provided comparator function.
- Use
Array.prototype.reduce()
andArray.prototype.some()
for an array containing only the first unique occurrence of each value, based on the comparator function,fn
. - The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsBy = (arr, fn) =>
arr.reduce((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);
uniqueElementsBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]
uniqueElementsByRight
- title: uniqueElementsByRight
- tags: array,intermediate
Finds all unique values of an array, based on a provided comparator function, starting from the right.
- Use
Array.prototype.reduceRight()
andArray.prototype.some()
for an array containing only the last unique occurrence of each value, based on the comparator function,fn
. - The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsByRight = (arr, fn) =>
arr.reduceRight((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);
uniqueElementsByRight(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]
uniqueSymmetricDifference
- title: uniqueSymmetricDifference
- tags: array,math,intermediate
Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.
- Use
Array.prototype.filter()
andArray.prototype.includes()
on each array to remove values contained in the other. - Create a
new Set()
from the results, removing duplicate values.
const uniqueSymmetricDifference = (a, b) => [
...new Set([
...a.filter(v => !b.includes(v)),
...b.filter(v => !a.includes(v)),
]),
];
uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]
untildify
- title: untildify
- tags: node,string,beginner
Converts a tilde path to an absolute path.
- Use
String.prototype.replace()
with a regular expression andos.homedir()
to replace the~
in the start of the path with the home directory.
const untildify = str =>
str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);
untildify('~/node'); // '/Users/aUser/node'
unzip
- title: unzip
- tags: array,intermediate
Creates an array of arrays, ungrouping the elements in an array produced by zip.
- Use
Math.max()
,Function.prototype.apply()
to get the longest subarray in the array,Array.prototype.map()
to make each element an array. - Use
Array.prototype.reduce()
andArray.prototype.forEach()
to map grouped values to individual arrays.
const unzip = arr =>
arr.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
);
unzip([['a', 1, true], ['b', 2, false]]); // [['a', 'b'], [1, 2], [true, false]]
unzip([['a', 1, true], ['b', 2]]); // [['a', 'b'], [1, 2], [true]]
unzipWith
- title: unzipWith
- tags: array,advanced
Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.
- Use
Math.max()
,Function.prototype.apply()
to get the longest subarray in the array,Array.prototype.map()
to make each element an array. - Use
Array.prototype.reduce()
andArray.prototype.forEach()
to map grouped values to individual arrays. - Use
Array.prototype.map()
and the spread operator (...
) to applyfn
to each individual group of elements.
const unzipWith = (arr, fn) =>
arr
.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
)
.map(val => fn(...val));
unzipWith(
[
[1, 10, 100],
[2, 20, 200],
],
(...args) => args.reduce((acc, v) => acc + v, 0)
);
// [3, 30, 300]
validateNumber
- title: validateNumber
- tags: math,intermediate
Checks if the given value is a number.
- Use
parseFloat()
to try to convertn
to a number. - Use
!Number.isNaN()
to check ifnum
is a number. - Use
Number.isFinite()
to check ifnum
is finite. - Use
Number()
and the loose equality operator (==
) to check if the coercion holds.
const validateNumber = n => {
const num = parseFloat(n);
return !Number.isNaN(num) && Number.isFinite(num) && Number(n) == n;
}
validateNumber('10'); // true
validateNumber('a'); // false
vectorAngle
- title: vectorAngle
- tags: math,beginner
Calculates the angle (theta) between two vectors.
- Use
Array.prototype.reduce()
,Math.pow()
andMath.sqrt()
to calculate the magnitude of each vector and the scalar product of the two vectors. - Use
Math.acos()
to calculate the arccosine and get the theta value.
const vectorAngle = (x, y) => {
let mX = Math.sqrt(x.reduce((acc, n) => acc + Math.pow(n, 2), 0));
let mY = Math.sqrt(y.reduce((acc, n) => acc + Math.pow(n, 2), 0));
return Math.acos(x.reduce((acc, n, i) => acc + n * y[i], 0) / (mX * mY));
};
vectorAngle([3, 4], [4, 3]); // 0.283794109208328
vectorDistance
- title: vectorDistance
- tags: math,algorithm,beginner
Calculates the distance between two vectors.
- Use
Array.prototype.reduce()
,Math.pow()
andMath.sqrt()
to calculate the Euclidean distance between two vectors.
const vectorDistance = (x, y) =>
Math.sqrt(x.reduce((acc, val, i) => acc + Math.pow(val - y[i], 2), 0));
vectorDistance([10, 0, 5], [20, 0, 10]); // 11.180339887498949
walkThrough
- title: walkThrough
- tags: object,recursion,generator,advanced
Creates a generator, that walks through all the keys of a given object.
- Use recursion.
- Define a generator function,
walk
, that takes an object and an array of keys. - Use a
for...of
loop andObject.keys()
to iterate over the keys of the object. - Use
typeof
to check if each value in the given object is itself an object. - If so, use the
yield*
expression to recursively delegate to the same generator function,walk
, appending the currentkey
to the array of keys. Otherwise,yield
the an array of keys representing the current path and the value of the givenkey
. - Use the
yield*
expression to delegate to thewalk
generator function.
const walkThrough = function* (obj) {
const walk = function* (x, previous = []) {
for (let key of Object.keys(x)) {
if (typeof x[key] === 'object') yield* walk(x[key], [...previous, key]);
else yield [[...previous, key], x[key]];
}
};
yield* walk(obj);
};
const obj = {
a: 10,
b: 20,
c: {
d: 10,
e: 20,
f: [30, 40]
},
g: [
{
h: 10,
i: 20
},
{
j: 30
},
40
]
};
[...walkThrough(obj)];
/*
[
[['a'], 10],
[['b'], 20],
[['c', 'd'], 10],
[['c', 'e'], 20],
[['c', 'f', '0'], 30],
[['c', 'f', '1'], 40],
[['g', '0', 'h'], 10],
[['g', '0', 'i'], 20],
[['g', '1', 'j'], 30],
[['g', '2'], 40]
]
*/
weightedAverage
- title: weightedAverage
- tags: math,intermediate
Calculates the weighted average of two or more numbers.
- Use
Array.prototype.reduce()
to create the weighted sum of the values and the sum of the weights. - Divide them with each other to get the weighted average.
const weightedAverage = (nums, weights) => {
const [sum, weightSum] = weights.reduce(
(acc, w, i) => {
acc[0] = acc[0] + nums[i] * w;
acc[1] = acc[1] + w;
return acc;
},
[0, 0]
);
return sum / weightSum;
};
weightedAverage([1, 2, 3], [0.6, 0.2, 0.3]); // 1.72727
weightedSample
- title: weightedSample
- tags: array,random,advanced
Gets a random element from an array, using the provided weights
as the probabilities for each element.
- Use
Array.prototype.reduce()
to create an array of partial sums for each value inweights
. - Use
Math.random()
to generate a random number andArray.prototype.findIndex()
to find the correct index based on the array previously produced. - Finally, return the element of
arr
with the produced index.
const weightedSample = (arr, weights) => {
let roll = Math.random();
return arr[
weights
.reduce(
(acc, w, i) => (i === 0 ? [w] : [...acc, acc[acc.length - 1] + w]),
[]
)
.findIndex((v, i, s) => roll >= (i === 0 ? 0 : s[i - 1]) && roll < v)
];
};
weightedSample([3, 7, 9, 11], [0.1, 0.2, 0.6, 0.1]); // 9
when
- title: when
- tags: function,logic,beginner
Returns a function that takes one argument and runs a callback if it's truthy or returns it if falsy.
- Return a function expecting a single value,
x
, that returns the appropriate value based onpred
.
const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);
const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1
without
- title: without
- tags: array,beginner
Filters out the elements of an array that have one of the specified values.
- Use
Array.prototype.includes()
to find values to exclude. - Use
Array.prototype.filter()
to create an array excluding them.
const without = (arr, ...args) => arr.filter(v => !args.includes(v));
without([2, 1, 2, 3], 1, 2); // [3]
wordWrap
- title: wordWrap
- tags: string,regexp,intermediate
Wraps a string to a given number of characters using a string break character.
- Use
String.prototype.replace()
and a regular expression to insert a given break character at the nearest whitespace ofmax
characters. - Omit the third argument,
br
, to use the default value of'\n'
.
const wordWrap = (str, max, br = '\n') => str.replace(
new RegExp(`(?![^\\n]{1,${max}}$)([^\\n]{1,${max}})\\s`, 'g'), '$1' + br
);
wordWrap(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce tempus.',
32
);
// 'Lorem ipsum dolor sit amet,\nconsectetur adipiscing elit.\nFusce tempus.'
wordWrap(
'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce tempus.',
32,
'\r\n'
);
// 'Lorem ipsum dolor sit amet,\r\nconsectetur adipiscing elit.\r\nFusce tempus.'
words
- title: words
- tags: string,regexp,intermediate
Converts a given string into an array of words.
- Use
String.prototype.split()
with a suppliedpattern
(defaults to non-alpha as a regexp) to convert to an array of strings. - Use
Array.prototype.filter()
to remove any empty strings. - Omit the second argument,
pattern
, to use the default regexp.
const words = (str, pattern = /[^a-zA-Z-]+/) =>
str.split(pattern).filter(Boolean);
words('I love javaScript!!'); // ['I', 'love', 'javaScript']
words('python, javaScript & coffee'); // ['python', 'javaScript', 'coffee']
xProd
- title: xProd
- tags: array,math,intermediate
Creates a new array out of the two supplied by creating each possible pair from the arrays.
- Use
Array.prototype.reduce()
,Array.prototype.map()
andArray.prototype.concat()
to produce every possible pair from the elements of the two arrays.
const xProd = (a, b) =>
a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);
xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]
xor
- title: xor
- tags: math,logic,beginner unlisted: true
Checks if only one of the arguments is true
.
- Use the logical or (
||
), and (&&
) and not (!
) operators on the two given values to create the logical xor.
const xor = (a, b) => (( a || b ) && !( a && b ));
xor(true, true); // false
xor(true, false); // true
xor(false, true); // true
xor(false, false); // false
yesNo
- title: yesNo
- tags: string,regexp,intermediate unlisted: true
Returns true
if the string is y
/yes
or false
if the string is n
/no
.
- Use
RegExp.prototype.test()
to check if the string evaluates toy/yes
orn/no
. - Omit the second argument,
def
to set the default answer asno
.
const yesNo = (val, def = false) =>
/^(y|yes)$/i.test(val) ? true : /^(n|no)$/i.test(val) ? false : def;
yesNo('Y'); // true
yesNo('yes'); // true
yesNo('No'); // false
yesNo('Foo', true); // true
yesterday
- title: yesterday
- tags: date,intermediate
Results in a string representation of yesterday's date.
- Use
new Date()
to get the current date. - Decrement it by one using
Date.prototype.getDate()
and set the value to the result usingDate.prototype.setDate()
. - Use
Date.prototype.toISOString()
to return a string inyyyy-mm-dd
format.
const yesterday = () => {
let d = new Date();
d.setDate(d.getDate() - 1);
return d.toISOString().split('T')[0];
};
yesterday(); // 2018-10-17 (if current date is 2018-10-18)
zip
- title: zip
- tags: array,intermediate
Creates an array of elements, grouped based on their position in the original arrays.
- Use
Math.max()
,Function.prototype.apply()
to get the longest array in the arguments. - Create an array with that length as return value and use
Array.from()
with a mapping function to create an array of grouped elements. - If lengths of the argument arrays vary,
undefined
is used where no value could be found.
const zip = (...arrays) => {
const maxLength = Math.max(...arrays.map(x => x.length));
return Array.from({ length: maxLength }).map((_, i) => {
return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
});
};
zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]
zipObject
- title: zipObject
- tags: array,object,intermediate
Associates properties to values, given array of valid property identifiers and an array of values.
- Use
Array.prototype.reduce()
to build an object from the two arrays. - If the length of
props
is longer thanvalues
, remaining keys will beundefined
. - If the length of
values
is longer thanprops
, remaining values will be ignored.
const zipObject = (props, values) =>
props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});
zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}
zipWith
- title: zipWith
- tags: array,advanced
Creates an array of elements, grouped based on the position in the original arrays and using a function to specify how grouped values should be combined.
- Check if the last argument provided is a function.
- Use
Math.max()
to get the longest array in the arguments. - Use
Array.from()
to create an array with appropriate length and a mapping function to create array of grouped elements. - If lengths of the argument arrays vary,
undefined
is used where no value could be found. - The function is invoked with the elements of each group.
const zipWith = (...array) => {
const fn =
typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
return Array.from({ length: Math.max(...array.map(a => a.length)) }, (_, i) =>
fn ? fn(...array.map(a => a[i])) : array.map(a => a[i])
);
};
zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111, 222]
zipWith(
[1, 2, 3],
[10, 20],
[100, 200],
(a, b, c) =>
(a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']
Accordion
- title: Accordion
- tags: components,children,state,advanced
Renders an accordion menu with multiple collapsible content elements.
- Define an
AccordionItem
component, that renders a<button>
which is used to update the component and notify its parent via thehandleClick
callback. - Use the
isCollapsed
prop inAccordionItem
to determine its appearance and set an appropriateclassName
. - Define an
Accordion
component that uses theuseState()
hook to initialize the value of thebindIndex
state variable todefaultIndex
. - Filter
children
to remove unnecessary nodes except forAccordionItem
by identifying the function's name. - Use
Array.prototype.map()
on the collected nodes to render the individual collapsible elements. - Define
changeItem
, which will be executed when clicking anAccordionItem
's<button>
. changeItem
executes the passed callback,onItemClick
, and updatesbindIndex
based on the clicked element.
.accordion-item.collapsed {
display: none;
}
.accordion-item.expanded {
display: block;
}
.accordion-button {
display: block;
width: 100%;
}
const AccordionItem = ({ label, isCollapsed, handleClick, children }) => {
return (
<>
<button className="accordion-button" onClick={handleClick}>
{label}
</button>
<div
className={`accordion-item ${isCollapsed ? 'collapsed' : 'expanded'}`}
aria-expanded={isCollapsed}
>
{children}
</div>
</>
);
};
const Accordion = ({ defaultIndex, onItemClick, children }) => {
const [bindIndex, setBindIndex] = React.useState(defaultIndex);
const changeItem = itemIndex => {
if (typeof onItemClick === 'function') onItemClick(itemIndex);
if (itemIndex !== bindIndex) setBindIndex(itemIndex);
};
const items = children.filter(item => item.type.name === 'AccordionItem');
return (
<>
{items.map(({ props }) => (
<AccordionItem
isCollapsed={bindIndex !== props.index}
label={props.label}
handleClick={() => changeItem(props.index)}
children={props.children}
/>
))}
</>
);
};
ReactDOM.render(
<Accordion defaultIndex="1" onItemClick={console.log}>
<AccordionItem label="A" index="1">
Lorem ipsum
</AccordionItem>
<AccordionItem label="B" index="2">
Dolor sit amet
</AccordionItem>
</Accordion>,
document.getElementById('root')
);
Alert
- title: Alert
- tags: components,state,effect,beginner
Renders an alert component with type
prop.
- Use the
useState()
hook to create theisShown
andisLeaving
state variables and set both tofalse
initially. - Define
timeoutId
to keep the timer instance for clearing on component unmount. - Use the
useEffect()
hook to update the value ofisShown
totrue
and clear the interval by usingtimeoutId
when the component is unmounted. - Define a
closeAlert
function to set the component as removed from the DOM by displaying a fading out animation and setisShown
tofalse
viasetTimeout()
.
@keyframes leave {
0% { opacity: 1 }
100% { opacity: 0 }
}
.alert {
padding: 0.75rem 0.5rem;
margin-bottom: 0.5rem;
text-align: left;
padding-right: 40px;
border-radius: 4px;
font-size: 16px;
position: relative;
}
.alert.warning {
color: ##856404;
background-color: ##fff3cd;
border-color: ##ffeeba;
}
.alert.error {
color: ##721c24;
background-color: ##f8d7da;
border-color: ##f5c6cb;
}
.alert.leaving {
animation: leave 0.5s forwards;
}
.alert .close {
position: absolute;
top: 0;
right: 0;
padding: 0 0.75rem;
color: ##333;
border: 0;
height: 100%;
cursor: pointer;
background: none;
font-weight: 600;
font-size: 16px;
}
.alert .close:after {
content: 'x';
}
const Alert = ({ isDefaultShown = false, timeout = 250, type, message }) => {
const [isShown, setIsShown] = React.useState(isDefaultShown);
const [isLeaving, setIsLeaving] = React.useState(false);
let timeoutId = null;
React.useEffect(() => {
setIsShown(true);
return () => {
clearTimeout(timeoutId);
};
}, [isDefaultShown, timeout, timeoutId]);
const closeAlert = () => {
setIsLeaving(true);
timeoutId = setTimeout(() => {
setIsLeaving(false);
setIsShown(false);
}, timeout);
};
return (
isShown && (
<div
className={`alert ${type} ${isLeaving ? 'leaving' : ''}`}
role="alert"
>
<button className="close" onClick={closeAlert} />
{message}
</div>
)
);
};
ReactDOM.render(
<Alert type="info" message="This is info" />,
document.getElementById('root')
);
AutoLink
- title: AutoLink
- tags: components,fragment,regexp,intermediate
Renders a string as plaintext, with URLs converted to appropriate link elements.
- Use
String.prototype.split()
andString.prototype.match()
with a regular expression to find URLs in a string. - Return matched URLs rendered as
<a>
elements, dealing with missing protocol prefixes if necessary. - Render the rest of the string as plaintext.
const AutoLink = ({ text }) => {
const delimiter = /((?:https?:\/\/)?(?:(?:[a-z0-9]?(?:[a-z0-9\-]{1,61}[a-z0-9])?\.[^\.|\s])+[a-z\.]*[a-z]+|(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3})(?::\d{1,5})*[a-z0-9.,_\/~##&=;%+?\-\\(\\)]*)/gi;
return (
<>
{text.split(delimiter).map(word => {
const match = word.match(delimiter);
if (match) {
const url = match[0];
return (
<a href={url.startsWith('http') ? url : `http://${url}`}>{url}</a>
);
}
return word;
})}
</>
);
};
ReactDOM.render(
<AutoLink text="foo bar baz http://example.org bar" />,
document.getElementById('root')
);
Callto
- title: Callto
- tags: components,beginner unlisted: true
Renders a link formatted to call a phone number (tel:
link).
- Use
phone
to create a<a>
element with an appropriatehref
attribute. - Render the link with
children
as its content.
const Callto = ({ phone, children }) => {
return <a href={`tel:${phone}`}>{children}</a>;
};
ReactDOM.render(
<Callto phone="+302101234567">Call me!</Callto>,
document.getElementById('root')
);
Carousel
- title: Carousel
- tags: components,children,state,effect,advanced
Renders a carousel component.
- Use the
useState()
hook to create theactive
state variable and give it a value of0
(index of the first item). - Use the
useEffect()
hook to update the value ofactive
to the index of the next item, usingsetTimeout
. - Compute the
className
for each carousel item while mapping over them and applying it accordingly. - Render the carousel items using
React.cloneElement()
and pass down...rest
along with the computedclassName
.
.carousel {
position: relative;
}
.carousel-item {
position: absolute;
visibility: hidden;
}
.carousel-item.visible {
visibility: visible;
}
const Carousel = ({ carouselItems, ...rest }) => {
const [active, setActive] = React.useState(0);
let scrollInterval = null;
React.useEffect(() => {
scrollInterval = setTimeout(() => {
setActive((active + 1) % carouselItems.length);
}, 2000);
return () => clearTimeout(scrollInterval);
});
return (
<div className="carousel">
{carouselItems.map((item, index) => {
const activeClass = active === index ? ' visible' : '';
return React.cloneElement(item, {
...rest,
className: `carousel-item${activeClass}`
});
})}
</div>
);
};
ReactDOM.render(
<Carousel
carouselItems={[
<div>carousel item 1</div>,
<div>carousel item 2</div>,
<div>carousel item 3</div>
]}
/>,
document.getElementById('root')
);
Collapse
- title: Collapse
- tags: components,children,state,beginner
Renders a component with collapsible content.
- Use the
useState()
hook to create theisCollapsed
state variable with an initial value ofcollapsed
. - Use the
<button>
to change the component'sisCollapsed
state and the content of the component, passed down viachildren
. - Determine the appearance of the content, based on
isCollapsed
and apply the appropriateclassName
. - Update the value of the
aria-expanded
attribute based onisCollapsed
to make the component accessible.
.collapse-button {
display: block;
width: 100%;
}
.collapse-content.collapsed {
display: none;
}
.collapsed-content.expanded {
display: block;
}
const Collapse = ({ collapsed, children }) => {
const [isCollapsed, setIsCollapsed] = React.useState(collapsed);
return (
<>
<button
className="collapse-button"
onClick={() => setIsCollapsed(!isCollapsed)}
>
{isCollapsed ? 'Show' : 'Hide'} content
</button>
<div
className={`collapse-content ${isCollapsed ? 'collapsed' : 'expanded'}`}
aria-expanded={isCollapsed}
>
{children}
</div>
</>
);
};
ReactDOM.render(
<Collapse>
<h1>This is a collapse</h1>
<p>Hello world!</p>
</Collapse>,
document.getElementById('root')
);
ControlledInput
- title: ControlledInput
- tags: components,input,intermediate
Renders a controlled <input>
element that uses a callback function to inform its parent about value updates.
- Use the
value
passed down from the parent as the controlled input field's value. - Use the
onChange
event to fire theonValueChange
callback and send the new value to the parent. - The parent must update the input field's
value
prop in order for its value to change on user input.
const ControlledInput = ({ value, onValueChange, ...rest }) => {
return (
<input
value={value}
onChange={({ target: { value } }) => onValueChange(value)}
{...rest}
/>
);
};
const Form = () => {
const [value, setValue] = React.useState('');
return (
<ControlledInput
type="text"
placeholder="Insert some text here..."
value={value}
onValueChange={setValue}
/>
);
};
ReactDOM.render(<Form />, document.getElementById('root'));
CountDown
- title: CountDown
- tags: components,state,advanced
Renders a countdown timer that prints a message when it reaches zero.
- Use the
useState()
hook to create a state variable to hold the time value, initialize it from the props and destructure it into its components. - Use the
useState()
hook to create thepaused
andover
state variables, used to prevent the timer from ticking if it's paused or the time has run out. - Create a method
tick
, that updates the time values based on the current value (i.e. decreasing the time by one second). - Create a method
reset
, that resets all state variables to their initial states. - Use the the
useEffect()
hook to call thetick
method every second via the use ofsetInterval()
and useclearInterval()
to clean up when the component is unmounted. - Use
String.prototype.padStart()
to pad each part of the time array to two characters to create the visual representation of the timer.
const CountDown = ({ hours = 0, minutes = 0, seconds = 0 }) => {
const [paused, setPaused] = React.useState(false);
const [over, setOver] = React.useState(false);
const [[h, m, s], setTime] = React.useState([hours, minutes, seconds]);
const tick = () => {
if (paused || over) return;
if (h === 0 && m === 0 && s === 0) setOver(true);
else if (m === 0 && s === 0) {
setTime([h - 1, 59, 59]);
} else if (s == 0) {
setTime([h, m - 1, 59]);
} else {
setTime([h, m, s - 1]);
}
};
const reset = () => {
setTime([parseInt(hours), parseInt(minutes), parseInt(seconds)]);
setPaused(false);
setOver(false);
};
React.useEffect(() => {
const timerID = setInterval(() => tick(), 1000);
return () => clearInterval(timerID);
});
return (
<div>
<p>{`${h.toString().padStart(2, '0')}:${m
.toString()
.padStart(2, '0')}:${s.toString().padStart(2, '0')}`}</p>
<div>{over ? "Time's up!" : ''}</div>
<button onClick={() => setPaused(!paused)}>
{paused ? 'Resume' : 'Pause'}
</button>
<button onClick={() => reset()}>Restart</button>
</div>
);
};
ReactDOM.render(
<CountDown hours={1} minutes={45} />,
document.getElementById('root')
);
DataList
- title: DataList
- tags: components,beginner
Renders a list of elements from an array of primitives.
- Use the value of the
isOrdered
prop to conditionally render an<ol>
or a<ul>
list. - Use
Array.prototype.map()
to render every item indata
as a<li>
element with an appropriatekey
.
const DataList = ({ isOrdered = false, data }) => {
const list = data.map((val, i) => <li key={`${i}_${val}`}>{val}</li>);
return isOrdered ? <ol>{list}</ol> : <ul>{list}</ul>;
};
const names = ['John', 'Paul', 'Mary'];
ReactDOM.render(<DataList data={names} />, document.getElementById('root'));
ReactDOM.render(
<DataList data={names} isOrdered />,
document.getElementById('root')
);
DataTable
- title: DataTable
- tags: components,beginner
Renders a table with rows dynamically created from an array of primitives.
- Render a
<table>
element with two columns (ID
andValue
). - Use
Array.prototype.map()
to render every item indata
as a<tr>
element with an appropriatekey
.
const DataTable = ({ data }) => {
return (
<table>
<thead>
<tr>
<th>ID</th>
<th>Value</th>
</tr>
</thead>
<tbody>
{data.map((val, i) => (
<tr key={`${i}_${val}`}>
<td>{i}</td>
<td>{val}</td>
</tr>
))}
</tbody>
</table>
);
};
const people = ['John', 'Jesse'];
ReactDOM.render(<DataTable data={people} />, document.getElementById('root'));
FileDrop
- title: FileDrop
- tags: components,input,state,effect,event,advanced
Renders a file drag and drop component for a single file.
- Create a ref, called
dropRef
and bind it to the component's wrapper. - Use the
useState()
hook to create thedrag
andfilename
variables, initialized tofalse
and''
respectively. - The variables
dragCounter
anddrag
are used to determine if a file is being dragged, whilefilename
is used to store the dropped file's name. - Create the
handleDrag
,handleDragIn
,handleDragOut
andhandleDrop
methods to handle drag and drop functionality. handleDrag
prevents the browser from opening the dragged file,handleDragIn
andhandleDragOut
handle the dragged file entering and exiting the component, whilehandleDrop
handles the file being dropped and passes it toonDrop
.- Use the
useEffect()
hook to handle each of the drag and drop events using the previously created methods.
.filedrop {
min-height: 120px;
border: 3px solid ##d3d3d3;
text-align: center;
font-size: 24px;
padding: 32px;
border-radius: 4px;
}
.filedrop.drag {
border: 3px dashed ##1e90ff;
}
.filedrop.ready {
border: 3px solid ##32cd32;
}
const FileDrop = ({ onDrop }) => {
const [drag, setDrag] = React.useState(false);
const [filename, setFilename] = React.useState('');
let dropRef = React.createRef();
let dragCounter = 0;
const handleDrag = e => {
e.preventDefault();
e.stopPropagation();
};
const handleDragIn = e => {
e.preventDefault();
e.stopPropagation();
dragCounter++;
if (e.dataTransfer.items && e.dataTransfer.items.length > 0) setDrag(true);
};
const handleDragOut = e => {
e.preventDefault();
e.stopPropagation();
dragCounter--;
if (dragCounter === 0) setDrag(false);
};
const handleDrop = e => {
e.preventDefault();
e.stopPropagation();
setDrag(false);
if (e.dataTransfer.files && e.dataTransfer.files.length > 0) {
onDrop(e.dataTransfer.files[0]);
setFilename(e.dataTransfer.files[0].name);
e.dataTransfer.clearData();
dragCounter = 0;
}
};
React.useEffect(() => {
let div = dropRef.current;
div.addEventListener('dragenter', handleDragIn);
div.addEventListener('dragleave', handleDragOut);
div.addEventListener('dragover', handleDrag);
div.addEventListener('drop', handleDrop);
return () => {
div.removeEventListener('dragenter', handleDragIn);
div.removeEventListener('dragleave', handleDragOut);
div.removeEventListener('dragover', handleDrag);
div.removeEventListener('drop', handleDrop);
};
});
return (
<div
ref={dropRef}
className={
drag ? 'filedrop drag' : filename ? 'filedrop ready' : 'filedrop'
}
>
{filename && !drag ? <div>{filename}</div> : <div>Drop a file here!</div>}
</div>
);
};
ReactDOM.render(
<FileDrop onDrop={console.log} />,
document.getElementById('root')
);
LimitedTextarea
- title: LimitedTextarea
- tags: components,state,callback,event,beginner
Renders a textarea component with a character limit.
- Use the
useState()
hook to create thecontent
state variable and set its value to that ofvalue
prop, trimmed down tolimit
characters. - Create a method
setFormattedContent
, which trims the content down tolimit
characters and memoize it, using theuseCallback()
hook. - Bind the
onChange
event of the<textarea>
to callsetFormattedContent
with the value of the fired event.
const LimitedTextarea = ({ rows, cols, value, limit }) => {
const [content, setContent] = React.useState(value.slice(0, limit));
const setFormattedContent = React.useCallback(
text => {
setContent(text.slice(0, limit));
},
[limit, setContent]
);
return (
<>
<textarea
rows={rows}
cols={cols}
onChange={event => setFormattedContent(event.target.value)}
value={content}
/>
<p>
{content.length}/{limit}
</p>
</>
);
};
ReactDOM.render(
<LimitedTextarea limit={32} value="Hello!" />,
document.getElementById('root')
);
LimitedWordTextarea
- title: LimitedWordTextarea
- tags: components,input,state,callback,effect,event,intermediate
Renders a textarea component with a word limit.
- Use the
useState()
hook to create a state variable, containingcontent
andwordCount
, using thevalue
prop and0
as the initial values respectively. - Use the
useCallback()
hooks to create a memoized function,setFormattedContent
, that usesString.prototype.split()
to turn the input into an array of words. - Check if the result of applying
Array.prototype.filter()
combined withBoolean
has alength
longer thanlimit
and, if so, trim the input, otherwise return the raw input, updating state accordingly in both cases. - Use the
useEffect()
hook to call thesetFormattedContent
method on the value of thecontent
state variable during the initial render. - Bind the
onChange
event of the<textarea>
to callsetFormattedContent
with the value ofevent.target.value
.
const LimitedWordTextarea = ({ rows, cols, value, limit }) => {
const [{ content, wordCount }, setContent] = React.useState({
content: value,
wordCount: 0
});
const setFormattedContent = React.useCallback(
text => {
let words = text.split(' ').filter(Boolean);
if (words.length > limit) {
setContent({
content: words.slice(0, limit).join(' '),
wordCount: limit
});
} else {
setContent({ content: text, wordCount: words.length });
}
},
[limit, setContent]
);
React.useEffect(() => {
setFormattedContent(content);
}, []);
return (
<>
<textarea
rows={rows}
cols={cols}
onChange={event => setFormattedContent(event.target.value)}
value={content}
/>
<p>
{wordCount}/{limit}
</p>
</>
);
};
ReactDOM.render(
<LimitedWordTextarea limit={5} value="Hello there!" />,
document.getElementById('root')
);
Loader
- title: Loader
- tags: components,beginner
Renders a spinning loader component.
- Render an SVG, whose
height
andwidth
are determined by thesize
prop. - Use CSS to animate the SVG, creating a spinning animation.
.loader {
animation: rotate 2s linear infinite;
}
@keyframes rotate {
100% {
transform: rotate(360deg);
}
}
.loader circle {
animation: dash 1.5s ease-in-out infinite;
}
@keyframes dash {
0% {
stroke-dasharray: 1, 150;
stroke-dashoffset: 0;
}
50% {
stroke-dasharray: 90, 150;
stroke-dashoffset: -35;
}
100% {
stroke-dasharray: 90, 150;
stroke-dashoffset: -124;
}
}
const Loader = ({ size }) => {
return (
<svg
className="loader"
xmlns="http://www.w3.org/2000/svg"
width={size}
height={size}
viewBox="0 0 24 24"
fill="none"
stroke="currentColor"
strokeWidth="2"
strokeLinecap="round"
strokeLinejoin="round"
>
<circle cx="12" cy="12" r="10" />
</svg>
);
};
ReactDOM.render(<Loader size={24} />, document.getElementById('root'));
Mailto
- title: Mailto
- tags: components,beginner
Renders a link formatted to send an email (mailto:
link).
- Use the
email
,subject
andbody
props to create a<a>
element with an appropriatehref
attribute. - Use
encodeURIcomponent
to safely encode thesubject
andbody
into the link URL. - Render the link with
children
as its content.
const Mailto = ({ email, subject = '', body = '', children }) => {
let params = subject || body ? '?' : '';
if (subject) params += `subject=${encodeURIComponent(subject)}`;
if (body) params += `${subject ? '&' : ''}body=${encodeURIComponent(body)}`;
return <a href={`mailto:${email}${params}`}>{children}</a>;
};
ReactDOM.render(
<Mailto email="foo@bar.baz" subject="Hello & Welcome" body="Hello world!">
Mail me!
</Mailto>,
document.getElementById('root')
);
MappedTable
- title: MappedTable
- tags: components,array,object,intermediate
Renders a table with rows dynamically created from an array of objects and a list of property names.
- Use
Object.keys()
,Array.prototype.filter()
,Array.prototype.includes()
andArray.prototype.reduce()
to produce afilteredData
array, containing all objects with the keys specified inpropertyNames
. - Render a
<table>
element with a set of columns equal to the amount of values inpropertyNames
. - Use
Array.prototype.map()
to render each value in thepropertyNames
array as a<th>
element. - Use
Array.prototype.map()
to render each object in thefilteredData
array as a<tr>
element, containing a<td>
for each key in the object.
This component does not work with nested objects and will break if there are nested objects inside any of the properties specified in propertyNames
const MappedTable = ({ data, propertyNames }) => {
let filteredData = data.map(v =>
Object.keys(v)
.filter(k => propertyNames.includes(k))
.reduce((acc, key) => ((acc[key] = v[key]), acc), {})
);
return (
<table>
<thead>
<tr>
{propertyNames.map(val => (
<th key={`h_${val}`}>{val}</th>
))}
</tr>
</thead>
<tbody>
{filteredData.map((val, i) => (
<tr key={`i_${i}`}>
{propertyNames.map(p => (
<td key={`i_${i}_${p}`}>{val[p]}</td>
))}
</tr>
))}
</tbody>
</table>
);
};
const people = [
{ name: 'John', surname: 'Smith', age: 42 },
{ name: 'Adam', surname: 'Smith', gender: 'male' }
];
const propertyNames = ['name', 'surname', 'age'];
ReactDOM.render(
<MappedTable data={people} propertyNames={propertyNames} />,
document.getElementById('root')
);
Modal
- title: Modal
- tags: components,effect,intermediate
Renders a Modal component, controllable through events.
- Define
keydownHandler
, a method which handles all keyboard events and is used to callonClose
when theEsc
key is pressed. - Use the
useEffect()
hook to add or remove thekeydown
event listener to thedocument
, callingkeydownHandler
for every event. - Add a styled
<span>
element that acts as a close button, callingonClose
when clicked. - Use the
isVisible
prop passed down from the parent to determine if the modal should be displayed or not. - To use the component, import
Modal
only once and then display it by passing a boolean value to theisVisible
attribute.
.modal {
position: fixed;
top: 0;
bottom: 0;
left: 0;
right: 0;
width: 100%;
z-index: 9999;
display: flex;
align-items: center;
justify-content: center;
background-color: rgba(0, 0, 0, 0.25);
animation-name: appear;
animation-duration: 300ms;
}
.modal-dialog {
width: 100%;
max-width: 550px;
background: white;
position: relative;
margin: 0 20px;
max-height: calc(100vh - 40px);
text-align: left;
display: flex;
flex-direction: column;
overflow: hidden;
box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19);
-webkit-animation-name: animatetop;
-webkit-animation-duration: 0.4s;
animation-name: slide-in;
animation-duration: 0.5s;
}
.modal-header,
.modal-footer {
display: flex;
align-items: center;
padding: 1rem;
}
.modal-header {
border-bottom: 1px solid ##dbdbdb;
justify-content: space-between;
}
.modal-footer {
border-top: 1px solid ##dbdbdb;
justify-content: flex-end;
}
.modal-close {
cursor: pointer;
padding: 1rem;
margin: -1rem -1rem -1rem auto;
}
.modal-body {
overflow: auto;
}
.modal-content {
padding: 1rem;
}
@keyframes appear {
from {
opacity: 0;
}
to {
opacity: 1;
}
}
@keyframes slide-in {
from {
transform: translateY(-150px);
}
to {
transform: translateY(0);
}
}
const Modal = ({ isVisible = false, title, content, footer, onClose }) => {
const keydownHandler = ({ key }) => {
switch (key) {
case 'Escape':
onClose();
break;
default:
}
};
React.useEffect(() => {
document.addEventListener('keydown', keydownHandler);
return () => document.removeEventListener('keydown', keydownHandler);
});
return !isVisible ? null : (
<div className="modal" onClick={onClose}>
<div className="modal-dialog" onClick={e => e.stopPropagation()}>
<div className="modal-header">
<h3 className="modal-title">{title}</h3>
<span className="modal-close" onClick={onClose}>
×
</span>
</div>
<div className="modal-body">
<div className="modal-content">{content}</div>
</div>
{footer && <div className="modal-footer">{footer}</div>}
</div>
</div>
);
};
const App = () => {
const [isModal, setModal] = React.useState(false);
return (
<>
<button onClick={() => setModal(true)}>Click Here</button>
<Modal
isVisible={isModal}
title="Modal Title"
content={<p>Add your content here</p>}
footer={<button>Cancel</button>}
onClose={() => setModal(false)}
/>
</>
);
};
ReactDOM.render(<App />, document.getElementById('root'));
MultiselectCheckbox
- title: MultiselectCheckbox
- tags: components,input,state,array,intermediate
Renders a checkbox list that uses a callback function to pass its selected value/values to the parent component.
- Use the
useState()
hook to create thedata
state variable and use theoptions
prop to initialize its value. - Create a
toggle
function that uses the spread operator (...
) andArray.prototype.splice()
to update thedata
state variable and call theonChange
callback with anychecked
options. - Use
Array.prototype.map()
to map thedata
state variable to individual<input type="checkbox">
elements, each one wrapped in a<label>
, binding theonClick
handler to thetoggle
function.
const MultiselectCheckbox = ({ options, onChange }) => {
const [data, setData] = React.useState(options);
const toggle = index => {
const newData = [...data];
newData.splice(index, 1, {
label: data[index].label,
checked: !data[index].checked
});
setData(newData);
onChange(newData.filter(x => x.checked));
};
return (
<>
{data.map((item, index) => (
<label key={item.label}>
<input
readOnly
type="checkbox"
checked={item.checked || false}
onClick={() => toggle(index)}
/>
{item.label}
</label>
))}
</>
);
};
const options = [{ label: 'Item One' }, { label: 'Item Two' }];
ReactDOM.render(
<MultiselectCheckbox
options={options}
onChange={data => {
console.log(data);
}}
/>,
document.getElementById('root')
);
PasswordRevealer
- title: PasswordRevealer
- tags: components,input,state,beginner
Renders a password input field with a reveal button.
- Use the
useState()
hook to create theshown
state variable and set its value tofalse
. - When the
<button>
is clicked, executesetShown
, toggling thetype
of the<input>
between"text"
and"password"
.
const PasswordRevealer = ({ value }) => {
const [shown, setShown] = React.useState(false);
return (
<>
<input type={shown ? 'text' : 'password'} value={value} />
<button onClick={() => setShown(!shown)}>Show/Hide</button>
</>
);
};
ReactDOM.render(<PasswordRevealer />, document.getElementById('root'));
RippleButton
- title: RippleButton
- tags: components,state,effect,intermediate
Renders a button that animates a ripple effect when clicked.
- Use the
useState()
hook to create thecoords
andisRippling
state variables for the pointer's coordinates and the animation state of the button respectively. - Use a
useEffect()
hook to change the value ofisRippling
every time thecoords
state variable changes, starting the animation. - Use
setTimeout()
in the previous hook to clear the animation after it's done playing. - Use a
useEffect()
hook to resetcoords
whenever theisRippling
state variable isfalse.
- Handle the
onClick
event by updating thecoords
state variable and calling the passed callback.
.ripple-button {
border-radius: 4px;
border: none;
margin: 8px;
padding: 14px 24px;
background: ##1976d2;
color: ##fff;
overflow: hidden;
position: relative;
cursor: pointer;
}
.ripple-button > .ripple {
width: 20px;
height: 20px;
position: absolute;
background: ##63a4ff;
display: block;
content: "";
border-radius: 9999px;
opacity: 1;
animation: 0.9s ease 1 forwards ripple-effect;
}
@keyframes ripple-effect {
0% {
transform: scale(1);
opacity: 1;
}
50% {
transform: scale(10);
opacity: 0.375;
}
100% {
transform: scale(35);
opacity: 0;
}
}
.ripple-button > .content {
position: relative;
z-index: 2;
}
const RippleButton = ({ children, onClick }) => {
const [coords, setCoords] = React.useState({ x: -1, y: -1 });
const [isRippling, setIsRippling] = React.useState(false);
React.useEffect(() => {
if (coords.x !== -1 && coords.y !== -1) {
setIsRippling(true);
setTimeout(() => setIsRippling(false), 300);
} else setIsRippling(false);
}, [coords]);
React.useEffect(() => {
if (!isRippling) setCoords({ x: -1, y: -1 });
}, [isRippling]);
return (
<button
className="ripple-button"
onClick={e => {
const rect = e.target.getBoundingClientRect();
setCoords({ x: e.clientX - rect.left, y: e.clientY - rect.top });
onClick && onClick(e);
}}
>
{isRippling ? (
<span
className="ripple"
style={{
left: coords.x,
top: coords.y
}}
/>
) : (
''
)}
<span className="content">{children}</span>
</button>
);
};
ReactDOM.render(
<RippleButton onClick={e => console.log(e)}>Click me</RippleButton>,
document.getElementById('root')
);
Select
- title: Select
- tags: components,input,beginner
Renders an uncontrolled <select>
element that uses a callback function to pass its value to the parent component.
- Use the the
selectedValue
prop as thedefaultValue
of the<select>
element to set its initial value.. - Use the
onChange
event to fire theonValueChange
callback and send the new value to the parent. - Use
Array.prototype.map()
on thevalues
array to create an<option>
element for each passed value. - Each item in
values
must be a 2-element array, where the first element is thevalue
of the item and the second one is the displayed text for it.
const Select = ({ values, onValueChange, selectedValue, ...rest }) => {
return (
<select
defaultValue={selectedValue}
onChange={({ target: { value } }) => onValueChange(value)}
{...rest}
>
{values.map(([value, text]) => (
<option key={value} value={value}>
{text}
</option>
))}
</select>
);
};
const choices = [
['grapefruit', 'Grapefruit'],
['lime', 'Lime'],
['coconut', 'Coconut'],
['mango', 'Mango'],
];
ReactDOM.render(
<Select
values={choices}
selectedValue="lime"
onValueChange={val => console.log(val)}
/>,
document.getElementById('root')
);
Slider
- title: Slider
- tags: components,input,beginner
Renders an uncontrolled range input element that uses a callback function to pass its value to the parent component.
- Set the
type
of the<input>
element to"range"
to create a slider. - Use the
defaultValue
passed down from the parent as the uncontrolled input field's initial value. - Use the
onChange
event to fire theonValueChange
callback and send the new value to the parent.
const Slider = ({
min = 0,
max = 100,
defaultValue,
onValueChange,
...rest
}) => {
return (
<input
type="range"
min={min}
max={max}
defaultValue={defaultValue}
onChange={({ target: { value } }) => onValueChange(value)}
{...rest}
/>
);
};
ReactDOM.render(
<Slider onValueChange={val => console.log(val)} />,
document.getElementById('root')
);
StarRating
- title: StarRating
- tags: components,children,input,state,intermediate
Renders a star rating component.
- Define a component, called
Star
that will render each individual star with the appropriate appearance, based on the parent component's state. - In the
StarRating
component, use theuseState()
hook to define therating
andselection
state variables with the appropriate initial values. - Create a method,
hoverOver
, that updatesselected
according to the providedevent
, using the .data-star-id
attribute of the event's target or resets it to0
if called with anull
argument. - Use
Array.from()
to create an array of5
elements andArray.prototype.map()
to create individual<Star>
components. - Handle the
onMouseOver
andonMouseLeave
events of the wrapping element usinghoverOver
and theonClick
event usingsetRating
.
.star {
color: ##ff9933;
cursor: pointer;
}
const Star = ({ marked, starId }) => {
return (
<span data-star-id={starId} className="star" role="button">
{marked ? '\u2605' : '\u2606'}
</span>
);
};
const StarRating = ({ value }) => {
const [rating, setRating] = React.useState(parseInt(value) || 0);
const [selection, setSelection] = React.useState(0);
const hoverOver = event => {
let val = 0;
if (event && event.target && event.target.getAttribute('data-star-id'))
val = event.target.getAttribute('data-star-id');
setSelection(val);
};
return (
<div
onMouseOut={() => hoverOver(null)}
onClick={e => setRating(e.target.getAttribute('data-star-id') || rating)}
onMouseOver={hoverOver}
>
{Array.from({ length: 5 }, (v, i) => (
<Star
starId={i + 1}
key={`star_${i + 1}`}
marked={selection ? selection >= i + 1 : rating >= i + 1}
/>
))}
</div>
);
};
ReactDOM.render(<StarRating value={2} />, document.getElementById('root'));
Tabs
- title: Tabs
- tags: components,state,children,intermediate
Renders a tabbed menu and view component.
- Define a
Tabs
component that uses theuseState()
hook to initialize the value of thebindIndex
state variable todefaultIndex
. - Define a
TabItem
component and filterchildren
passed to theTabs
component to remove unnecessary nodes except forTabItem
by identifying the function's name. - Define
changeTab
, which will be executed when clicking a<button>
from the menu. changeTab
executes the passed callback,onTabClick
, and updatesbindIndex
based on the clicked element.- Use
Array.prototype.map()
on the collected nodes to render the menu and view of the tabs, using the value ofbinIndex
to determine the active tab and apply the correctclassName
.
.tab-menu > button {
cursor: pointer;
padding: 8px 16px;
border: 0;
border-bottom: 2px solid transparent;
background: none;
}
.tab-menu > button.focus {
border-bottom: 2px solid ##007bef;
}
.tab-menu > button:hover {
border-bottom: 2px solid ##007bef;
}
.tab-content {
display: none;
}
.tab-content.selected {
display: block;
}
const TabItem = props => <div {...props} />;
const Tabs = ({ defaultIndex = 0, onTabClick, children }) => {
const [bindIndex, setBindIndex] = React.useState(defaultIndex);
const changeTab = newIndex => {
if (typeof onItemClick === 'function') onItemClick(itemIndex);
setBindIndex(newIndex);
};
const items = children.filter(item => item.type.name === 'TabItem');
return (
<div className="wrapper">
<div className="tab-menu">
{items.map(({ props: { index, label } }) => (
<button
key={`tab-btn-${index}`}
onClick={() => changeTab(index)}
className={bindIndex === index ? 'focus' : ''}
>
{label}
</button>
))}
</div>
<div className="tab-view">
{items.map(({ props }) => (
<div
{...props}
className={`tab-content ${
bindIndex === props.index ? 'selected' : ''
}`}
key={`tab-content-${props.index}`}
/>
))}
</div>
</div>
);
};
ReactDOM.render(
<Tabs defaultIndex="1" onTabClick={console.log}>
<TabItem label="A" index="1">
Lorem ipsum
</TabItem>
<TabItem label="B" index="2">
Dolor sit amet
</TabItem>
</Tabs>,
document.getElementById('root')
);
TagInput
- title: TagInput
- tags: components,input,state,intermediate
Renders a tag input field.
- Define a
TagInput
component and use theuseState()
hook to initialize an array fromtags
. - Use
Array.prototype.map()
on the collected nodes to render the list of tags. - Define the
addTagData
method, which will be executed when pressing theEnter
key. - The
addTagData
method callssetTagData
to add the new tag using the spread (...
) operator to prepend the existing tags and add the new tag at the end of thetagData
array. - Define the
removeTagData
method, which will be executed on clicking the delete icon in the tag. - Use
Array.prototype.filter()
in theremoveTagData
method to remove the tag using itsindex
to filter it out from thetagData
array.
.tag-input {
display: flex;
flex-wrap: wrap;
min-height: 48px;
padding: 0 8px;
border: 1px solid ##d6d8da;
border-radius: 6px;
}
.tag-input input {
flex: 1;
border: none;
height: 46px;
font-size: 14px;
padding: 4px 0 0;
}
.tag-input input:focus {
outline: transparent;
}
.tags {
display: flex;
flex-wrap: wrap;
padding: 0;
margin: 8px 0 0;
}
.tag {
width: auto;
height: 32px;
display: flex;
align-items: center;
justify-content: center;
color: ##fff;
padding: 0 8px;
font-size: 14px;
list-style: none;
border-radius: 6px;
margin: 0 8px 8px 0;
background: ##0052cc;
}
.tag-title {
margin-top: 3px;
}
.tag-close-icon {
display: block;
width: 16px;
height: 16px;
line-height: 16px;
text-align: center;
font-size: 14px;
margin-left: 8px;
color: ##0052cc;
border-radius: 50%;
background: ##fff;
cursor: pointer;
}
const TagInput = ({ tags }) => {
const [tagData, setTagData] = React.useState(tags);
const removeTagData = indexToRemove => {
setTagData([...tagData.filter((_, index) => index !== indexToRemove)]);
};
const addTagData = event => {
if (event.target.value !== '') {
setTagData([...tagData, event.target.value]);
event.target.value = '';
}
};
return (
<div className="tag-input">
<ul className="tags">
{tagData.map((tag, index) => (
<li key={index} className="tag">
<span className="tag-title">{tag}</span>
<span
className="tag-close-icon"
onClick={() => removeTagData(index)}
>
x
</span>
</li>
))}
</ul>
<input
type="text"
onKeyUp={event => (event.key === 'Enter' ? addTagData(event) : null)}
placeholder="Press enter to add a tag"
/>
</div>
);
};
ReactDOM.render(
<TagInput tags={['Nodejs', 'MongoDB']} />,
document.getElementById('root')
);
TextArea
- title: TextArea
- tags: components,input,beginner
Renders an uncontrolled <textarea>
element that uses a callback function to pass its value to the parent component.
- Use the
defaultValue
passed down from the parent as the uncontrolled input field's initial value. - Use the
onChange
event to fire theonValueChange
callback and send the new value to the parent.
const TextArea = ({
cols = 20,
rows = 2,
defaultValue,
onValueChange,
...rest
}) => {
return (
<textarea
cols={cols}
rows={rows}
defaultValue={defaultValue}
onChange={({ target: { value } }) => onValueChange(value)}
{...rest}
/>
);
};
ReactDOM.render(
<TextArea
placeholder="Insert some text here..."
onValueChange={val => console.log(val)}
/>,
document.getElementById('root')
);
Toggle
- title: Toggle
- tags: components,state,beginner
Renders a toggle component.
- Use the
useState()
hook to initialize theisToggleOn
state variable todefaultToggled
. - Render an
<input>
and bind itsonClick
event to update theisToggledOn
state variable, applying the appropriateclassName
to the wrapping<label>
.
.toggle input[type="checkbox"] {
display: none;
}
.toggle.on {
background-color: green;
}
.toggle.off {
background-color: red;
}
const Toggle = ({ defaultToggled = false }) => {
const [isToggleOn, setIsToggleOn] = React.useState(defaultToggled);
return (
<label className={isToggleOn ? 'toggle on' : 'toggle off'}>
<input
type="checkbox"
checked={isToggleOn}
onChange={() => setIsToggleOn(!isToggleOn)}
/>
{isToggleOn ? 'ON' : 'OFF'}
</label>
);
};
ReactDOM.render(<Toggle />, document.getElementById('root'));
Tooltip
- title: Tooltip
- tags: components,state,children,beginner
Renders a tooltip component.
- Use the
useState()
hook to create theshow
variable and initialize it tofalse
. - Render a container element that contains the tooltip element and the
children
passed to the component. - Handle the
onMouseEnter
andonMouseLeave
methods, by altering the value of theshow
variable, toggling theclassName
of the tooltip.
.tooltip-container {
position: relative;
}
.tooltip-box {
position: absolute;
background: rgba(0, 0, 0, 0.7);
color: ##fff;
padding: 5px;
border-radius: 5px;
top: calc(100% + 5px);
display: none;
}
.tooltip-box.visible {
display: block;
}
.tooltip-arrow {
position: absolute;
top: -10px;
left: 50%;
border-width: 5px;
border-style: solid;
border-color: transparent transparent rgba(0, 0, 0, 0.7) transparent;
}
const Tooltip = ({ children, text, ...rest }) => {
const [show, setShow] = React.useState(false);
return (
<div className="tooltip-container">
<div className={show ? 'tooltip-box visible' : 'tooltip-box'}>
{text}
<span className="tooltip-arrow" />
</div>
<div
onMouseEnter={() => setShow(true)}
onMouseLeave={() => setShow(false)}
{...rest}
>
{children}
</div>
</div>
);
};
ReactDOM.render(
<Tooltip text="Simple tooltip">
<button>Hover me!</button>
</Tooltip>,
document.getElementById('root')
);
TreeView
- title: TreeView
- tags: components,object,state,recursion,advanced
Renders a tree view of a JSON object or array with collapsible content.
- Use the value of the
toggled
prop to determine the initial state of the content (collapsed/expanded). - Use the
useState()
hook to create theisToggled
state variable and give it the value of thetoggled
prop initially. - Render a
<span>
element and bind itsonClick
event to alter the component'sisToggled
state. - Determine the appearance of the component, based on
isParentToggled
,isToggled
,name
and checking forArray.isArray()
ondata
. - For each child in
data
, determine if it is an object or array and recursively render a sub-tree or a text element with the appropriate style.
.tree-element {
margin: 0 0 0 4px;
position: relative;
}
.tree-element.is-child {
margin-left: 16px;
}
div.tree-element:before {
content: '';
position: absolute;
top: 24px;
left: 1px;
height: calc(100% - 48px);
border-left: 1px solid gray;
}
p.tree-element {
margin-left: 16px;
}
.toggler {
position: absolute;
top: 10px;
left: 0px;
width: 0;
height: 0;
border-top: 4px solid transparent;
border-bottom: 4px solid transparent;
border-left: 5px solid gray;
cursor: pointer;
}
.toggler.closed {
transform: rotate(90deg);
}
.collapsed {
display: none;
}
const TreeView = ({
data,
toggled = true,
name = null,
isLast = true,
isChildElement = false,
isParentToggled = true
}) => {
const [isToggled, setIsToggled] = React.useState(toggled);
const isDataArray = Array.isArray(data);
return (
<div
className={`tree-element ${isParentToggled && 'collapsed'} ${
isChildElement && 'is-child'
}`}
>
<span
className={isToggled ? 'toggler' : 'toggler closed'}
onClick={() => setIsToggled(!isToggled)}
/>
{name ? <strong> {name}: </strong> : <span> </span>}
{isDataArray ? '[' : '{'}
{!isToggled && '...'}
{Object.keys(data).map((v, i, a) =>
typeof data[v] === 'object' ? (
<TreeView
key={`${name}-${v}-${i}`}
data={data[v]}
isLast={i === a.length - 1}
name={isDataArray ? null : v}
isChildElement
isParentToggled={isParentToggled && isToggled}
/>
) : (
<p
key={`${name}-${v}-${i}`}
className={isToggled ? 'tree-element' : 'tree-element collapsed'}
>
{isDataArray ? '' : <strong>{v}: </strong>}
{data[v]}
{i === a.length - 1 ? '' : ','}
</p>
)
)}
{isDataArray ? ']' : '}'}
{!isLast ? ',' : ''}
</div>
);
};
const data = {
lorem: {
ipsum: 'dolor sit',
amet: {
consectetur: 'adipiscing',
elit: [
'duis',
'vitae',
{
semper: 'orci'
},
{
est: 'sed ornare'
},
'etiam',
['laoreet', 'tincidunt'],
['vestibulum', 'ante']
]
},
ipsum: 'primis'
}
};
ReactDOM.render(
<TreeView data={data} name="data" />,
document.getElementById('root')
);
UncontrolledInput
- title: UncontrolledInput
- tags: components,input,intermediate
Renders an uncontrolled <input>
element that uses a callback function to inform its parent about value updates.
- Use the
defaultValue
passed down from the parent as the uncontrolled input field's initial value. - Use the
onChange
event to fire theonValueChange
callback and send the new value to the parent.
const UncontrolledInput = ({ defaultValue, onValueChange, ...rest }) => {
return (
<input
defaultValue={defaultValue}
onChange={({ target: { value } }) => onValueChange(value)}
{...rest}
/>
);
};
ReactDOM.render(
<UncontrolledInput
type="text"
placeholder="Insert some text here..."
onValueChange={console.log}
/>,
document.getElementById('root')
);
useAsync
- title: useAsync
- tags: hooks,state,reducer,advanced
Handles asynchronous calls.
- Create a custom hook that takes a handler function,
fn
. - Define a reducer function and an initial state for the custom hook's state.
- Use the
useReducer()
hook to initialize thestate
variable and thedispatch
function. - Define an asynchronous
run
function that will run the provided callback,fn
, while usingdispatch
to updatestate
as necessary. - Return an object containing the properties of
state
(value
,error
andloading
) and therun
function.
const useAsync = fn => {
const initialState = { loading: false, error: null, value: null };
const stateReducer = (_, action) => {
switch (action.type) {
case 'start':
return { loading: true, error: null, value: null };
case 'finish':
return { loading: false, error: null, value: action.value };
case 'error':
return { loading: false, error: action.error, value: null };
}
};
const [state, dispatch] = React.useReducer(stateReducer, initialState);
const run = async (args = null) => {
try {
dispatch({ type: 'start' });
const value = await fn(args);
dispatch({ type: 'finish', value });
} catch (error) {
dispatch({ type: 'error', error });
}
};
return { ...state, run };
};
const RandomImage = props => {
const imgFetch = useAsync(url =>
fetch(url).then(response => response.json())
);
return (
<div>
<button
onClick={() => imgFetch.run('https://dog.ceo/api/breeds/image/random')}
disabled={imgFetch.isLoading}
>
Load image
</button>
<br />
{imgFetch.loading && <div>Loading...</div>}
{imgFetch.error && <div>Error {imgFetch.error}</div>}
{imgFetch.value && (
<img
src={imgFetch.value.message}
alt="avatar"
width={400}
height="auto"
/>
)}
</div>
);
};
ReactDOM.render(<RandomImage />, document.getElementById('root'));
useClickInside
- title: useClickInside
- tags: hooks,effect,event,intermediate
Handles the event of clicking inside the wrapped component.
- Create a custom hook that takes a
ref
and acallback
to handle the'click'
event. - Use the
useEffect()
hook to append and clean up theclick
event. - Use the
useRef()
hook to create aref
for your click component and pass it to theuseClickInside
hook.
const useClickInside = (ref, callback) => {
const handleClick = e => {
if (ref.current && ref.current.contains(e.target)) {
callback();
}
};
React.useEffect(() => {
document.addEventListener('click', handleClick);
return () => {
document.removeEventListener('click', handleClick);
};
});
};
const ClickBox = ({ onClickInside }) => {
const clickRef = React.useRef();
useClickInside(clickRef, onClickInside);
return (
<div
className="click-box"
ref={clickRef}
style={{
border: '2px dashed orangered',
height: 200,
width: 400,
display: 'flex',
justifyContent: 'center',
alignItems: 'center'
}}
>
<p>Click inside this element</p>
</div>
);
};
ReactDOM.render(
<ClickBox onClickInside={() => alert('click inside')} />,
document.getElementById('root')
);
useClickOutside
- title: useClickOutside
- tags: hooks,effect,event,intermediate
Handles the event of clicking outside of the wrapped component.
- Create a custom hook that takes a
ref
and acallback
to handle theclick
event. - Use the
useEffect()
hook to append and clean up theclick
event. - Use the
useRef()
hook to create aref
for your click component and pass it to theuseClickOutside
hook.
const useClickOutside = (ref, callback) => {
const handleClick = e => {
if (ref.current && !ref.current.contains(e.target)) {
callback();
}
};
React.useEffect(() => {
document.addEventListener('click', handleClick);
return () => {
document.removeEventListener('click', handleClick);
};
});
};
const ClickBox = ({ onClickOutside }) => {
const clickRef = React.useRef();
useClickOutside(clickRef, onClickOutside);
return (
<div
className="click-box"
ref={clickRef}
style={{
border: '2px dashed orangered',
height: 200,
width: 400,
display: 'flex',
justifyContent: 'center',
alignItems: 'center'
}}
>
<p>Click out of this element</p>
</div>
);
};
ReactDOM.render(
<ClickBox onClickOutside={() => alert('click outside')} />,
document.getElementById('root')
);
useComponentDidMount
- title: useComponentDidMount
- tags: hooks,effect,beginner
Executes a callback immediately after a component is mounted.
- Use
useEffect()
with an empty array as the second argument to execute the provided callback only once when the component is mounted. - Behaves like the
componentDidMount()
lifecycle method of class components.
const useComponentDidMount = onMountHandler => {
React.useEffect(() => {
onMountHandler();
}, []);
};
const Mounter = () => {
useComponentDidMount(() => console.log('Component did mount'));
return <div>Check the console!</div>;
};
ReactDOM.render(<Mounter />, document.getElementById('root'));
useComponentWillUnmount
- title: useComponentWillUnmount
- tags: hooks,effect,beginner
Executes a callback immediately before a component is unmounted and destroyed.
- Use
useEffect()
with an empty array as the second argument and return the provided callback to be executed only once before cleanup. - Behaves like the
componentWillUnmount()
lifecycle method of class components.
const useComponentWillUnmount = onUnmountHandler => {
React.useEffect(
() => () => {
onUnmountHandler();
},
[]
);
};
const Unmounter = () => {
useComponentWillUnmount(() => console.log('Component will unmount'));
return <div>Check the console!</div>;
};
ReactDOM.render(<Unmounter />, document.getElementById('root'));
useCopyToClipboard
- title: useCopyToClipboard
- tags: hooks,effect,state,callback,advanced
Copies the given text to the clipboard.
- Use the copyToClipboard snippet to copy the text to clipboard.
- Use the
useState()
hook to initialize thecopied
variable. - Use the
useCallback()
hook to create a callback for thecopyToClipboard
method. - Use the
useEffect()
hook to reset thecopied
state variable if thetext
changes. - Return the
copied
state variable and thecopy
callback.
const useCopyToClipboard = text => {
const copyToClipboard = str => {
const el = document.createElement('textarea');
el.value = str;
el.setAttribute('readonly', '');
el.style.position = 'absolute';
el.style.left = '-9999px';
document.body.appendChild(el);
const selected =
document.getSelection().rangeCount > 0
? document.getSelection().getRangeAt(0)
: false;
el.select();
const success = document.execCommand('copy');
document.body.removeChild(el);
if (selected) {
document.getSelection().removeAllRanges();
document.getSelection().addRange(selected);
}
return success;
};
const [copied, setCopied] = React.useState(false);
const copy = React.useCallback(() => {
if (!copied) setCopied(copyToClipboard(text));
}, [text]);
React.useEffect(() => () => setCopied(false), [text]);
return [copied, copy];
};
const TextCopy = props => {
const [copied, copy] = useCopyToClipboard('Lorem ipsum');
return (
<div>
<button onClick={copy}>Click to copy</button>
<span>{copied && 'Copied!'}</span>
</div>
);
};
ReactDOM.render(<TextCopy />, document.getElementById('root'));
useDebounce
- title: useDebounce
- tags: hooks,state,effect,intermediate
Debounces the given value.
- Create a custom hook that takes a
value
and adelay
. - Use the
useState()
hook to store the debounced value. - Use the
useEffect()
hook to update the debounced value every timevalue
is updated. - Use
setTimeout()
to create a timeout that delays invoking the setter of the previous state variable bydelay
ms. - Use
clearTimeout()
to clean up when dismounting the component. - This is particularly useful when dealing with user input.
const useDebounce = (value, delay) => {
const [debouncedValue, setDebouncedValue] = React.useState(value);
React.useEffect(() => {
const handler = setTimeout(() => {
setDebouncedValue(value);
}, delay);
return () => {
clearTimeout(handler);
};
}, [value]);
return debouncedValue;
};
const Counter = () => {
const [value, setValue] = React.useState(0);
const lastValue = useDebounce(value, 500);
return (
<div>
<p>
Current: {value} - Debounced: {lastValue}
</p>
<button onClick={() => setValue(value + 1)}>Increment</button>
</div>
);
};
ReactDOM.render(<Counter />, document.getElementById('root'));
useFetch
- title: useFetch
- tags: hooks,effect,state,intermediate
Implements fetch
in a declarative manner.
- Create a custom hook that takes a
url
andoptions
. - Use the
useState()
hook to initialize theresponse
anderror
state variables. - Use the
useEffect()
hook to asynchronously callfetch()
and update the state variables accordingly. - Return an object containing the
response
anderror
state variables.
const useFetch = (url, options) => {
const [response, setResponse] = React.useState(null);
const [error, setError] = React.useState(null);
React.useEffect(() => {
const fetchData = async () => {
try {
const res = await fetch(url, options);
const json = await res.json();
setResponse(json);
} catch (error) {
setError(error);
}
};
fetchData();
}, []);
return { response, error };
};
const ImageFetch = props => {
const res = useFetch('https://dog.ceo/api/breeds/image/random', {});
if (!res.response) {
return <div>Loading...</div>;
}
const imageUrl = res.response.message;
return (
<div>
<img src={imageUrl} alt="avatar" width={400} height="auto" />
</div>
);
};
ReactDOM.render(<ImageFetch />, document.getElementById('root'));
useInterval
- title: useInterval
- tags: hooks,effect,intermediate
Implements setInterval
in a declarative manner.
- Create a custom hook that takes a
callback
and adelay
. - Use the
useRef()
hook to create aref
for the callback function. - Use a
useEffect()
hook to remember the latestcallback
whenever it changes. - Use a
useEffect()
hook dependent ondelay
to set up the interval and clean up.
const useInterval = (callback, delay) => {
const savedCallback = React.useRef();
React.useEffect(() => {
savedCallback.current = callback;
}, [callback]);
React.useEffect(() => {
function tick() {
savedCallback.current();
}
if (delay !== null) {
let id = setInterval(tick, delay);
return () => clearInterval(id);
}
}, [delay]);
};
const Timer = props => {
const [seconds, setSeconds] = React.useState(0);
useInterval(() => {
setSeconds(seconds + 1);
}, 1000);
return <p>{seconds}</p>;
};
ReactDOM.render(<Timer />, document.getElementById('root'));
useMediaQuery
- title: useMediaQuery
- tags: hooks,state,effect,intermediate
Checks if the current environment matches a given media query and returns the appropriate value.
- Check if
window
andwindow.matchMedia
exist, returnwhenFalse
if not (e.g. SSR environment or unsupported browser). - Use
window.matchMedia()
to match the givenquery
, cast itsmatches
property to a boolean and store in a state variable,match
, using theuseState()
hook. - Use the
useEffect()
hook to add a listener for changes and to clean up the listeners after the hook is destroyed. - Return either
whenTrue
orwhenFalse
based on the value ofmatch
.
const useMediaQuery = (query, whenTrue, whenFalse) => {
if (typeof window === 'undefined' || typeof window.matchMedia === 'undefined')
return whenFalse;
const mediaQuery = window.matchMedia(query);
const [match, setMatch] = React.useState(!!mediaQuery.matches);
React.useEffect(() => {
const handler = () => setMatch(!!mediaQuery.matches);
mediaQuery.addListener(handler);
return () => mediaQuery.removeListener(handler);
}, []);
return match ? whenTrue : whenFalse;
};
const ResponsiveText = () => {
const text = useMediaQuery(
'(max-width: 400px)',
'Less than 400px wide',
'More than 400px wide'
);
return <span>{text}</span>;
};
ReactDOM.render(<ResponsiveText />, document.getElementById('root'));
useNavigatorOnLine
- title: useNavigatorOnLine
- tags: hooks,state,effect,intermediate
Checks if the client is online or offline.
- Create a function,
getOnLineStatus
, that uses theNavigatorOnLine
web API to get the online status of the client. - Use the
useState()
hook to create an appropriate state variable,status
, and setter. - Use the
useEffect()
hook to add listeners for appropriate events, updating state, and cleanup those listeners when unmounting. - Finally return the
status
state variable.
const getOnLineStatus = () =>
typeof navigator !== 'undefined' && typeof navigator.onLine === 'boolean'
? navigator.onLine
: true;
const useNavigatorOnLine = () => {
const [status, setStatus] = React.useState(getOnLineStatus());
const setOnline = () => setStatus(true);
const setOffline = () => setStatus(false);
React.useEffect(() => {
window.addEventListener('online', setOnline);
window.addEventListener('offline', setOffline);
return () => {
window.removeEventListener('online', setOnline);
window.removeEventListener('offline', setOffline);
};
}, []);
return status;
};
const StatusIndicator = () => {
const isOnline = useNavigatorOnLine();
return <span>You are {isOnline ? 'online' : 'offline'}.</span>;
};
ReactDOM.render(<StatusIndicator />, document.getElementById('root'));
usePersistedState
- title: usePersistedState
- tags: hooks,state,effect,advanced
Returns a stateful value, persisted in localStorage
, and a function to update it.
- Use the
useState()
hook to initialize thevalue
todefaultValue
. - Use the
useRef()
hook to create a ref that will hold thename
of the value inlocalStorage
. - Use 3 instances of the
useEffect()
hook for initialization,value
change andname
change respectively. - When the component is first mounted, use
Storage.getItem()
to updatevalue
if there's a stored value orStorage.setItem()
to persist the current value. - When
value
is updated, useStorage.setItem()
to store the new value. - When
name
is updated, useStorage.setItem()
to create the new key, update thenameRef
and useStorage.removeItem()
to remove the previous key fromlocalStorage
. - NOTE: The hook is meant for use with primitive values (i.e. not objects) and doesn't account for changes to
localStorage
due to other code. Both of these issues can be easily handled (e.g. JSON serialization and handling the'storage'
event).
const usePersistedState = (name, defaultValue) => {
const [value, setValue] = React.useState(defaultValue);
const nameRef = React.useRef(name);
React.useEffect(() => {
try {
const storedValue = localStorage.getItem(name);
if (storedValue !== null) setValue(storedValue);
else localStorage.setItem(name, defaultValue);
} catch {
setValue(defaultValue);
}
}, []);
React.useEffect(() => {
try {
localStorage.setItem(nameRef.current, value);
} catch {}
}, [value]);
React.useEffect(() => {
const lastName = nameRef.current;
if (name !== lastName) {
try {
localStorage.setItem(name, value);
nameRef.current = name;
localStorage.removeItem(lastName);
} catch {}
}
}, [name]);
return [value, setValue];
};
const MyComponent = ({ name }) => {
const [val, setVal] = usePersistedState(name, 10);
return (
<input
value={val}
onChange={e => {
setVal(e.target.value);
}}
/>
);
};
const MyApp = () => {
const [name, setName] = React.useState('my-value');
return (
<>
<MyComponent name={name} />
<input
value={name}
onChange={e => {
setName(e.target.value);
}}
/>
</>
);
};
ReactDOM.render(<MyApp />, document.getElementById('root'));
usePrevious
- title: usePrevious
- tags: hooks,state,effect,beginner
Stores the previous state or props.
- Create a custom hook that takes a
value
. - Use the
useRef()
hook to create aref
for thevalue
. - Use the
useEffect()
hook to remember the latestvalue
.
const usePrevious = value => {
const ref = React.useRef();
React.useEffect(() => {
ref.current = value;
});
return ref.current;
};
const Counter = () => {
const [value, setValue] = React.useState(0);
const lastValue = usePrevious(value);
return (
<div>
<p>
Current: {value} - Previous: {lastValue}
</p>
<button onClick={() => setValue(value + 1)}>Increment</button>
</div>
);
};
ReactDOM.render(<Counter />, document.getElementById('root'));
useSSR
- title: useSSR
- tags: hooks,effect,state,memo,intermediate
Checks if the code is running on the browser or the server.
- Create a custom hook that returns an appropriate object.
- Use
typeof window
,window.document
andDocument.createElement()
to check if the code is running on the browser. - Use the
useState()
hook to define theinBrowser
state variable. - Use the
useEffect()
hook to update theinBrowser
state variable and clean up at the end. - Use the
useMemo()
hook to memoize the return values of the custom hook.
const isDOMavailable = !!(
typeof window !== 'undefined' &&
window.document &&
window.document.createElement
);
const useSSR = (callback, delay) => {
const [inBrowser, setInBrowser] = React.useState(isDOMavailable);
React.useEffect(() => {
setInBrowser(isDOMavailable);
return () => {
setInBrowser(false);
};
}, []);
const useSSRObject = React.useMemo(
() => ({
isBrowser: inBrowser,
isServer: !inBrowser,
canUseWorkers: typeof Worker !== 'undefined',
canUseEventListeners: inBrowser && !!window.addEventListener,
canUseViewport: inBrowser && !!window.screen
}),
[inBrowser]
);
return React.useMemo(
() => Object.assign(Object.values(useSSRObject), useSSRObject),
[inBrowser]
);
};
const SSRChecker = props => {
let { isBrowser, isServer } = useSSR();
return <p>{isBrowser ? 'Running on browser' : 'Running on server'}</p>;
};
ReactDOM.render(<SSRChecker />, document.getElementById('root'));
useTimeout
- title: useTimeout
- tags: hooks,effect,intermediate
Implements setTimeout
in a declarative manner.
- Create a custom hook that takes a
callback
and adelay
. - Use the
useRef()
hook to create aref
for the callback function. - Use the
useEffect()
hook to remember the latest callback. - Use the
useEffect()
hook to set up the timeout and clean up.
const useTimeout = (callback, delay) => {
const savedCallback = React.useRef();
React.useEffect(() => {
savedCallback.current = callback;
}, [callback]);
React.useEffect(() => {
function tick() {
savedCallback.current();
}
if (delay !== null) {
let id = setTimeout(tick, delay);
return () => clearTimeout(id);
}
}, [delay]);
};
const OneSecondTimer = props => {
const [seconds, setSeconds] = React.useState(0);
useTimeout(() => {
setSeconds(seconds + 1);
}, 1000);
return <p>{seconds}</p>;
};
ReactDOM.render(<OneSecondTimer />, document.getElementById('root'));
useToggler
- title: useToggler
- tags: hooks,state,callback,beginner
Provides a boolean state variable that can be toggled between its two states.
- Use the
useState()
hook to create thevalue
state variable and its setter. - Create a function that toggles the value of the
value
state variable and memoize it, using theuseCallback()
hook. - Return the
value
state variable and the memoized toggler function.
const useToggler = initialState => {
const [value, setValue] = React.useState(initialState);
const toggleValue = React.useCallback(() => setValue(prev => !prev), []);
return [value, toggleValue];
};
const Switch = () => {
const [val, toggleVal] = useToggler(false);
return <button onClick={toggleVal}>{val ? 'ON' : 'OFF'}</button>;
};
ReactDOM.render(<Switch />, document.getElementById('root'));
useUnload
- title: useUnload
- tags: hooks,effect,event,intermediate
Handles the beforeunload
window event.
- Use the
useRef()
hook to create a ref for the callback function,fn
. - Use the
useEffect()
hook andEventTarget.addEventListener()
to handle the'beforeunload'
(when the user is about to close the window). - Use
EventTarget.removeEventListener()
to perform cleanup after the component is unmounted.
const useUnload = fn => {
const cb = React.useRef(fn);
React.useEffect(() => {
const onUnload = cb.current;
window.addEventListener('beforeunload', onUnload);
return () => {
window.removeEventListener('beforeunload', onUnload);
};
}, [cb]);
};
const App = () => {
useUnload(e => {
e.preventDefault();
const exit = confirm('Are you sure you want to leave?');
if (exit) window.close();
});
return <div>Try closing the window.</div>;
};
ReactDOM.render(<App />, document.getElementById('root'));
all
- title: all
- tags: array,beginner
Returns true
if the provided function returns true
for all elements of an array, false
otherwise.
- Use
array_filter()
andcount()
to check if$func
returnstrue
for all the elements in$items
.
function all($items, $func)
{
return count(array_filter($items, $func)) === count($items);
}
all([2, 3, 4, 5], function ($item) {
return $item > 1;
}); // true
any
- title: any
- tags: array,beginner
Returns true
if the provided function returns true
for at least one element of an array, false
otherwise.
- Use
array_filter()
andcount()
to check if$func
returnstrue
for any of the elements in$items
.
function any($items, $func)
{
return count(array_filter($items, $func)) > 0;
}
any([1, 2, 3, 4], function ($item) {
return $item < 2;
}); // true
approximatelyEqual
- title: approximatelyEqual
- tags: math,beginner
Checks if two numbers are approximately equal to each other.
- Use
abs()
to compare the absolute difference of the two values to$epsilon
. - Omit the third parameter,
$epsilon
, to use a default value of0.001
.
function approximatelyEqual($number1, $number2, $epsilon = 0.001)
{
return abs($number1 - $number2) < $epsilon;
}
approximatelyEqual(10.0, 10.00001); // true
approximatelyEqual(10.0, 10.01); // false
average
- title: average
- tags: math,beginner
Returns the average of two or more numbers.
- Use
array_sum()
for all the values in$items
and return the result divided by theircount()
.
function average(...$items)
{
$count = count($items);
return $count === 0 ? 0 : array_sum($items) / $count;
}
average(1, 2, 3); // 2
clampNumber
- title: clampNumber
- tags: math,beginner
Clamps $num
within the inclusive range specified by the boundary values $a
and $b
.
- If
$num
falls within the range, return$num
. - Otherwise, return the nearest number in the range, using
min()
andmax()
.
function clampNumber($num, $a, $b)
{
return max(min($num, max($a, $b)), min($a, $b));
}
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1
compose
- title: compose
- tags: function,intermediate
Return a new function that composes multiple functions into a single callable.
- Use
array_reduce()
to perform right-to-left function composition.
function compose(...$functions)
{
return array_reduce(
$functions,
function ($carry, $function) {
return function ($x) use ($carry, $function) {
return $function($carry($x));
};
},
function ($x) {
return $x;
}
);
}
$compose = compose(
// add 2
function ($x) {
return $x + 2;
},
// multiply 4
function ($x) {
return $x * 4;
}
);
$compose(3); // 20
countVowels
- title: countVowels
- tags: string,regexp,beginner
Returns number of vowels in the provided string.
- Use a regular expression to count the number of vowels (
a
,e
,i
,o
andu
a) in a string.
function countVowels($string)
{
preg_match_all('/[aeiou]/i', $string, $matches);
return count($matches[0]);
}
countVowels('sampleInput'); // 4
curry
- title: curry
- tags: function,advanced
Curries a function to take arguments in multiple calls.
- If the number of provided arguments (
$args
) is sufficient, call the passed function,$function
. - Otherwise, return a curried function that expects the rest of the arguments.
function curry($function)
{
$accumulator = function ($arguments) use ($function, &$accumulator) {
return function (...$args) use ($function, $arguments, $accumulator) {
$arguments = array_merge($arguments, $args);
$reflection = new ReflectionFunction($function);
$totalArguments = $reflection->getNumberOfRequiredParameters();
if ($totalArguments <= count($arguments)) {
return $function(...$arguments);
}
return $accumulator($arguments);
};
};
return $accumulator([]);
}
$curriedAdd = curry(
function ($a, $b) {
return $a + $b;
}
);
$add10 = $curriedAdd(10);
var_dump($add10(15)); // 25
decapitalize
- title: decapitalize
- tags: string,beginner
Decapitalizes the first letter of a string.
- Decapitalizes the first letter of the string and then adds it with rest of the string.
- Omit the
$upperRest
parameter to keep the rest of the string intact, or set it totrue
to convert to uppercase.
function decapitalize($string, $upperRest = false)
{
return lcfirst($upperRest ? strtoupper($string) : $string);
}
decapitalize('FooBar'); // 'fooBar'
deepFlatten
- title: deepFlatten
- tags: array,recursion,intermediate
Deep flattens an array.
- Use recursion.
- Use
array_push
, splat operator and an empty array to flatten the array. - Recursively flatten each element that is an array.
function deepFlatten($items)
{
$result = [];
foreach ($items as $item) {
if (!is_array($item)) {
$result[] = $item;
} else {
array_push($result, ...deepFlatten($item));
}
}
return $result;
}
deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]
drop
- title: drop
- tags: array,beginner
Returns a new array with $n
elements removed from the left.
- Use
array_slice()
to remove$n
elements from the left. - Omit the second argument,
$n
, to only remove one element.
function drop($items, $n = 1)
{
return array_slice($items, $n);
}
drop([1, 2, 3]); // [2,3]
drop([1, 2, 3], 2); // [3]
endsWith
- title: endsWith
- tags: string,beginner
Checks if a string is ends with a given substring.
- Use
strrpos()
in combination withstrlen
to find the position of$needle
in$haystack
.
function endsWith($haystack, $needle)
{
return strrpos($haystack, $needle) === (strlen($haystack) - strlen($needle));
}
endsWith('Hi, this is me', 'me'); // true
factorial
- title: factorial
- tags: math,recursion,beginner
Calculates the factorial of a number.
- Use recursion.
- If
$n
is less then or equal to1
, return1
. - Otherwise, return the product of
$n
and the factorial of$n -1
.
function factorial($n)
{
if ($n <= 1) {
return 1;
}
return $n * factorial($n - 1);
}
factorial(6); // 720
fibonacci
- title: fibonacci
- tags: math,intermediate
Generates an array, containing the Fibonacci sequence, up until the nth term.
- Create an empty array, initializing the first two values (
0
and1
). - Loop from 2 through
$n
and add values into the array, using the sum of the last two values.
function fibonacci($n)
{
$sequence = [0, 1];
for ($i = 2; $i < $n; $i++) {
$sequence[$i] = $sequence[$i-1] + $sequence[$i-2];
}
return $sequence;
}
fibonacci(6); // [0, 1, 1, 2, 3, 5]
findLast
- title: findLast
- tags: array,beginner
Returns the last element for which the provided function returns a truthy value.
- Use
array_filter()
to remove elements for which$func
returns falsy values,array_pop()
to get the last one.
function findLast($items, $func)
{
$filteredItems = array_filter($items, $func);
return array_pop($filteredItems);
}
findLast([1, 2, 3, 4], function ($n) {
return ($n % 2) === 1;
});
// 3
findLastIndex
- title: findLastIndex
- tags: array,beginner
Returns the index of the last element for which the provided function returns a truthy value.
- Use
array_keys()
andarray_filter()
to remove elements for which$func
returns falsy values,array_pop()
to get the last one.
function findLastIndex($items, $func)
{
$keys = array_keys(array_filter($items, $func));
return array_pop($keys);
}
findLastIndex([1, 2, 3, 4], function ($n) {
return ($n % 2) === 1;
});
// 2
firstStringBetween
- title: firstStringBetween
- tags: string,beginner
Returns the first string there is between the strings from the parameter $start
and $end
.
- Use
trim()
andstrstr()
to find the string contained between$start
and$end
.
function firstStringBetween($haystack, $start, $end)
{
return trim(strstr(strstr($haystack, $start), $end, true), $start . $end);
}
firstStringBetween('This is a [custom] string', '[', ']'); // custom
flatten
- title: flatten
- tags: array,intermediate
Flattens an array up to the one level depth.
- Use
array_push()
, splat operator andarray_values()
to flatten the array.
function flatten($items)
{
$result = [];
foreach ($items as $item) {
if (!is_array($item)) {
$result[] = $item;
} else {
array_push($result, ...array_values($item));
}
}
return $result;
}
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
gcd
- title: gcd
- tags: math,recursion,intermediate
Calculates the greatest common divisor between two or more numbers.
- Use recursion.
- Use
array_reduce()
with thegcd
function to appy to all elements in the$numbers
list. - Base case is when
y
equals0
. In this case, returnx
. - Otherwise, return the gcd of
y
and the remainder of the divisionx/y
.
function gcd(...$numbers)
{
if (count($numbers) > 2) {
return array_reduce($numbers, 'gcd');
}
$r = $numbers[0] % $numbers[1];
return $r === 0 ? abs($numbers[1]) : gcd($numbers[1], $r);
}
gcd(8, 36); // 4
gcd(12, 8, 32); // 4
groupBy
- title: groupBy
- tags: array,intermediate
Groups the elements of an array based on the given function.
- Use
call_use_func()
with$func
on$items
to group them based on$func
.
function groupBy($items, $func)
{
$group = [];
foreach ($items as $item) {
if ((!is_string($func) && is_callable($func)) || function_exists($func)) {
$key = call_user_func($func, $item);
$group[$key][] = $item;
} elseif (is_object($item)) {
$group[$item->{$func}][] = $item;
} elseif (isset($item[$func])) {
$group[$item[$func]][] = $item;
}
}
return $group;
}
groupBy(['one', 'two', 'three'], 'strlen'); // [3 => ['one', 'two'], 5 => ['three']]
hasDuplicates
- title: hasDuplicates
- tags: array,beginner
Checks a flat list for duplicate values, returning true
if duplicate values exists and false
if values are all unique.
- Use
count()
andarray_unique()
to check$items
for duplicate values.
function hasDuplicates($items)
{
return count($items) > count(array_unique($items));
}
hasDuplicates([1, 2, 3, 4, 5, 5]); // true
head
- title: head
- tags: array,beginner
Returns the head of a list.
- Use
reset()
to return the first item in the array.
function head($items)
{
return reset($items);
}
head([1, 2, 3]); // 1
isAnagram
- title: isAnagram
- tags: string,beginner
Compare two strings and returns true
if both strings are anagram, false
otherwise.
- Use
count_chars()
to compare$string1
and$string2
.
function isAnagram($string1, $string2)
{
return count_chars($string1, 1) === count_chars($string2, 1);
}
isAnagram('act', 'cat'); // true
isContains
- title: isContains
- tags: string,beginner
Check if a word / substring exists in a given string input.
- Using
strpos()
to find the position of the first occurrence of a substring in a string.
function isContains($string, $needle)
{
return strpos($string, $needle) === false ? false : true;
}
isContains('This is an example string', 'example'); // true
isContains('This is an example string', 'hello'); // false
isEven
- title: isEven
- tags: math,beginner
Returns true
if the given number is even, false
otherwise.
- Checks whether a number is odd or even using the modulo (
%
) operator. - Returns
true
if the number is even,false
if the number is odd.
function isEven($number)
{
return ($number % 2) === 0;
}
isEven(4); // true
isLowerCase
- title: isLowerCase
- tags: string,beginner
Returns true
if the given string is lower case, false
otherwise.
- Convert the given string to lower case, using
strtolower
and compare it to the original.
function isLowerCase($string)
{
return $string === strtolower($string);
}
isLowerCase('Morning shows the day!'); // false
isLowerCase('hello'); // true
isPrime
- title: isPrime
- tags: math,beginner
Checks if the provided integer is a prime number.
- Check numbers from
2
to the square root of the given number. - Return
false
if any of them divides the given number, else returntrue
, unless the number is less than2
.
function isPrime($number)
{
$boundary = floor(sqrt($number));
for ($i = 2; $i <= $boundary; $i++) {
if ($number % $i === 0) {
return false;
}
}
return $number >= 2;
}
isPrime(3); // true
isUpperCase
- title: isUpperCase
- tags: string,beginner
Returns true
if the given string is upper case, false otherwise.
- Convert the given string to upper case, using
strtoupper
and compare it to the original.
function isUpperCase($string)
{
return $string === strtoupper($string);
}
isUpperCase('MORNING SHOWS THE DAY!'); // true
isUpperCase('qUick Fox'); // false
last
- title: last
- tags: array,beginner
Returns the last element in an array.
- Use
end()
to return the last item in the array.
function last($items)
{
return end($items);
}
last([1, 2, 3]); // 3
lcm
- title: lcm
- tags: math,intermediate
Returns the least common multiple of two or more numbers.
- Use the greatest common divisor (GCD) formula and the fact that
lcm(x,y) = x * y / gcd(x,y)
to determine the least common multiple. - The GCD formula uses recursion.
function lcm(...$numbers)
{
$ans = $numbers[0];
for ($i = 1, $max = count($numbers); $i < $max; $i++) {
$ans = (($numbers[$i] * $ans) / gcd($numbers[$i], $ans));
}
return $ans;
}
lcm(12, 7); // 84
lcm(1, 3, 4, 5); // 60
maxN
- title: maxN
- tags: math,array,intermediate
Returns the maximum value from the provided array.
- Use
array_filter()
andmax()
to find the maximum value in an array.
function maxN($numbers)
{
$maxValue = max($numbers);
$maxValueArray = array_filter($numbers, function ($value) use ($maxValue) {
return $maxValue === $value;
});
return count($maxValueArray);
}
maxN([1, 2, 3, 4, 5, 5]); // 2
maxN([1, 2, 3, 4, 5]); // 1
median
- title: median
- tags: math,array,beginner
Returns the median of an array of numbers.
- Find the middle of the array, use
sort()
to sort the values. - Return the number at the midpoint if the array's length is odd, otherwise the average of the two middle numbers.
function median($numbers)
{
sort($numbers);
$totalNumbers = count($numbers);
$mid = floor($totalNumbers / 2);
return ($totalNumbers % 2) === 0 ? ($numbers[$mid - 1] + $numbers[$mid]) / 2 : $numbers[$mid];
}
median([1, 3, 3, 6, 7, 8, 9]); // 6
median([1, 2, 3, 6, 7, 9]); // 4.5
memoize
- title: memoize
- tags: function,advanced
Returns the memoized (cached) function.
- Create an empty cache by instantiating a new array.
- Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not.
- Allow access to the cache by setting it as a property on the returned function.
function memoize($func)
{
return function () use ($func) {
static $cache = [];
$args = func_get_args();
$key = serialize($args);
$cached = true;
if (!isset($cache[$key])) {
$cache[$key] = $func(...$args);
$cached = false;
}
return ['result' => $cache[$key], 'cached' => $cached];
};
}
$memoizedAdd = memoize(
function ($num) {
return $num + 10;
}
);
var_dump($memoizedAdd(5)); // ['result' => 15, 'cached' => false]
var_dump($memoizedAdd(6)); // ['result' => 16, 'cached' => false]
var_dump($memoizedAdd(5)); // ['result' => 15, 'cached' => true]
minN
- title: minN
- tags: math,array,intermediate
Returns the minimum value from the provided array.
- Use
array_filter()
andmin()
to find the minimum value in an array.
function minN($numbers)
{
$minValue = min($numbers);
$minValueArray = array_filter($numbers, function ($value) use ($minValue) {
return $minValue === $value;
});
return count($minValueArray);
}
minN([1, 1, 2, 3, 4, 5, 5]); // 2
minN([1, 2, 3, 4, 5]); // 1
once
- title: once
- tags: function,intermediate
Call a function only once.
- Return a function, which only calls the provided function,
$function
, if$called
isfalse
and sets$called
totrue
.
function once($function)
{
return function (...$args) use ($function) {
static $called = false;
if ($called) {
return;
}
$called = true;
return $function(...$args);
};
}
$add = function ($a, $b) {
return $a + $b;
};
$once = once($add);
var_dump($once(10, 5)); // 15
var_dump($once(20, 10)); // null
orderBy
- title: orderBy
- tags: array,advanced
Sorts a collection of arrays or objects by key.
- Uses
sort()
on the provided array to sort the array based on$order
and$attr
.
function orderBy($items, $attr, $order)
{
$sortedItems = [];
foreach ($items as $item) {
$key = is_object($item) ? $item->{$attr} : $item[$attr];
$sortedItems[$key] = $item;
}
if ($order === 'desc') {
krsort($sortedItems);
} else {
ksort($sortedItems);
}
return array_values($sortedItems);
}
orderBy(
[
['id' => 2, 'name' => 'Joy'],
['id' => 3, 'name' => 'Khaja'],
['id' => 1, 'name' => 'Raja']
],
'id',
'desc'
); // [['id' => 3, 'name' => 'Khaja'], ['id' => 2, 'name' => 'Joy'], ['id' => 1, 'name' => 'Raja']]
palindrome
- title: palindrome
- tags: string,beginner
Returns true
if the given string is a palindrome, false
otherwise.
- Check if the value of
strrev($string)
is equal to the passed$string
.
function palindrome($string)
{
return strrev($string) === (string) $string;
}
palindrome('racecar'); // true
palindrome(2221222); // true
pluck
- title: pluck
- tags: array,beginner
Retrieves all of the values for a given key.
- Use
array_map()
to map each object in the$items
array to the provided$key
.
function pluck($items, $key)
{
return array_map( function($item) use ($key) {
return is_object($item) ? $item->$key : $item[$key];
}, $items);
}
pluck([
['product_id' => 'prod-100', 'name' => 'Desk'],
['product_id' => 'prod-200', 'name' => 'Chair'],
], 'name');
// ['Desk', 'Chair']
pull
- title: pull
- tags: array,beginner
Mutates the original array to filter out the values specified.
- Use
array_values()
andarray_diff()
to remove the specified values from$items
.
function pull(&$items, ...$params)
{
$items = array_values(array_diff($items, $params));
return $items;
}
$items = ['a', 'b', 'c', 'a', 'b', 'c'];
pull($items, 'a', 'c'); // $items will be ['b', 'b']
reject
- title: reject
- tags: array,beginner
Filters the collection using the given callback.
- Use
array_values()
,array_diff()
andarray_filter()
to filter$items
based on$func
.
function reject($items, $func)
{
return array_values(array_diff($items, array_filter($items, $func)));
}
reject(['Apple', 'Pear', 'Kiwi', 'Banana'], function ($item) {
return strlen($item) > 4;
}); // ['Pear', 'Kiwi']
remove
- title: remove
- tags: array,beginner
Removes elements from an array for which the given function returns false
.
- Use
array_filter()
to find array elements that return truthy values andarray_diff_keys()
to remove the elements not contained in$filtered
.
function remove($items, $func)
{
$filtered = array_filter($items, $func);
return array_diff_key($items, $filtered);
}
remove([1, 2, 3, 4], function ($n) {
return ($n % 2) === 0;
});
// [0 => 1, 2 => 3]
rotate
- title: rotate
- tags: array,beginner
Rotates the array (in left direction) by the number of shifts.
- Given the
$shift
index, merge the array values after$shift
with the values before$shift
.
function rotate($array, $shift = 1)
{
for ($i = 0; $i < $shift; $i++) {
array_push($array, array_shift($array));
}
return $array;
}
rotate([1, 3, 5, 2, 4]); // [3, 5, 2, 4, 1]
rotate([1, 3, 5, 2, 4], 2); // [5, 2, 4, 1, 3]
shorten
- title: shorten
- tags: string,beginner
Returns a shortened string.
- Use
mb_strlen()
,mb_substr()
andrtrim()
to shorten a string to a give number of characters.
function shorten($input, $length = 100, $end = '...')
{
if (mb_strlen($input) <= $length) {
return $input;
}
return rtrim(mb_substr($input, 0, $length, 'UTF-8')) . $end;
}
shorten('The quick brown fox jumped over the lazy dog', 15); // The quick brown...
slugify
- title: slugify
- tags: string,intermediate
Converts a string to a URL-friendly slug.
- Uses
preg_replace()
to replace invalid chars with dashes,iconv()
to convert the text to ASCII,strtolower()
andtrim()
to convert to lowercase and remove extra whitespace.
function slugify($text) {
$text = preg_replace('~[^\pL\d]+~u', '-', $text);
$text = iconv('utf-8', 'us-ascii//TRANSLIT', $text);
$text = preg_replace('~[^-\w]+~', '', $text);
$text = preg_replace('~-+~', '-', $text);
$text = strtolower($text);
$text = trim($text, " \t\n\r\0\x0B-");
if (empty($text)) {
return 'n-a';
}
return $text;
}
slugify('Hello World'); // 'hello-world'
startsWith
- title: startsWith
- tags: string,beginner
Check if a string starts with a given substring.
- Use
strpos()
to find the position of$needle
in$haystack
.
function startsWith($haystack, $needle)
{
return strpos($haystack, $needle) === 0;
}
startsWith('Hi, this is me', 'Hi'); // true
tail
- title: tail
- tags: array,beginner
Returns all elements in an array except for the first one.
- Use
array_slice()
andcount()
to return all the items in the array except for the first one.
function tail($items)
{
return count($items) > 1 ? array_slice($items, 1) : $items;
}
tail([1, 2, 3]); // [2, 3]
take
- title: take
- tags: array,beginner
Returns an array with $n
elements removed from the beginning.
- Use
array_slice()
to remove$n
items from the beginning of the array.
function take($items, $n = 1)
{
return array_slice($items, 0, $n);
}
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3, 4, 5], 2); // [1, 2]
without
- title: without
- tags: array,beginner
Filters out the elements of an array, that have one of the specified values.
- Use
array_values()
andarray_diff()
to remove any values in$params
from$items
.
function without($items, ...$params)
{
return array_values(array_diff($items, $params));
}
without([2, 1, 2, 3], 1, 2); // [3]
add_days
- title: add_days
- tags: date,intermediate
Calculates the date of n
days from the given date.
- Use
datetime.timedelta
and the+
operator to calculate the newdatetime.datetime
value after addingn
days tod
. - Omit the second argument,
d
, to use a default value ofdatetime.today()
.
from datetime import datetime, timedelta
def add_days(n, d = datetime.today()):
return d + timedelta(n)
from datetime import date
add_days(5, date(2020, 10, 25)) ## date(2020, 10, 30)
add_days(-5, date(2020, 10, 25)) ## date(2020, 10, 20)
all_equal
- title: all_equal
- tags: list,beginner
Checks if all elements in a list are equal.
- Use
set()
to eliminate duplicate elements and then uselen()
to check if length is1
.
def all_equal(lst):
return len(set(lst)) == 1
all_equal([1, 2, 3, 4, 5, 6]) ## False
all_equal([1, 1, 1, 1]) ## True
all_unique
- title: all_unique
- tags: list,beginner
Checks if all the values in a list are unique.
- Use
set()
on the given list to keep only unique occurrences. - Use
len()
to compare the length of the unique values to the original list.
def all_unique(lst):
return len(lst) == len(set(lst))
x = [1, 2, 3, 4, 5, 6]
y = [1, 2, 2, 3, 4, 5]
all_unique(x) ## True
all_unique(y) ## False
arithmetic_progression
- title: arithmetic_progression
- tags: math,beginner
Generates a list of numbers in the arithmetic progression starting with the given positive integer and up to the specified limit.
- Use
range()
andlist()
with the appropriate start, step and end values.
def arithmetic_progression(n, lim):
return list(range(n, lim + 1, n))
arithmetic_progression(5, 25) ## [5, 10, 15, 20, 25]
average
- title: average
- tags: math,list,beginner
Calculates the average of two or more numbers.
- Use
sum()
to sum all of theargs
provided, divide bylen()
.
def average(*args):
return sum(args, 0.0) / len(args)
average(*[1, 2, 3]) ## 2.0
average(1, 2, 3) ## 2.0
average_by
- title: average_by
- tags: math,list,intermediate
Calculates the average of a list, after mapping each element to a value using the provided function.
- Use
map()
to map each element to the value returned byfn
. - Use
sum()
to sum all of the mapped values, divide bylen(lst)
. - Omit the last argument,
fn
, to use the default identity function.
def average_by(lst, fn = lambda x: x):
return sum(map(fn, lst), 0.0) / len(lst)
average_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda x: x['n'])
## 5.0
bifurcate
- title: bifurcate
- tags: list,intermediate
Splits values into two groups, based on the result of the given filter
list.
- Use a list comprehension and
zip()
to add elements to groups, based onfilter
. - If
filter
has a truthy value for any element, add it to the first group, otherwise add it to the second group.
def bifurcate(lst, filter):
return [
[x for x, flag in zip(lst, filter) if flag],
[x for x, flag in zip(lst, filter) if not flag]
]
bifurcate(['beep', 'boop', 'foo', 'bar'], [True, True, False, True])
## [ ['beep', 'boop', 'bar'], ['foo'] ]
bifurcate_by
- title: bifurcate_by
- tags: list,intermediate
Splits values into two groups, based on the result of the given filtering function.
- Use a list comprehension to add elements to groups, based on the value returned by
fn
for each element. - If
fn
returns a truthy value for any element, add it to the first group, otherwise add it to the second group.
def bifurcate_by(lst, fn):
return [
[x for x in lst if fn(x)],
[x for x in lst if not fn(x)]
]
bifurcate_by(['beep', 'boop', 'foo', 'bar'], lambda x: x[0] == 'b')
## [ ['beep', 'boop', 'bar'], ['foo'] ]
binomial_coefficient
- title: binomial_coefficient
- tags: math,beginner
Calculates the number of ways to choose k
items from n
items without repetition and without order.
- Use
math.comb()
to calculate the binomial coefficient.
from math import comb
def binomial_coefficient(n, k):
return comb(n, k)
binomial_coefficient(8, 2) ## 28
byte_size
- title: byte_size
- tags: string,beginner
Returns the length of a string in bytes.
- Use
str.encode('utf-8')
to encode the given string and return its length.
def byte_size(s):
return len(s.encode('utf-8'))
byte_size('😀') ## 4
byte_size('Hello World') ## 11
camel
- title: camel
- tags: string,regexp,intermediate
Converts a string to camelcase.
- Use
re.sub()
to replace any-
or_
with a space, using the regexpr"(_|-)+"
. - Use
str.title()
to capitalize the first letter of each word and convert the rest to lowercase. - Finally, use
str.replace()
to remove spaces between words.
from re import sub
def camel(s):
s = sub(r"(_|-)+", " ", s).title().replace(" ", "")
return ''.join([s[0].lower(), s[1:]])
camel('some_database_field_name') ## 'someDatabaseFieldName'
camel('Some label that needs to be camelized')
## 'someLabelThatNeedsToBeCamelized'
camel('some-javascript-property') ## 'someJavascriptProperty'
camel('some-mixed_string with spaces_underscores-and-hyphens')
## 'someMixedStringWithSpacesUnderscoresAndHyphens'
capitalize
- title: capitalize
- tags: string,intermediate
Capitalizes the first letter of a string.
- Use list slicing and
str.upper()
to capitalize the first letter of the string. - Use
str.join()
to combine the capitalized first letter with the rest of the characters. - Omit the
lower_rest
parameter to keep the rest of the string intact, or set it toTrue
to convert to lowercase.
def capitalize(s, lower_rest = False):
return ''.join([s[:1].upper(), (s[1:].lower() if lower_rest else s[1:])])
capitalize('fooBar') ## 'FooBar'
capitalize('fooBar', True) ## 'Foobar'
capitalize_every_word
- title: capitalize_every_word
- tags: string,beginner
Capitalizes the first letter of every word in a string.
- Use
str.title()
to capitalize the first letter of every word in the string.
def capitalize_every_word(s):
return s.title()
capitalize_every_word('hello world!') ## 'Hello World!'
cast_list
- title: cast_list
- tags: list,intermediate
Casts the provided value as a list if it's not one.
- Use
isinstance()
to check if the given value is enumerable. - Return it by using
list()
or encapsulated in a list accordingly.
def cast_list(val):
return list(val) if isinstance(val, (tuple, list, set, dict)) else [val]
cast_list('foo') ## ['foo']
cast_list([1]) ## [1]
cast_list(('foo', 'bar')) ## ['foo', 'bar']
celsius_to_fahrenheit
- title: celsius_to_fahrenheit
- tags: math,beginner unlisted: true
Converts Celsius to Fahrenheit.
- Follow the conversion formula
F = 1.8 * C + 32
.
def celsius_to_fahrenheit(degrees):
return ((degrees * 1.8) + 32)
celsius_to_fahrenheit(180) ## 356.0
check_prop
- title: check_prop
- tags: function,intermediate
Creates a function that will invoke a predicate function for the specified property on a given object.
- Return a
lambda
function that takes an object and applies the predicate function,fn
to the specified property.
def check_prop(fn, prop):
return lambda obj: fn(obj[prop])
check_age = check_prop(lambda x: x >= 18, 'age')
user = {'name': 'Mark', 'age': 18}
check_age(user) ## True
chunk
- title: chunk
- tags: list,intermediate
Chunks a list into smaller lists of a specified size.
- Use
list()
andrange()
to create a list of the desiredsize
. - Use
map()
on the list and fill it with splices of the given list. - Finally, return the created list.
from math import ceil
def chunk(lst, size):
return list(
map(lambda x: lst[x * size:x * size + size],
list(range(ceil(len(lst) / size)))))
chunk([1, 2, 3, 4, 5], 2) ## [[1, 2], [3, 4], [5]]
chunk_into_n
- title: chunk_into_n
- tags: list,intermediate
Chunks a list into n
smaller lists.
- Use
math.ceil()
andlen()
to get the size of each chunk. - Use
list()
andrange()
to create a new list of sizen
. - Use
map()
to map each element of the new list to a chunk the length ofsize
. - If the original list can't be split evenly, the final chunk will contain the remaining elements.
from math import ceil
def chunk_into_n(lst, n):
size = ceil(len(lst) / n)
return list(
map(lambda x: lst[x * size:x * size + size],
list(range(n)))
)
chunk_into_n([1, 2, 3, 4, 5, 6, 7], 4) ## [[1, 2], [3, 4], [5, 6], [7]]
clamp_number
- title: clamp_number
- tags: math,beginner
Clamps num
within the inclusive range specified by the boundary values.
- If
num
falls within the range (a
,b
), returnnum
. - Otherwise, return the nearest number in the range.
def clamp_number(num, a, b):
return max(min(num, max(a, b)), min(a, b))
clamp_number(2, 3, 5) ## 3
clamp_number(1, -1, -5) ## -1
collect_dictionary
- title: collect_dictionary
- tags: dictionary,intermediate
Inverts a dictionary with non-unique hashable values.
- Create a
collections.defaultdict
withlist
as the default value for each key. - Use
dictionary.items()
in combination with a loop to map the values of the dictionary to keys usingdict.append()
. - Use
dict()
to convert thecollections.defaultdict
to a regular dictionary.
from collections import defaultdict
def collect_dictionary(obj):
inv_obj = defaultdict(list)
for key, value in obj.items():
inv_obj[value].append(key)
return dict(inv_obj)
ages = {
'Peter': 10,
'Isabel': 10,
'Anna': 9,
}
collect_dictionary(ages) ## { 10: ['Peter', 'Isabel'], 9: ['Anna'] }
compact
- title: compact
- tags: list,beginner
Removes falsy values from a list.
- Use
filter()
to filter out falsy values (False
,None
,0
, and""
).
def compact(lst):
return list(filter(None, lst))
compact([0, 1, False, 2, '', 3, 'a', 's', 34]) ## [ 1, 2, 3, 'a', 's', 34 ]
compose
- title: compose
- tags: function,advanced
Performs right-to-left function composition.
- Use
functools.reduce()
to perform right-to-left function composition. - The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.
from functools import reduce
def compose(*fns):
return reduce(lambda f, g: lambda *args: f(g(*args)), fns)
add5 = lambda x: x + 5
multiply = lambda x, y: x * y
multiply_and_add_5 = compose(add5, multiply)
multiply_and_add_5(5, 2) ## 15
compose_right
- title: compose_right
- tags: function,advanced
Performs left-to-right function composition.
- Use
functools.reduce()
to perform left-to-right function composition. - The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
from functools import reduce
def compose_right(*fns):
return reduce(lambda f, g: lambda *args: g(f(*args)), fns)
add = lambda x, y: x + y
square = lambda x: x * x
add_and_square = compose_right(add, square)
add_and_square(1, 2) ## 9
count_by
- title: count_by
- tags: list,intermediate
Groups the elements of a list based on the given function and returns the count of elements in each group.
- Use
collections.defaultdict
to initialize a dictionary. - Use
map()
to map the values of the given list using the given function. - Iterate over the map and increase the element count each time it occurs.
from collections import defaultdict
def count_by(lst, fn = lambda x: x):
count = defaultdict(int)
for val in map(fn, lst):
count[val] += 1
return dict(count)
from math import floor
count_by([6.1, 4.2, 6.3], floor) ## {6: 2, 4: 1}
count_by(['one', 'two', 'three'], len) ## {3: 2, 5: 1}
count_occurrences
- title: count_occurrences
- tags: list,beginner
Counts the occurrences of a value in a list.
- Use
list.count()
to count the number of occurrences ofval
inlst
.
def count_occurrences(lst, val):
return lst.count(val)
count_occurrences([1, 1, 2, 1, 2, 3], 1) ## 3
cumsum
- title: cumsum
- tags: list,intermediate
Creates a list of partial sums.
- Use
itertools.accumulate()
to create the accumulated sum for each element. - Use
list()
to convert the result into a list.
from itertools import accumulate
def cumsum(lst):
return list(accumulate(lst))
cumsum(range(0, 15, 3)) ## [0, 3, 9, 18, 30]
curry
- title: curry
- tags: function,intermediate
Curries a function.
- Use
functools.partial()
to return a new partial object which behaves likefn
with the given arguments,args
, partially applied.
from functools import partial
def curry(fn, *args):
return partial(fn, *args)
add = lambda x, y: x + y
add10 = curry(add, 10)
add10(20) ## 30
daterange
- title: daterange
- tags: date,intermediate
Creates a list of dates between start
(inclusive) and end
(not inclusive).
- Use
datetime.timedelta.days
to get the days betweenstart
andend
. - Use
int()
to convert the result to an integer andrange()
to iterate over each day. - Use a list comprehension and
datetime.timedelta()
to create a list ofdatetime.date
objects.
from datetime import timedelta, date
def daterange(start, end):
return [start + timedelta(n) for n in range(int((end - start).days))]
from datetime import date
daterange(date(2020, 10, 1), date(2020, 10, 5))
## [date(2020, 10, 1), date(2020, 10, 2), date(2020, 10, 3), date(2020, 10, 4)]
days_ago
- title: days_ago
- tags: date,intermediate
Calculates the date of n
days ago from today.
- Use
datetime.date.today()
to get the current day. - Use
datetime.timedelta
to subtractn
days from today's date.
from datetime import timedelta, date
def days_ago(n):
return date.today() - timedelta(n)
days_ago(5) ## date(2020, 10, 23)
days_diff
- title: days_diff
- tags: date,beginner
Calculates the day difference between two dates.
- Subtract
start
fromend
and usedatetime.timedelta.days
to get the day difference.
def days_diff(start, end):
return (end - start).days
from datetime import date
days_diff(date(2020, 10, 25), date(2020, 10, 28)) ## 3
days_from_now
- title: days_from_now
- tags: date,intermediate
Calculates the date of n
days from today.
- Use
datetime.date.today()
to get the current day. - Use
datetime.timedelta
to addn
days from today's date.
from datetime import timedelta, date
def days_from_now(n):
return date.today() + timedelta(n)
days_from_now(5) ## date(2020, 11, 02)
decapitalize
- title: decapitalize
- tags: string,intermediate
Decapitalizes the first letter of a string.
- Use list slicing and
str.lower()
to decapitalize the first letter of the string. - Use
str.join()
to combine the lowercase first letter with the rest of the characters. - Omit the
upper_rest
parameter to keep the rest of the string intact, or set it toTrue
to convert to uppercase.
def decapitalize(s, upper_rest = False):
return ''.join([s[:1].lower(), (s[1:].upper() if upper_rest else s[1:])])
decapitalize('FooBar') ## 'fooBar'
decapitalize('FooBar', True) ## 'fOOBAR'
deep_flatten
- title: deep_flatten
- tags: list,recursion,intermediate
Deep flattens a list.
- Use recursion.
- Use
isinstance()
withcollections.abc.Iterable
to check if an element is iterable. - If it is iterable, apply
deep_flatten()
recursively, otherwise return[lst]
.
from collections.abc import Iterable
def deep_flatten(lst):
return ([a for i in lst for a in
deep_flatten(i)] if isinstance(lst, Iterable) else [lst])
deep_flatten([1, [2], [[3], 4], 5]) ## [1, 2, 3, 4, 5]
degrees_to_rads
- title: degrees_to_rads
- tags: math,beginner
Converts an angle from degrees to radians.
- Use
math.pi
and the degrees to radians formula to convert the angle from degrees to radians.
from math import pi
def degrees_to_rads(deg):
return (deg * pi) / 180.0
degrees_to_rads(180) ## ~3.1416
delay
- title: delay
- tags: function,intermediate
Invokes the provided function after ms
milliseconds.
- Use
time.sleep()
to delay the execution offn
byms / 1000
seconds.
from time import sleep
def delay(fn, ms, *args):
sleep(ms / 1000)
return fn(*args)
delay(lambda x: print(x), 1000, 'later') ## prints 'later' after one second
dict_to_list
- title: dict_to_list
- tags: dictionary,list,intermediate
Converts a dictionary to a list of tuples.
- Use
dict.items()
andlist()
to get a list of tuples from the given dictionary.
def dict_to_list(d):
return list(d.items())
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
dict_to_list(d)
## [('one', 1), ('three', 3), ('five', 5), ('two', 2), ('four', 4)]
difference
- title: difference
- tags: list,beginner
Calculates the difference between two iterables, without filtering duplicate values.
- Create a
set
fromb
. - Use a list comprehension on
a
to only keep values not contained in the previously created set,_b
.
def difference(a, b):
_b = set(b)
return [item for item in a if item not in _b]
difference([1, 2, 3], [1, 2, 4]) ## [3]
difference_by
- title: difference_by
- tags: list,function,intermediate
Returns the difference between two lists, after applying the provided function to each list element of both.
- Create a
set
, usingmap()
to applyfn
to each element inb
. - Use a list comprehension in combination with
fn
ona
to only keep values not contained in the previously created set,_b
.
def difference_by(a, b, fn):
_b = set(map(fn, b))
return [item for item in a if fn(item) not in _b]
from math import floor
difference_by([2.1, 1.2], [2.3, 3.4], floor) ## [1.2]
difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x'])
## [ { x: 2 } ]
digitize
- title: digitize
- tags: math,list,beginner
Converts a number to a list of digits.
- Use
map()
combined withint
on the string representation ofn
and return a list from the result.
def digitize(n):
return list(map(int, str(n)))
digitize(123) ## [1, 2, 3]
drop
- title: drop
- tags: list,beginner
Returns a list with n
elements removed from the left.
- Use slice notation to remove the specified number of elements from the left.
- Omit the last argument,
n
, to use a default value of1
.
def drop(a, n = 1):
return a[n:]
drop([1, 2, 3]) ## [2, 3]
drop([1, 2, 3], 2) ## [3]
drop([1, 2, 3], 42) ## []
drop_right
- title: drop_right
- tags: list,beginner
Returns a list with n
elements removed from the right.
- Use slice notation to remove the specified number of elements from the right.
- Omit the last argument,
n
, to use a default value of1
.
def drop_right(a, n = 1):
return a[:-n]
drop_right([1, 2, 3]) ## [1, 2]
drop_right([1, 2, 3], 2) ## [1]
drop_right([1, 2, 3], 42) ## []
every
- title: every
- tags: list,intermediate
Checks if the provided function returns True
for every element in the list.
- Use
all()
in combination withmap()
andfn
to check iffn
returnsTrue
for all elements in the list.
def every(lst, fn = lambda x: x):
return all(map(fn, lst))
every([4, 2, 3], lambda x: x > 1) ## True
every([1, 2, 3]) ## True
every_nth
- title: every_nth
- tags: list,beginner
Returns every nth
element in a list.
- Use slice notation to create a new list that contains every
nth
element of the given list.
def every_nth(lst, nth):
return lst[nth - 1::nth]
every_nth([1, 2, 3, 4, 5, 6], 2) ## [ 2, 4, 6 ]
factorial
- title: factorial
- tags: math,recursion,beginner
Calculates the factorial of a number.
- Use recursion.
- If
num
is less than or equal to1
, return1
. - Otherwise, return the product of
num
and the factorial ofnum - 1
. - Throws an exception if
num
is a negative or a floating point number.
def factorial(num):
if not ((num >= 0) and (num % 1 == 0)):
raise Exception("Number can't be floating point or negative.")
return 1 if num == 0 else num * factorial(num - 1)
factorial(6) ## 720
fahrenheit_to_celsius
- title: fahrenheit_to_celsius
- tags: math,beginner unlisted: true
Converts Fahrenheit to Celsius.
- Follow the conversion formula
C = (F - 32) * 5/9
.
def fahrenheit_to_celsius(degrees):
return ((degrees - 32) * 5/9)
fahrenheit_to_celsius(77) ## 25.0
fibonacci
- title: fibonacci
- tags: math,list,intermediate
Generates a list, containing the Fibonacci sequence, up until the nth term.
- Starting with
0
and1
, uselist.append()
to add the sum of the last two numbers of the list to the end of the list, until the length of the list reachesn
. - If
n
is less or equal to0
, return a list containing0
.
def fibonacci(n):
if n <= 0:
return [0]
sequence = [0, 1]
while len(sequence) <= n:
next_value = sequence[len(sequence) - 1] + sequence[len(sequence) - 2]
sequence.append(next_value)
return sequence
fibonacci(7) ## [0, 1, 1, 2, 3, 5, 8, 13]
filter_non_unique
- title: filter_non_unique
- tags: list,beginner
Creates a list with the non-unique values filtered out.
- Use
collections.Counter
to get the count of each value in the list. - Use a list comprehension to create a list containing only the unique values.
from collections import Counter
def filter_non_unique(lst):
return [item for item, count in Counter(lst).items() if count == 1]
filter_non_unique([1, 2, 2, 3, 4, 4, 5]) ## [1, 3, 5]
filter_unique
- title: filter_unique
- tags: list,beginner
Creates a list with the unique values filtered out.
- Use
collections.Counter
to get the count of each value in the list. - Use a list comprehension to create a list containing only the non-unique values.
from collections import Counter
def filter_unique(lst):
return [item for item, count in Counter(lst).items() if count > 1]
filter_unique([1, 2, 2, 3, 4, 4, 5]) ## [2, 4]
find
- title: find
- tags: list,beginner
Finds the value of the first element in the given list that satisfies the provided testing function.
- Use a list comprehension and
next()
to return the first element inlst
for whichfn
returnsTrue
.
def find(lst, fn):
return next(x for x in lst if fn(x))
find([1, 2, 3, 4], lambda n: n % 2 == 1) ## 1
find_index
- title: find_index
- tags: list,intermediate
Finds the index of the first element in the given list that satisfies the provided testing function.
- Use a list comprehension,
enumerate()
andnext()
to return the index of the first element inlst
for whichfn
returnsTrue
.
def find_index(lst, fn):
return next(i for i, x in enumerate(lst) if fn(x))
find_index([1, 2, 3, 4], lambda n: n % 2 == 1) ## 0
find_index_of_all
- title: find_index_of_all
- tags: list,intermediate
Finds the indexes of all elements in the given list that satisfy the provided testing function.
- Use
enumerate()
and a list comprehension to return the indexes of the all element inlst
for whichfn
returnsTrue
.
def find_index_of_all(lst, fn):
return [i for i, x in enumerate(lst) if fn(x)]
find_index_of_all([1, 2, 3, 4], lambda n: n % 2 == 1) ## [0, 2]
find_key
- title: find_key
- tags: dictionary,intermediate
Finds the first key in the provided dictionary that has the given value.
- Use
dictionary.items()
andnext()
to return the first key that has a value equal toval
.
def find_key(dict, val):
return next(key for key, value in dict.items() if value == val)
ages = {
'Peter': 10,
'Isabel': 11,
'Anna': 9,
}
find_key(ages, 11) ## 'Isabel'
find_keys
- title: find_keys
- tags: dictionary,intermediate
Finds all keys in the provided dictionary that have the given value.
- Use
dictionary.items()
, a generator andlist()
to return all keys that have a value equal toval
.
def find_keys(dict, val):
return list(key for key, value in dict.items() if value == val)
ages = {
'Peter': 10,
'Isabel': 11,
'Anna': 10,
}
find_keys(ages, 10) ## [ 'Peter', 'Anna' ]
find_last
- title: find_last
- tags: list,beginner
Finds the value of the last element in the given list that satisfies the provided testing function.
- Use a list comprehension and
next()
to return the last element inlst
for whichfn
returnsTrue
.
def find_last(lst, fn):
return next(x for x in lst[::-1] if fn(x))
find_last([1, 2, 3, 4], lambda n: n % 2 == 1) ## 3
find_last_index
- title: find_last_index
- tags: list,beginner
Finds the index of the last element in the given list that satisfies the provided testing function.
- Use a list comprehension,
enumerate()
andnext()
to return the index of the last element inlst
for whichfn
returnsTrue
.
def find_last_index(lst, fn):
return len(lst) - 1 - next(i for i, x in enumerate(lst[::-1]) if fn(x))
find_last_index([1, 2, 3, 4], lambda n: n % 2 == 1) ## 2
find_parity_outliers
- title: find_parity_outliers
- tags: list,math,intermediate
Finds the items that are parity outliers in a given list.
- Use
collections.Counter
with a list comprehension to count even and odd values in the list. - Use
collections.Counter.most_common()
to get the most common parity. - Use a list comprehension to find all elements that do not match the most common parity.
from collections import Counter
def find_parity_outliers(nums):
return [
x for x in nums
if x % 2 != Counter([n % 2 for n in nums]).most_common()[0][0]
]
find_parity_outliers([1, 2, 3, 4, 6]) ## [1, 3]
flatten
- title: flatten
- tags: list,intermediate
Flattens a list of lists once.
- Use a list comprehension to extract each value from sub-lists in order.
def flatten(lst):
return [x for y in lst for x in y]
flatten([[1, 2, 3, 4], [5, 6, 7, 8]]) ## [1, 2, 3, 4, 5, 6, 7, 8]
for_each
- title: for_each
- tags: list,beginner
Executes the provided function once for each list element.
- Use a
for
loop to executefn
for each element initr
.
def for_each(itr, fn):
for el in itr:
fn(el)
for_each([1, 2, 3], print) ## 1 2 3
for_each_right
- title: for_each_right
- tags: list,beginner
Executes the provided function once for each list element, starting from the list's last element.
- Use a
for
loop in combination with slice notation to executefn
for each element initr
, starting from the last one.
def for_each_right(itr, fn):
for el in itr[::-1]:
fn(el)
for_each_right([1, 2, 3], print) ## 3 2 1
frequencies
- title: frequencies
- tags: list,intermediate
Creates a dictionary with the unique values of a list as keys and their frequencies as the values.
- Use
collections.defaultdict()
to store the frequencies of each unique element. - Use
dict()
to return a dictionary with the unique elements of the list as keys and their frequencies as the values.
from collections import defaultdict
def frequencies(lst):
freq = defaultdict(int)
for val in lst:
freq[val] += 1
return dict(freq)
frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']) ## { 'a': 4, 'b': 2, 'c': 1 }
from_iso_date
- title: from_iso_date
- tags: date,intermediate
Converts a date from its ISO-8601 representation.
- Use
datetime.datetime.fromisoformat()
to convert the given ISO-8601 date to adatetime.datetime
object.
from datetime import datetime
def from_iso_date(d):
return datetime.fromisoformat(d)
from_iso_date('2020-10-28T12:30:59.000000') ## 2020-10-28 12:30:59
gcd
- title: gcd
- tags: math,beginner
Calculates the greatest common divisor of a list of numbers.
- Use
functools.reduce()
andmath.gcd()
over the given list.
from functools import reduce
from math import gcd as _gcd
def gcd(numbers):
return reduce(_gcd, numbers)
gcd([8, 36, 28]) ## 4
geometric_progression
- title: geometric_progression
- tags: math,list,intermediate
Initializes a list containing the numbers in the specified range where start
and end
are inclusive and the ratio between two terms is step
.
Returns an error if step
equals 1
.
- Use
range()
,math.log()
andmath.floor()
and a list comprehension to create a list of the appropriate length, applying the step for each element. - Omit the second argument,
start
, to use a default value of1
. - Omit the third argument,
step
, to use a default value of2
.
from math import floor, log
def geometric_progression(end, start=1, step=2):
return [start * step ** i for i in range(floor(log(end / start)
/ log(step)) + 1)]
geometric_progression(256) ## [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometric_progression(256, 3) ## [3, 6, 12, 24, 48, 96, 192]
geometric_progression(256, 1, 4) ## [1, 4, 16, 64, 256]
get
- title: get
- tags: dictionary,list,intermediate
Retrieves the value of the nested key indicated by the given selector list from a dictionary or list.
- Use
functools.reduce()
to iterate over theselectors
list. - Apply
operator.getitem()
for each key inselectors
, retrieving the value to be used as the iteratee for the next iteration.
from functools import reduce
from operator import getitem
def get(d, selectors):
return reduce(getitem, selectors, d)
users = {
'freddy': {
'name': {
'first': 'fred',
'last': 'smith'
},
'postIds': [1, 2, 3]
}
}
get(users, ['freddy', 'name', 'last']) ## 'smith'
get(users, ['freddy', 'postIds', 1]) ## 2
group_by
- title: group_by
- tags: list,dictionary,intermediate
Groups the elements of a list based on the given function.
- Use
collections.defaultdict
to initialize a dictionary. - Use
fn
in combination with afor
loop anddict.append()
to populate the dictionary. - Use
dict()
to convert it to a regular dictionary.
from collections import defaultdict
def group_by(lst, fn):
d = defaultdict(list)
for el in lst:
d[fn(el)].append(el)
return dict(d)
from math import floor
group_by([6.1, 4.2, 6.3], floor) ## {4: [4.2], 6: [6.1, 6.3]}
group_by(['one', 'two', 'three'], len) ## {3: ['one', 'two'], 5: ['three']}
hamming_distance
- title: hamming_distance
- tags: math,intermediate
Calculates the Hamming distance between two values.
- Use the XOR operator (
^
) to find the bit difference between the two numbers. - Use
bin()
to convert the result to a binary string. - Convert the string to a list and use
list.count()
to count and return the number of1
s in it.
def hamming_distance(a, b):
return list(bin(a ^ b)).count('1')
hamming_distance(2, 3) ## 1
has_duplicates
- title: has_duplicates
- tags: list,beginner
Checks if there are duplicate values in a flat list.
- Use
set()
on the given list to remove duplicates, compare its length with the length of the list.
def has_duplicates(lst):
return len(lst) != len(set(lst))
x = [1, 2, 3, 4, 5, 5]
y = [1, 2, 3, 4, 5]
has_duplicates(x) ## True
has_duplicates(y) ## False
have_same_contents
- title: have_same_contents
- tags: list,intermediate
Checks if two lists contain the same elements regardless of order.
- Use
set()
on the combination of both lists to find the unique values. - Iterate over them with a
for
loop comparing thecount()
of each unique value in each list. - Return
False
if the counts do not match for any element,True
otherwise.
def have_same_contents(a, b):
for v in set(a + b):
if a.count(v) != b.count(v):
return False
return True
have_same_contents([1, 2, 4], [2, 4, 1]) ## True
head
- title: head
- tags: list,beginner
Returns the head of a list.
- Use
lst[0]
to return the first element of the passed list.
def head(lst):
return lst[0]
head([1, 2, 3]) ## 1
hex_to_rgb
- title: hex_to_rgb
- tags: string,math,intermediate
Converts a hexadecimal color code to a tuple of integers corresponding to its RGB components.
- Use a list comprehension in combination with
int()
and list slice notation to get the RGB components from the hexadecimal string. - Use
tuple()
to convert the resulting list to a tuple.
def hex_to_rgb(hex):
return tuple(int(hex[i:i+2], 16) for i in (0, 2, 4))
hex_to_rgb('FFA501') ## (255, 165, 1)
in_range
- title: in_range
- tags: math,beginner
Checks if the given number falls within the given range.
- Use arithmetic comparison to check if the given number is in the specified range.
- If the second parameter,
end
, is not specified, the range is considered to be from0
tostart
.
def in_range(n, start, end = 0):
return start <= n <= end if end >= start else end <= n <= start
in_range(3, 2, 5) ## True
in_range(3, 4) ## True
in_range(2, 3, 5) ## False
in_range(3, 2) ## False
includes_all
- title: includes_all
- tags: list,intermediate
Checks if all the elements in values
are included in lst
.
- Check if every value in
values
is contained inlst
using afor
loop. - Return
False
if any one value is not found,True
otherwise.
def includes_all(lst, values):
for v in values:
if v not in lst:
return False
return True
includes_all([1, 2, 3, 4], [1, 4]) ## True
includes_all([1, 2, 3, 4], [1, 5]) ## False
includes_any
- title: includes_any
- tags: list,intermediate
Checks if any element in values
is included in lst
.
- Check if any value in
values
is contained inlst
using afor
loop. - Return
True
if any one value is found,False
otherwise.
def includes_any(lst, values):
for v in values:
if v in lst:
return True
return False
includes_any([1, 2, 3, 4], [2, 9]) ## True
includes_any([1, 2, 3, 4], [8, 9]) ## False
index_of_all
- title: index_of_all
- tags: list,intermediate
Returns a list of indexes of all the occurrences of an element in a list.
- Use
enumerate()
and a list comprehension to check each element for equality withvalue
and addingi
to the result.
def index_of_all(lst, value):
return [i for i, x in enumerate(lst) if x == value]
index_of_all([1, 2, 1, 4, 5, 1], 1) ## [0, 2, 5]
index_of_all([1, 2, 3, 4], 6) ## []
initial
- title: initial
- tags: list,beginner
Returns all the elements of a list except the last one.
- Use
lst[:-1]
to return all but the last element of the list.
def initial(lst):
return lst[:-1]
initial([1, 2, 3]) ## [1, 2]
initialize_2d_list
- title: initialize_2d_list
- tags: list,intermediate
Initializes a 2D list of given width and height and value.
- Use a list comprehension and
range()
to generateh
rows where each is a list with lengthh
, initialized withval
. - Omit the last argument,
val
, to set the default value toNone
.
def initialize_2d_list(w, h, val = None):
return [[val for x in range(w)] for y in range(h)]
initialize_2d_list(2, 2, 0) ## [[0, 0], [0, 0]]
initialize_list_with_range
- title: initialize_list_with_range
- tags: list,beginner
Initializes a list containing the numbers in the specified range where start
and end
are inclusive with their common difference step
.
- Use
list()
andrange()
to generate a list of the appropriate length, filled with the desired values in the given range. - Omit
start
to use the default value of0
. - Omit
step
to use the default value of1
.
def initialize_list_with_range(end, start = 0, step = 1):
return list(range(start, end + 1, step))
initialize_list_with_range(5) ## [0, 1, 2, 3, 4, 5]
initialize_list_with_range(7, 3) ## [3, 4, 5, 6, 7]
initialize_list_with_range(9, 0, 2) ## [0, 2, 4, 6, 8]
initialize_list_with_values
- title: initialize_list_with_values
- tags: list,beginner
Initializes and fills a list with the specified value.
- Use a list comprehension and
range()
to generate a list of length equal ton
, filled with the desired values. - Omit
val
to use the default value of0
.
def initialize_list_with_values(n, val = 0):
return [val for x in range(n)]
initialize_list_with_values(5, 2) ## [2, 2, 2, 2, 2]
intersection
- title: intersection
- tags: list,beginner
Returns a list of elements that exist in both lists.
- Create a
set
froma
andb
. - Use the built-in set operator
&
to only keep values contained in both sets, then transform theset
back into alist
.
def intersection(a, b):
_a, _b = set(a), set(b)
return list(_a & _b)
intersection([1, 2, 3], [4, 3, 2]) ## [2, 3]
intersection_by
- title: intersection_by
- tags: list,function,intermediate
Returns a list of elements that exist in both lists, after applying the provided function to each list element of both.
- Create a
set
, usingmap()
to applyfn
to each element inb
. - Use a list comprehension in combination with
fn
ona
to only keep values contained in both lists.
def intersection_by(a, b, fn):
_b = set(map(fn, b))
return [item for item in a if fn(item) in _b]
from math import floor
intersection_by([2.1, 1.2], [2.3, 3.4], floor) ## [2.1]
invert_dictionary
- title: invert_dictionary
- tags: dictionary,intermediate
Inverts a dictionary with unique hashable values.
- Use
dictionary.items()
in combination with a list comprehension to create a new dictionary with the values and keys inverted.
def invert_dictionary(obj):
return { value: key for key, value in obj.items() }
ages = {
'Peter': 10,
'Isabel': 11,
'Anna': 9,
}
invert_dictionary(ages) ## { 10: 'Peter', 11: 'Isabel', 9: 'Anna' }
is_anagram
- title: is_anagram
- tags: string,intermediate
Checks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).
- Use
str.isalnum()
to filter out non-alphanumeric characters,str.lower()
to transform each character to lowercase. - Use
collections.Counter
to count the resulting characters for each string and compare the results.
from collections import Counter
def is_anagram(s1, s2):
return Counter(
c.lower() for c in s1 if c.isalnum()
) == Counter(
c.lower() for c in s2 if c.isalnum()
)
is_anagram('##anagram', 'Nag a ram!') ## True
is_contained_in
- title: is_contained_in
- tags: list,intermediate
Checks if the elements of the first list are contained in the second one regardless of order.
- Use
count()
to check if any value ina
has more occurrences than it has inb
. - Return
False
if any such value is found,True
otherwise.
def is_contained_in(a, b):
for v in set(a):
if a.count(v) > b.count(v):
return False
return True
is_contained_in([1, 4], [2, 4, 1]) ## True
is_divisible
- title: is_divisible
- tags: math,beginner unlisted: true
Checks if the first numeric argument is divisible by the second one.
- Use the modulo operator (
%
) to check if the remainder is equal to0
.
def is_divisible(dividend, divisor):
return dividend % divisor == 0
is_divisible(6, 3) ## True
is_even
- title: is_even
- tags: math,beginner unlisted: true
Checks if the given number is even.
- Check whether a number is odd or even using the modulo (
%
) operator. - Return
True
if the number is even,False
if the number is odd.
def is_even(num):
return num % 2 == 0
is_even(3) ## False
is_odd
- title: is_odd
- tags: math,beginner unlisted: true
Checks if the given number is odd.
- Checks whether a number is even or odd using the modulo (
%
) operator. - Returns
True
if the number is odd,False
if the number is even.
def is_odd(num):
return num % 2 != 0
is_odd(3) ## True
is_prime
- title: is_prime
- tags: math,intermediate
Checks if the provided integer is a prime number.
- Return
False
if the number is0
,1
, a negative number or a multiple of2
. - Use
all()
andrange()
to check numbers from3
to the square root of the given number. - Return
True
if none divides the given number,False
otherwise.
from math import sqrt
def is_prime(n):
if n <= 1 or (n % 2 == 0 and n > 2):
return False
return all(n % i for i in range(3, int(sqrt(n)) + 1, 2))
is_prime(11) ## True
is_weekday
- title: is_weekday
- tags: date,beginner
Checks if the given date is a weekday.
- Use
datetime.datetime.weekday()
to get the day of the week as an integer. - Check if the day of the week is less than or equal to
4
. - Omit the second argument,
d
, to use a default value ofdatetime.today()
.
from datetime import datetime
def is_weekday(d = datetime.today()):
return d.weekday() <= 4
from datetime import date
is_weekday(date(2020, 10, 25)) ## False
is_weekday(date(2020, 10, 28)) ## True
is_weekend
- title: is_weekend
- tags: date,beginner
Checks if the given date is a weekend.
- Use
datetime.datetime.weekday()
to get the day of the week as an integer. - Check if the day of the week is greater than
4
. - Omit the second argument,
d
, to use a default value ofdatetime.today()
.
from datetime import datetime
def is_weekend(d = datetime.today()):
return d.weekday() > 4
from datetime import date
is_weekend(date(2020, 10, 25)) ## True
is_weekend(date(2020, 10, 28)) ## False
kebab
- title: kebab
- tags: string,regexp,intermediate
Converts a string to kebab case.
- Use
re.sub()
to replace any-
or_
with a space, using the regexpr"(_|-)+"
. - Use
re.sub()
to match all words in the string,str.lower()
to lowercase them. - Finally, use
str.join()
to combine all word using-
as the separator.
from re import sub
def kebab(s):
return '-'.join(
sub(r"(\s|_|-)+"," ",
sub(r"[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+",
lambda mo: ' ' + mo.group(0).lower(), s)).split())
kebab('camelCase') ## 'camel-case'
kebab('some text') ## 'some-text'
kebab('some-mixed_string With spaces_underscores-and-hyphens')
## 'some-mixed-string-with-spaces-underscores-and-hyphens'
kebab('AllThe-small Things') ## 'all-the-small-things'
key_in_dict
- title: key_in_dict
- tags: dictionary,beginner
Checks if the given key exists in a dictionary.
- Use the
in
operator to check ifd
containskey
.
def key_in_dict(d, key):
return (key in d)
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
key_in_dict(d, 'three') ## True
key_of_max
- title: key_of_max
- tags: dictionary,beginner
Finds the key of the maximum value in a dictionary.
- Use
max()
with thekey
parameter set todict.get()
to find and return the key of the maximum value in the given dictionary.
def key_of_max(d):
return max(d, key = d.get)
key_of_max({'a':4, 'b':0, 'c':13}) ## c
key_of_min
- title: key_of_min
- tags: dictionary,beginner
Finds the key of the minimum value in a dictionary.
- Use
min()
with thekey
parameter set todict.get()
to find and return the key of the minimum value in the given dictionary.
def key_of_min(d):
return min(d, key = d.get)
key_of_min({'a':4, 'b':0, 'c':13}) ## b
keys_only
- title: keys_only
- tags: dictionary,list,beginner
Creates a flat list of all the keys in a flat dictionary.
- Use
dict.keys()
to return the keys in the given dictionary. - Return a
list()
of the previous result.
def keys_only(flat_dict):
return list(flat_dict.keys())
ages = {
'Peter': 10,
'Isabel': 11,
'Anna': 9,
}
keys_only(ages) ## ['Peter', 'Isabel', 'Anna']
km_to_miles
- title: km_to_miles
- tags: math,beginner unlisted: true
Converts kilometers to miles.
- Follows the conversion formula
mi = km * 0.621371
.
def km_to_miles(km):
return km * 0.621371
km_to_miles(8.1) ## 5.0331051
last
- title: last
- tags: list,beginner
Returns the last element in a list.
- Use
lst[-1]
to return the last element of the passed list.
def last(lst):
return lst[-1]
last([1, 2, 3]) ## 3
lcm
- title: lcm
- tags: math,list,intermediate
Returns the least common multiple of a list of numbers.
- Use
functools.reduce()
,math.gcd()
andlcm(x,y) = x * y / gcd(x,y)
over the given list.
from functools import reduce
from math import gcd
def lcm(numbers):
return reduce((lambda x, y: int(x * y / gcd(x, y))), numbers)
lcm([12, 7]) ## 84
lcm([1, 3, 4, 5]) ## 60
longest_item
- title: longest_item
- tags: list,string,intermediate
Takes any number of iterable objects or objects with a length property and returns the longest one.
- Use
max()
withlen()
as thekey
to return the item with the greatest length. - If multiple objects have the same length, the first one will be returned.
def longest_item(*args):
return max(args, key = len)
longest_item('this', 'is', 'a', 'testcase') ## 'testcase'
longest_item([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]) ## [1, 2, 3, 4, 5]
longest_item([1, 2, 3], 'foobar') ## 'foobar'
map_dictionary
- title: map_dictionary
- tags: list,dictionary,intermediate
Maps the values of a list to a dictionary using a function, where the key-value pairs consist of the original value as the key and the result of the function as the value.
- Use
map()
to applyfn
to each value of the list. - Use
zip()
to pair original values to the values produced byfn
. - Use
dict()
to return an appropriate dictionary.
def map_dictionary(itr, fn):
return dict(zip(itr, map(fn, itr)))
map_dictionary([1, 2, 3], lambda x: x * x) ## { 1: 1, 2: 4, 3: 9 }
map_values
- title: map_values
- tags: dictionary,intermediate
Creates a dictionary with the same keys as the provided dictionary and values generated by running the provided function for each value.
- Use
dict.items()
to iterate over the dictionary, assigning the values produced byfn
to each key of a new dictionary.
def map_values(obj, fn):
return dict((k, fn(v)) for k, v in obj.items())
users = {
'fred': { 'user': 'fred', 'age': 40 },
'pebbles': { 'user': 'pebbles', 'age': 1 }
}
map_values(users, lambda u : u['age']) ## {'fred': 40, 'pebbles': 1}
max_by
- title: max_by
- tags: math,list,beginner
Returns the maximum value of a list, after mapping each element to a value using the provided function.
- Use
map()
withfn
to map each element to a value using the provided function. - Use
max()
to return the maximum value.
def max_by(lst, fn):
return max(map(fn, lst))
max_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) ## 8
max_element_index
- title: max_element_index
- tags: math,list,beginner
Returns the index of the element with the maximum value in a list.
- Use
max()
andlist.index()
to get the maximum value in the list and return its index.
def max_element_index(arr):
return arr.index(max(arr))
max_element_index([5, 8, 9, 7, 10, 3, 0]) ## 4
max_n
- title: max_n
- tags: list,math,beginner
Returns the n
maximum elements from the provided list.
- Use
sorted()
to sort the list. - Use slice notation to get the specified number of elements.
- Omit the second argument,
n
, to get a one-element list. - If
n
is greater than or equal to the provided list's length, then return the original list (sorted in descending order).
def max_n(lst, n = 1):
return sorted(lst, reverse = True)[:n]
max_n([1, 2, 3]) ## [3]
max_n([1, 2, 3], 2) ## [3, 2]
median
- title: median
- tags: math,beginner
Finds the median of a list of numbers.
- Sort the numbers of the list using
list.sort()
. - Find the median, which is either the middle element of the list if the list length is odd or the average of the two middle elements if the list length is even.
statistics.median()
provides similar functionality to this snippet.
def median(list):
list.sort()
list_length = len(list)
if list_length % 2 == 0:
return (list[int(list_length / 2) - 1] + list[int(list_length / 2)]) / 2
return float(list[int(list_length / 2)])
median([1, 2, 3]) ## 2.0
median([1, 2, 3, 4]) ## 2.5
merge
- title: merge
- tags: list,advanced
Merges two or more lists into a list of lists, combining elements from each of the input lists based on their positions.
- Use
max()
combined with a list comprehension to get the length of the longest list in the arguments. - Use
range()
in combination with themax_length
variable to loop as many times as there are elements in the longest list. - If a list is shorter than
max_length
, usefill_value
for the remaining items (defaults toNone
). zip()
anditertools.zip_longest()
provide similar functionality to this snippet.
def merge(*args, fill_value = None):
max_length = max([len(lst) for lst in args])
result = []
for i in range(max_length):
result.append([
args[k][i] if i < len(args[k]) else fill_value for k in range(len(args))
])
return result
merge(['a', 'b'], [1, 2], [True, False]) ## [['a', 1, True], ['b', 2, False]]
merge(['a'], [1, 2], [True, False]) ## [['a', 1, True], [None, 2, False]]
merge(['a'], [1, 2], [True, False], fill_value = '_')
## [['a', 1, True], ['_', 2, False]]
merge_dictionaries
- title: merge_dictionaries
- tags: dictionary,intermediate
Merges two or more dictionaries.
- Create a new
dict
and loop overdicts
, usingdictionary.update()
to add the key-value pairs from each one to the result.
def merge_dictionaries(*dicts):
res = dict()
for d in dicts:
res.update(d)
return res
ages_one = {
'Peter': 10,
'Isabel': 11,
}
ages_two = {
'Anna': 9
}
merge_dictionaries(ages_one, ages_two)
## { 'Peter': 10, 'Isabel': 11, 'Anna': 9 }
miles_to_km
- title: miles_to_km
- tags: math,beginner unlisted: true
Converts miles to kilometers.
- Follows the conversion formula
km = mi * 1.609344
.
def miles_to_km(miles):
return miles * 1.609344
miles_to_km(5.03) ## 8.09500032
min_by
- title: min_by
- tags: math,list,beginner
Returns the minimum value of a list, after mapping each element to a value using the provided function.
- Use
map()
withfn
to map each element to a value using the provided function. - Use
min()
to return the minimum value.
def min_by(lst, fn):
return min(map(fn, lst))
min_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) ## 2
min_element_index
- title: min_element_index
- tags: math,list,beginner
Returns the index of the element with the minimum value in a list.
- Use
min()
andlist.index()
to obtain the minimum value in the list and then return its index.
def min_element_index(arr):
return arr.index(min(arr))
min_element_index([3, 5, 2, 6, 10, 7, 9]) ## 2
min_n
- title: min_n
- tags: list,math,beginner
Returns the n
minimum elements from the provided list.
- Use
sorted()
to sort the list. - Use slice notation to get the specified number of elements.
- Omit the second argument,
n
, to get a one-element list. - If
n
is greater than or equal to the provided list's length, then return the original list (sorted in ascending order).
def min_n(lst, n = 1):
return sorted(lst, reverse = False)[:n]
min_n([1, 2, 3]) ## [1]
min_n([1, 2, 3], 2) ## [1, 2]
months_diff
- title: months_diff
- tags: date,beginner
Calculates the month difference between two dates.
- Subtract
start
fromend
and usedatetime.timedelta.days
to get the day difference. - Divide by
30
and usemath.ceil()
to get the difference in months (rounded up).
from math import ceil
def months_diff(start, end):
return ceil((end - start).days / 30)
from datetime import date
months_diff(date(2020, 10, 28), date(2020, 11, 25)) ## 1
most_frequent
- title: most_frequent
- tags: list,beginner
Returns the most frequent element in a list.
- Use
set()
to get the unique values inlst
. - Use
max()
to find the element that has the most appearances.
def most_frequent(lst):
return max(set(lst), key = lst.count)
most_frequent([1, 2, 1, 2, 3, 2, 1, 4, 2]) ##2
n_times_string
- title: n_times_string
- tags: string,beginner
Generates a string with the given string value repeated n
number of times.
- Repeat the string
n
times, using the*
operator.
def n_times_string(s, n):
return (s * n)
n_times_string('py', 4) ##'pypypypy'
none
- title: none
- tags: list,intermediate
Checks if the provided function returns True
for at least one element in the list.
- Use
all()
andfn
to check iffn
returnsFalse
for all the elements in the list.
def none(lst, fn = lambda x: x):
return all(not fn(x) for x in lst)
none([0, 1, 2, 0], lambda x: x >= 2 ) ## False
none([0, 0, 0]) ## True
num_to_range
- title: num_to_range
- tags: math,beginner
Maps a number from one range to another range.
- Return
num
mapped betweenoutMin
-outMax
frominMin
-inMax
.
def num_to_range(num, inMin, inMax, outMin, outMax):
return outMin(float(num - inMin) / float(inMax - inMin) * (outMax
- outMin))
num_to_range(5, 0, 10, 0, 100) ## 50.0
offset
- title: offset
- tags: list,beginner
Moves the specified amount of elements to the end of the list.
- Use slice notation to get the two slices of the list and combine them before returning.
def offset(lst, offset):
return lst[offset:] + lst[:offset]
offset([1, 2, 3, 4, 5], 2) ## [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2) ## [4, 5, 1, 2, 3]
pad
- title: pad
- tags: string,beginner
Pads a string on both sides with the specified character, if it's shorter than the specified length.
- Use
str.ljust()
andstr.rjust()
to pad both sides of the given string. - Omit the third argument,
char
, to use the whitespace character as the default padding character.
from math import floor
def pad(s, length, char = ' '):
return s.rjust(floor((len(s) + length)/2), char).ljust(length, char)
pad('cat', 8) ## ' cat '
pad('42', 6, '0') ## '004200'
pad('foobar', 3) ## 'foobar'
pad_number
- title: pad_number
- tags: string,math,beginner
Pads a given number to the specified length.
- Use
str.zfill()
to pad the number to the specified length, after converting it to a string.
def pad_number(n, l):
return str(n).zfill(l)
pad_number(1234, 6); ## '001234'
palindrome
- title: palindrome
- tags: string,intermediate
Checks if the given string is a palindrome.
- Use
str.lower()
andre.sub()
to convert to lowercase and remove non-alphanumeric characters from the given string. - Then, compare the new string with its reverse, using slice notation.
from re import sub
def palindrome(s):
s = sub('[\W_]', '', s.lower())
return s == s[::-1]
palindrome('taco cat') ## True
pluck
- title: pluck
- tags: list,dictionary,beginner
Converts a list of dictionaries into a list of values corresponding to the specified key
.
- Use a list comprehension and
dict.get()
to get the value ofkey
for each dictionary inlst
.
def pluck(lst, key):
return [x.get(key) for x in lst]
simpsons = [
{ 'name': 'lisa', 'age': 8 },
{ 'name': 'homer', 'age': 36 },
{ 'name': 'marge', 'age': 34 },
{ 'name': 'bart', 'age': 10 }
]
pluck(simpsons, 'age') ## [8, 36, 34, 10]
powerset
- title: powerset
- tags: math,list,advanced
Returns the powerset of a given iterable.
- Use
list()
to convert the given value to a list. - Use
range()
anditertools.combinations()
to create a generator that returns all subsets. - Use
itertools.chain.from_iterable()
andlist()
to consume the generator and return a list.
from itertools import chain, combinations
def powerset(iterable):
s = list(iterable)
return list(chain.from_iterable(combinations(s, r) for r in range(len(s)+1)))
powerset([1, 2]) ## [(), (1,), (2,), (1, 2)]
rads_to_degrees
- title: rads_to_degrees
- tags: math,beginner
Converts an angle from radians to degrees.
- Use
math.pi
and the radian to degree formula to convert the angle from radians to degrees.
from math import pi
def rads_to_degrees(rad):
return (rad * 180.0) / pi
from math import pi
rads_to_degrees(pi / 2) ## 90.0
reverse
- title: reverse
- tags: list,string,beginner
Reverses a list or a string.
- Use slice notation to reverse the list or string.
def reverse(itr):
return itr[::-1]
reverse([1, 2, 3]) ## [3, 2, 1]
reverse('snippet') ## 'teppins'
reverse_number
- title: reverse_number
- tags: math,intermediate
Reverses a number.
- Use
str()
to convert the number to a string, slice notation to reverse it andstr.replace()
to remove the sign. - Use
float()
to convert the result to a number andmath.copysign()
to copy the original sign.
from math import copysign
def reverse_number(n):
return copysign(float(str(n)[::-1].replace('-', '')), n)
reverse_number(981) ## 189
reverse_number(-500) ## -5
reverse_number(73.6) ## 6.37
reverse_number(-5.23) ## -32.5
rgb_to_hex
- title: rgb_to_hex
- tags: string,math,intermediate
Converts the values of RGB components to a hexadecimal color code.
- Create a placeholder for a zero-padded hexadecimal value using
'{:02X}'
and copy it three times. - Use
str.format()
on the resulting string to replace the placeholders with the given values.
def rgb_to_hex(r, g, b):
return ('{:02X}' * 3).format(r, g, b)
rgb_to_hex(255, 165, 1) ## 'FFA501'
roll
- title: roll
- tags: list,beginner
Moves the specified amount of elements to the start of the list.
- Use slice notation to get the two slices of the list and combine them before returning.
def roll(lst, offset):
return lst[-offset:] + lst[:-offset]
roll([1, 2, 3, 4, 5], 2) ## [4, 5, 1, 2, 3]
roll([1, 2, 3, 4, 5], -2) ## [3, 4, 5, 1, 2]
sample
- title: sample
- tags: list,random,beginner
Returns a random element from a list.
- Use
random.choice()
to get a random element fromlst
.
from random import choice
def sample(lst):
return choice(lst)
sample([3, 7, 9, 11]) ## 9
shuffle
- title: shuffle
- tags: list,random,advanced
Randomizes the order of the values of an list, returning a new list.
- Uses the Fisher-Yates algorithm to reorder the elements of the list.
random.shuffle
provides similar functionality to this snippet.
from copy import deepcopy
from random import randint
def shuffle(lst):
temp_lst = deepcopy(lst)
m = len(temp_lst)
while (m):
m -= 1
i = randint(0, m)
temp_lst[m], temp_lst[i] = temp_lst[i], temp_lst[m]
return temp_lst
foo = [1, 2, 3]
shuffle(foo) ## [2, 3, 1], foo = [1, 2, 3]
similarity
- title: similarity
- tags: list,beginner
Returns a list of elements that exist in both lists.
- Use a list comprehension on
a
to only keep values contained in both lists.
def similarity(a, b):
return [item for item in a if item in b]
similarity([1, 2, 3], [1, 2, 4]) ## [1, 2]
slugify
- title: slugify
- tags: string,regexp,intermediate
Converts a string to a URL-friendly slug.
- Use
str.lower()
andstr.strip()
to normalize the input string. - Use
re.sub()
to to replace spaces, dashes and underscores with-
and remove special characters.
import re
def slugify(s):
s = s.lower().strip()
s = re.sub(r'[^\w\s-]', '', s)
s = re.sub(r'[\s_-]+', '-', s)
s = re.sub(r'^-+|-+$', '', s)
return s
slugify('Hello World!') ## 'hello-world'
snake
- title: snake
- tags: string,regexp,intermediate
Converts a string to snake case.
- Use
re.sub()
to match all words in the string,str.lower()
to lowercase them. - Use
re.sub()
to replace any-
characters with spaces. - Finally, use
str.join()
to combine all words using-
as the separator.
from re import sub
def snake(s):
return '_'.join(
sub('([A-Z][a-z]+)', r' \1',
sub('([A-Z]+)', r' \1',
s.replace('-', ' '))).split()).lower()
snake('camelCase') ## 'camel_case'
snake('some text') ## 'some_text'
snake('some-mixed_string With spaces_underscores-and-hyphens')
## 'some_mixed_string_with_spaces_underscores_and_hyphens'
snake('AllThe-small Things') ## 'all_the_small_things'
some
- title: some
- tags: list,intermediate
Checks if the provided function returns True
for at least one element in the list.
- Use
any()
in combination withmap()
to check iffn
returnsTrue
for any element in the list.
def some(lst, fn = lambda x: x):
return any(map(fn, lst))
some([0, 1, 2, 0], lambda x: x >= 2 ) ## True
some([0, 0, 1, 0]) ## True
sort_by_indexes
- title: sort_by_indexes
- tags: list,intermediate
Sorts one list based on another list containing the desired indexes.
- Use
zip()
andsorted()
to combine and sort the two lists, based on the values ofindexes
. - Use a list comprehension to get the first element of each pair from the result.
- Use the
reverse
parameter insorted()
to sort the dictionary in reverse order, based on the third argument.
def sort_by_indexes(lst, indexes, reverse=False):
return [val for (_, val) in sorted(zip(indexes, lst), key=lambda x: \
x[0], reverse=reverse)]
a = ['eggs', 'bread', 'oranges', 'jam', 'apples', 'milk']
b = [3, 2, 6, 4, 1, 5]
sort_by_indexes(a, b) ## ['apples', 'bread', 'eggs', 'jam', 'milk', 'oranges']
sort_by_indexes(a, b, True)
## ['oranges', 'milk', 'jam', 'eggs', 'bread', 'apples']
sort_dict_by_key
- title: sort_dict_by_key
- tags: dictionary,intermediate
Sorts the given dictionary by key.
- Use
dict.items()
to get a list of tuple pairs fromd
and sort it usingsorted()
. - Use
dict()
to convert the sorted list back to a dictionary. - Use the
reverse
parameter insorted()
to sort the dictionary in reverse order, based on the second argument.
def sort_dict_by_key(d, reverse = False):
return dict(sorted(d.items(), reverse = reverse))
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
sort_dict_by_key(d) ## {'five': 5, 'four': 4, 'one': 1, 'three': 3, 'two': 2}
sort_dict_by_key(d, True)
## {'two': 2, 'three': 3, 'one': 1, 'four': 4, 'five': 5}
sort_dict_by_value
- title: sort_dict_by_value
- tags: dictionary,intermediate
Sorts the given dictionary by value.
- Use
dict.items()
to get a list of tuple pairs fromd
and sort it using a lambda function andsorted()
. - Use
dict()
to convert the sorted list back to a dictionary. - Use the
reverse
parameter insorted()
to sort the dictionary in reverse order, based on the second argument. - ⚠️ NOTICE: Dictionary values must be of the same type.
def sort_dict_by_value(d, reverse = False):
return dict(sorted(d.items(), key = lambda x: x[1], reverse = reverse))
d = {'one': 1, 'three': 3, 'five': 5, 'two': 2, 'four': 4}
sort_dict_by_value(d) ## {'one': 1, 'two': 2, 'three': 3, 'four': 4, 'five': 5}
sort_dict_by_value(d, True)
## {'five': 5, 'four': 4, 'three': 3, 'two': 2, 'one': 1}
split_lines
- title: split_lines
- tags: string,beginner
Splits a multiline string into a list of lines.
- Use
str.split()
and'\n'
to match line breaks and create a list. str.splitlines()
provides similar functionality to this snippet.
def split_lines(s):
return s.split('\n')
split_lines('This\nis a\nmultiline\nstring.\n')
## ['This', 'is a', 'multiline', 'string.' , '']
spread
- title: spread
- tags: list,intermediate
Flattens a list, by spreading its elements into a new list.
- Loop over elements, use
list.extend()
if the element is a list,list.append()
otherwise.
def spread(arg):
ret = []
for i in arg:
ret.extend(i) if isinstance(i, list) else ret.append(i)
return ret
spread([1, 2, 3, [4, 5, 6], [7], 8, 9]) ## [1, 2, 3, 4, 5, 6, 7, 8, 9]
sum_by
- title: sum_by
- tags: math,list,beginner
Calculates the sum of a list, after mapping each element to a value using the provided function.
- Use
map()
withfn
to map each element to a value using the provided function. - Use
sum()
to return the sum of the values.
def sum_by(lst, fn):
return sum(map(fn, lst))
sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) ## 20
sum_of_powers
- title: sum_of_powers
- tags: math,intermediate
Returns the sum of the powers of all the numbers from start
to end
(both inclusive).
- Use
range()
in combination with a list comprehension to create a list of elements in the desired range raised to the givenpower
. - Use
sum()
to add the values together. - Omit the second argument,
power
, to use a default power of2
. - Omit the third argument,
start
, to use a default starting value of1
.
def sum_of_powers(end, power = 2, start = 1):
return sum([(i) ** power for i in range(start, end + 1)])
sum_of_powers(10) ## 385
sum_of_powers(10, 3) ## 3025
sum_of_powers(10, 3, 5) ## 2925
symmetric_difference
- title: symmetric_difference
- tags: list,intermediate
Returns the symmetric difference between two iterables, without filtering out duplicate values.
- Create a
set
from each list. - Use a list comprehension on each of them to only keep values not contained in the previously created set of the other.
def symmetric_difference(a, b):
(_a, _b) = (set(a), set(b))
return [item for item in a if item not in _b] + [item for item in b
if item not in _a]
symmetric_difference([1, 2, 3], [1, 2, 4]) ## [3, 4]
symmetric_difference_by
- title: symmetric_difference_by
- tags: list,intermediate
Returns the symmetric difference between two lists, after applying the provided function to each list element of both.
- Create a
set
by applyingfn
to each element in every list. - Use a list comprehension in combination with
fn
on each of them to only keep values not contained in the previously created set of the other.
def symmetric_difference_by(a, b, fn):
(_a, _b) = (set(map(fn, a)), set(map(fn, b)))
return [item for item in a if fn(item) not in _b] + [item
for item in b if fn(item) not in _a]
from math import floor
symmetric_difference_by([2.1, 1.2], [2.3, 3.4], floor) ## [1.2, 3.4]
tail
- title: tail
- tags: list,beginner
Returns all elements in a list except for the first one.
- Use slice notation to return the last element if the list's length is more than
1
. - Otherwise, return the whole list.
def tail(lst):
return lst[1:] if len(lst) > 1 else lst
tail([1, 2, 3]) ## [2, 3]
tail([1]) ## [1]
take
- title: take
- tags: list,beginner
Returns a list with n
elements removed from the beginning.
- Use slice notation to create a slice of the list with
n
elements taken from the beginning.
def take(itr, n = 1):
return itr[:n]
take([1, 2, 3], 5) ## [1, 2, 3]
take([1, 2, 3], 0) ## []
take_right
- title: take_right
- tags: list,beginner
Returns a list with n
elements removed from the end.
- Use slice notation to create a slice of the list with
n
elements taken from the end.
def take_right(itr, n = 1):
return itr[-n:]
take_right([1, 2, 3], 2) ## [2, 3]
take_right([1, 2, 3]) ## [3]
to_binary
- title: to_binary
- tags: math,beginner
Returns the binary representation of the given number.
- Use
bin()
to convert a given decimal number into its binary equivalent.
def to_binary(n):
return bin(n)
to_binary(100) ## 0b1100100
to_dictionary
- title: to_dictionary
- tags: list,dictionary,intermediate
Combines two lists into a dictionary, where the elements of the first one serve as the keys and the elements of the second one serve as the values. The values of the first list need to be unique and hashable.
- Use
zip()
in combination withdict()
to combine the values of the two lists into a dictionary.
def to_dictionary(keys, values):
return dict(zip(keys, values))
to_dictionary(['a', 'b'], [1, 2]) ## { a: 1, b: 2 }
to_hex
- title: to_hex
- tags: math,beginner
Returns the hexadecimal representation of the given number.
- Use
hex()
to convert a given decimal number into its hexadecimal equivalent.
def to_hex(dec):
return hex(dec)
to_hex(41) ## 0x29
to_hex(332) ## 0x14c
to_iso_date
- title: to_iso_date
- tags: date,intermediate
Converts a date to its ISO-8601 representation.
- Use
datetime.datetime.isoformat()
to convert the givendatetime.datetime
object to an ISO-8601 date.
from datetime import datetime
def to_iso_date(d):
return d.isoformat()
from datetime import datetime
to_iso_date(datetime(2020, 10, 25)) ## 2020-10-25T00:00:00
to_roman_numeral
- title: to_roman_numeral
- tags: math,string,intermediate
Converts an integer to its roman numeral representation.
Accepts value between 1
and 3999
(both inclusive).
- Create a lookup list containing tuples in the form of (roman value, integer).
- Use a
for
loop to iterate over the values inlookup
. - Use
divmod()
to updatenum
with the remainder, adding the roman numeral representation to the result.
def to_roman_numeral(num):
lookup = [
(1000, 'M'),
(900, 'CM'),
(500, 'D'),
(400, 'CD'),
(100, 'C'),
(90, 'XC'),
(50, 'L'),
(40, 'XL'),
(10, 'X'),
(9, 'IX'),
(5, 'V'),
(4, 'IV'),
(1, 'I'),
]
res = ''
for (n, roman) in lookup:
(d, num) = divmod(num, n)
res += roman * d
return res
to_roman_numeral(3) ## 'III'
to_roman_numeral(11) ## 'XI'
to_roman_numeral(1998) ## 'MCMXCVIII'
transpose
- title: transpose
- tags: list,intermediate
Transposes a two-dimensional list.
- Use
*lst
to get the provided list as tuples. - Use
zip()
in combination withlist()
to create the transpose of the given two-dimensional list.
def transpose(lst):
return list(zip(*lst))
transpose([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
## [(1, 4, 7, 10), (2, 5, 8, 11), (3, 6, 9, 12)]
unfold
- title: unfold
- tags: function,list,advanced
Builds a list, using an iterator function and an initial seed value.
- The iterator function accepts one argument (
seed
) and must always return a list with two elements ([value
,nextSeed
]) orFalse
to terminate. - Use a generator function,
fn_generator
, that uses awhile
loop to call the iterator function andyield
thevalue
until it returnsFalse
. - Use a list comprehension to return the list that is produced by the generator, using the iterator function.
def unfold(fn, seed):
def fn_generator(val):
while True:
val = fn(val[1])
if val == False: break
yield val[0]
return [i for i in fn_generator([None, seed])]
f = lambda n: False if n > 50 else [-n, n + 10]
unfold(f, 10) ## [-10, -20, -30, -40, -50]
union
- title: union
- tags: list,beginner
Returns every element that exists in any of the two lists once.
- Create a
set
with all values ofa
andb
and convert to alist
.
def union(a, b):
return list(set(a + b))
union([1, 2, 3], [4, 3, 2]) ## [1, 2, 3, 4]
union_by
- title: union_by
- tags: list,intermediate
Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.
- Create a
set
by applyingfn
to each element ina
. - Use a list comprehension in combination with
fn
onb
to only keep values not contained in the previously created set,_a
. - Finally, create a
set
from the previous result anda
and transform it into alist
def union_by(a, b, fn):
_a = set(map(fn, a))
return list(set(a + [item for item in b if fn(item) not in _a]))
from math import floor
union_by([2.1], [1.2, 2.3], floor) ## [2.1, 1.2]
unique_elements
- title: unique_elements
- tags: list,beginner
Returns the unique elements in a given list.
- Create a
set
from the list to discard duplicated values, then return alist
from it.
def unique_elements(li):
return list(set(li))
unique_elements([1, 2, 2, 3, 4, 3]) ## [1, 2, 3, 4]
values_only
- title: values_only
- tags: dictionary,list,beginner
Returns a flat list of all the values in a flat dictionary.
- Use
dict.values()
to return the values in the given dictionary. - Return a
list()
of the previous result.
def values_only(flat_dict):
return list(flat_dict.values())
ages = {
'Peter': 10,
'Isabel': 11,
'Anna': 9,
}
values_only(ages) ## [10, 11, 9]
weighted_average
- title: weighted_average
- tags: math,list,intermediate
Returns the weighted average of two or more numbers.
- Use
sum()
to sum the products of the numbers by their weight and to sum the weights. - Use
zip()
and a list comprehension to iterate over the pairs of values and weights.
def weighted_average(nums, weights):
return sum(x * y for x, y in zip(nums, weights)) / sum(weights)
weighted_average([1, 2, 3], [0.6, 0.2, 0.3]) ## 1.72727
when
- title: when
- tags: function,intermediate
Tests a value, x
, against a testing function, conditionally applying a function.
- Check if the value of
predicate(x)
isTrue
and if so returnwhen_true(x)
, otherwise returnx
.
def when(predicate, when_true):
return lambda x: when_true(x) if predicate(x) else x
double_even_numbers = when(lambda x: x % 2 == 0, lambda x : x * 2)
double_even_numbers(2) ## 4
double_even_numbers(1) ## 1
words
- title: words
- tags: string,regexp,beginner
Converts a given string into a list of words.
- Use
re.findall()
with the suppliedpattern
to find all matching substrings. - Omit the second argument to use the default regexp, which matches alphanumeric and hyphens.
import re
def words(s, pattern = '[a-zA-Z-]+'):
return re.findall(pattern, s)
words('I love Python!!') ## ['I', 'love', 'Python']
words('python, javaScript & coffee') ## ['python', 'javaScript', 'coffee']
words('build -q --out one-item', r'\b[a-zA-Z-]+\b')
## ['build', 'q', 'out', 'one-item']
All
- title: All
- tags: array,list,lambda,overload,intermediate
Returns true
if the provided predicate function returns true
for all elements in a collection, false
otherwise.
- Use
IEnumerable.ToArray()
,Array.TrueForAll()
to test if all elements in the collection returntrue
based on the predicate function,match
. - Omit the predicate function,
match
, to use the overload that checks if each value is different fromnull
by default.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static bool All<T>(IEnumerable<T> data, Predicate<T> match)
{
return Array.TrueForAll(data.ToArray(), match);
}
public static bool All<T>(IEnumerable<T> data)
{
return Array.TrueForAll(data.ToArray(), val => val != null);
}
}
int[] nums = { 4, 2, 3 };
_30s.All(nums, x => x > 1); // true
_30s.All(nums); // true
AverageBy
- title: AverageBy
- tags: math,list,array,lambda,intermediate
Returns the average of a collection, after mapping each element to a value using the provided function.
- Use
IEnumerable.Select()
to map each element to the value returned by the provided selector function,fn
. - Use
IEnumerable.Average()
to get the average of the resulting values.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static double AverageBy<T>(IEnumerable<T> values, Func<T,int> fn)
{
return values.Select(fn).Average();
}
}
var p = new [] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
_30s.AverageBy(p, v => v.a); // 2.5
_30s.AverageBy(p, v => v.b); // 1.5
Bifurcate
- title: Bifurcate
- tags: array,list,intermediate
Splits values into two groups.
If an element in filter
is true
, the corresponding element in the collection belongs to the first group; otherwise, it belongs to the second group.
- Use
IEnumerable.Where()
to separate values into two groups and assign them to the two passedout
arrays.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static void Bifurcate<T>(IEnumerable<T> items, IList<bool> filter, out T[] filteredTrue, out T[] filteredFalse)
{
filteredTrue = items.Where((val, i) => filter[i] == true).ToArray();
filteredFalse = items.Where((val, i) => filter[i] == false).ToArray();
}
}
int[] nums = {1, 2, 3, 4};
bool[] filter = {true, true, false, true};
int[] n1;
int[] n2;
_30s.Bifurcate(nums, filter, out n1, out n2); // // n1 = {1, 2, 4}, n2 = {3}
BifurcateBy
- title: BifurcateBy
- tags: array,list,lambda,advanced
Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.
- Use
IEnumerable.Where()
to separate values into two groups and assign them to the two passedout
arrays.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static void BifurcateBy<T>(IEnumerable<T> items, Predicate<T> filter, out T[] filteredTrue, out T[] filteredFalse)
{
filteredTrue = items.Where(i => filter(i) == true).ToArray();
filteredFalse = items.Where(i => filter(i) == false).ToArray();
}
}
int[] nums = {1, 2, 3, 4};
int[] n1;
int[] n2;
_30s.BifurcateBy(nums, x => x % 2 == 0, out n1, out n2); // n1 = {2, 4}, n2 = {1, 3}
ByteArrayToHex
- title: ByteArrayToHex
- tags: array,utility,beginner
Converts a byte
array to its hexadecimal string representation.
- Use
BitConverter.ToString()
to convert thebyte
array to a string. - Use
string.Replace()
to remove dashes in the produced string.
public static partial class _30s
{
public static string ByteArrayToHex(byte[] bytes)
{
return BitConverter.ToString(bytes).Replace("-", "");
}
}
byte[] data = { 241, 89, 54 };
_30s.ByteArrayToHex(data); // "F15936"
Capitalize
- title: Capitalize
- tags: string,beginner
Capitalizes the first letter of a string.
- Use
string.ToCharArray()
to convert the string to an array ofchar
,chars
. - Use
char.ToUpper(chars[0])
to capitalize the first letter. - Finally, return a
new string()
from thechars
array.
public static partial class _30s
{
public static string Capitalize(string str)
{
char[] chars = str.ToCharArray();
chars[0] = char.ToUpper(chars[0]);
return new string(chars);
}
}
string s = "fooBar";
_30s.Capitalize(s); // "FooBar"
Chunk
- title: Chunk
- tags: array,list,lambda,advanced
Chunks a collection into smaller lists of a specified size.
- Use
IEnumerable.Select()
to convert the given list to index-value pairs. - Use
IEnumerable.GroupBy()
to split elements into groups based on their index. - Use
IEnumerable.Select()
a second time to map each group's elements to their values andIEnumerable.ToList()
to convert the result to a list. - Finally, use
IEnumerable.ToList()
on the result to convert everything to a list and return it. - If the original list can't be split evenly, the final chunk will contain the remaining elements.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static List<List<T>> Chunk<T>(IEnumerable<T> data, int size)
{
return data
.Select((x, i) => new { Index = i, Value = x })
.GroupBy(x => x.Index / size)
.Select(x => x.Select(v => v.Value).ToList())
.ToList();
}
}
List<int> nums = new List<int> { 1, 2, 3, 4, 5 };
_30s.Chunk(nums, 2); // { {1, 2}, {3, 4}, {5} }
CompactWhitespace
- title: CompactWhitespace
- tags: string,regex,intermediate
Returns a string with whitespaces compacted.
- Use
Regex.Replace()
with a regular expression to replace all occurences of 2 or more subsequent whitespace characters with a single space.
using System.Text.RegularExpressions;
public static partial class _30s
{
public static string CompactWhitespace(string str)
{
return Regex.Replace(str, @"\s{2,}", " ");
}
}
string s = "Lorem ipsum\n dolor sit amet";
_30s.CompactWhitespace(s); // "Lorem ipsum dolor sit amet"
CountBy
- title: CountBy
- tags: array,list,lambda,intermediate
Groups the elements of a collection based on the given function and returns the count of elements in each group.
- Use
IEnumerable.GroupBy()
to create groups for each distinct value in the collection, after applying the provided function. - Use
IEnumerable.ToDictionary()
to convert the result of the previous operation to aDictionary
.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static Dictionary<R,int> CountBy<T,R>(IEnumerable<T> values, Func<T,R> map)
{
return values
.GroupBy(map)
.ToDictionary(v => v.Key, v => v.Count());
}
}
var p = new[] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
_30s.CountBy(p, x => x.a); // { [3, 1], [2, 1] }
CountOccurences
- title: CountOccurences
- tags: array,list,intermediate
Counts the occurences of a value in a collection.
- Use
IEnumerable.Count()
in combination withEqualityComparer<T>.Default.Equals()
to compare each value in theIEnumerable
withel
.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static int CountOccurences<T>(IEnumerable<T> obj, T el)
{
return obj.Count(f => EqualityComparer<T>.Default.Equals(f, el));
}
}
string s = "fooBar";
List<int> nums = new List<int> { 1, 2, 3, 3, 3, 4, 5, 6 };
_30s.CountOccurences(s,'o'); // 2
_30s.CountOccurences(nums,3); // 3
DayOfTheWeek
- title: DayOfTheWeek
- tags: date,utility,beginner
Returns the string representation of the weekday for the given DateTime
.
- Use
DateTime.ToString()
with an appropriate format modifier to return the day of the week.
public static partial class _30s
{
public static string DayOfTheWeek(DateTime date)
{
return date.ToString("dddd");
}
}
_30s.DayOfTheWeek(new DateTime(2020, 1, 15)); // "Wednesday"
Decapitalize
- title: Decapitalize
- tags: string,beginner
Decapitalizes the first letter of a string.
- Use
string.ToCharArray()
to convert the string to an array ofchar
,chars
. - Use
char.ToLower(chars[0])
to decapitalize the first letter. - Finally, return a
new string()
from thechars
array.
public static partial class _30s
{
public static string Decapitalize(string str)
{
char[] chars = str.ToCharArray();
chars[0] = char.ToLower(chars[0]);
return new string(chars);
}
}
string s = "FooBar";
_30s.Decapitalize(s); // "fooBar"
Difference
- title: Difference
- tags: array,list,beginner
Returns the difference betweend two collections.
- Use
IEnumerable.Except()
to only return elements in the second enumerable object and not the first one.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> Difference<T>(IEnumerable<T> a, IEnumerable<T> b)
{
return a.Except(b);
}
}
int[] a = { 1, 2, 3, 5 };
int[] b = { 1, 2, 4 };
_30s.Difference(a, b); // { 3, 5 }
DifferenceBy
- title: DifferenceBy
- tags: array,list,lambda,advanced
Returns the difference between two collections, after applying the provided function to each element of both.
- Use
IEnumerable.Select()
to map each element of either collection to the desired type. - Use
IEnumerable.Except()
to only return elements in the second enumerable object and not the first one.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<R> DifferenceBy<T,R>(IEnumerable<T> a, IEnumerable<T> b, Func<T,R> map)
{
return a.Select(map).Except(b.Select(map));
}
}
var p = new[] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
var q = new[] {
new { a = 6, b = 2}
};
_30s.DifferenceBy(p, q, x => x.b); // { 1 }
DistinctValues
- title: DistinctValues
- tags: array,list,beginner
Returns all distinct values in a collection.
- Use
IEnumerable.Distinct()
to get the distinct values in the given collection.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> DistinctValues<T>(IEnumerable<T> data)
{
return data.Distinct();
}
}
int[] nums = { 1, 2, 1, 3, 3, 4, 5 };
_30s.DistinctValues(nums); // { 1, 2, 3, 4, 5 }
DuplicateValues
- title: DuplicateValues
- tags: array,list,intermediate
Returns all distinct values in a collection.
- Use
IEnumerable.GroupBy()
to create groups for each distinct value in the enumerable. - Use
IEnumerable.Where()
to create select only the groups with a count greater than1
. - Use
IEnumerable.Select()
to return theKey
property of each group.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> DuplicateValues<T>(IEnumerable<T> items)
{
return items
.GroupBy(c => c)
.Where(g => g.Count() > 1)
.Select(i => i.Key);
}
}
int[] arr = {1, 2, 1, 3, 2, 4};
_30s.DuplicateValues(arr); // {1, 2}
Fibonacci
- title: Fibonacci
- tags: math,array,intermediate
Generates an array, containing the Fibonacci sequence, up until the nth term.
- Starting with
0
and1
, loop from2
throughn
adding the sum of the last two numbers and appending to the sequence. - If
n
is less or equal to0
, return a list containing0
.
public static partial class _30s
{
public static int[] Fibonacci(int n)
{
if (n <= 0 ) return new [] { 0 };
int[] fib = new int[n + 1];
fib[0] = 0;
fib[1] = 1;
for (int i = 2; i <= n; i ++)
{
fib[i] = fib[i - 1] + fib[i - 2];
}
return fib;
}
}
_30s.Fibonacci(7); // { 0, 1, 1, 2, 3, 5, 8, 13 }
FilterString
- title: FilterString
- tags: string,utility,intermediate
Filter a string's contents to include only alphanumeric and allowed characters.
- Use
string.ToCharArray()
in combination withArray.FindAll()
to check if each character in the string is alphanumeric or contained in thefilter
. - Omit the second argument,
filter
, to only allow alphanumeric characters.
using System.Collections.Generic;
public static partial class _30s
{
public static string FilterString(string s, string filter = "")
{
return new string(
Array.FindAll(s.ToCharArray(), c => char.IsLetterOrDigit(c) || filter.Contains(c))
);
}
}
string s = "@30_seconds_of_code##-$";
_30s.FilterString(s); // "30secondsofcode"
_30s.FilterString(s,"_"); // "30_seconds_of_code"
_30s.FilterString(s,"_@"); // "@30_seconds_of_code"
FindFirstBy
- title: FindFirstBy
- tags: array,list,lambda,intermediate
Returns the first element in a collection that matches the given predicate function, match
.
- Use
IEnumerable.Where()
to filter out all values indata
for whichmatch
returnsfalse
. - Use
IEnumerable.First()
to return only the first matching element.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static T FindFirstBy<T>(IEnumerable<T> data, Predicate<T> match)
{
return data.Where(i => match(i)).First();
}
}
int[] nums = {1, 2, 4, 5, 2, 2, 4};
_30s.FindFirstBy(nums, x => x % 2 == 0); // 2
FindIndexOfAll
- title: FindIndexOfAll
- tags: array,list,lambda,intermediate
Returns all indices in an IList
that match the given predicate function, match
.
- Use
Enumerable.Range()
to iterate over all indices indata
. - Use
IEnumerable.Where()
to filter out all values indata
for whichmatch
returnsfalse
and return only matching indices.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<int> FindIndexOfAll<T>(IList<T> data, Predicate<T> match)
{
return Enumerable
.Range(0, data.Count())
.Where(i => match(data[i]));
}
}
int[] nums = {1, 2, 4, 5, 2, 2, 4};
_30s.FindIndexOfAll(nums, x => x % 2 != 0); // {0, 3}
FindIndexOfFirstBy
- title: FindIndexOfFirstBy
- tags: array,list,lambda,intermediate
Returns the first index in an IList
that matches the given predicate function, match
.
- Use
Enumerable.Range()
to iterate over all indices indata
. - Use
IEnumerable.Where()
to filter out all values indata
for whichmatch
returnsfalse
. - Use
IEnumerable.First()
to return only the first matching index.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static int FindIndexOfFirstBy<T>(IList<T> data, Predicate<T> match)
{
return Enumerable
.Range(0, data.Count())
.Where(i => match(data[i]))
.First();
}
}
int[] nums = {1, 2, 4, 5, 2, 2, 4};
_30s.FindIndexOfFirstBy(nums, x => x % 2 == 0); // 1
FindIndexOfLastBy
- title: FindIndexOfLastBy
- tags: array,list,lambda,intermediate
Returns the last index in an IList
that matches the given predicate function, match
.
- Use
Enumerable.Range()
to iterate over all indices indata
. - Use
IEnumerable.Where()
to filter out all values indata
for whichmatch
returnsfalse
. - Use
IEnumerable.Last()
to return only the last matching index.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static int FindIndexOfLastBy<T>(IList<T> data, Predicate<T> match)
{
return Enumerable
.Range(0, data.Count())
.Where(i => match(data[i]))
.Last();
}
}
int[] nums = {1, 2, 4, 5, 2, 2, 4};
_30s.FindIndexOfLastBy(nums, x => x % 2 == 0); // 6
FindLastBy
- title: FindLastBy
- tags: array,list,lambda,intermediate
Returns the last element in a collection that matches the given predicate function, match
.
- Use
IEnumerable.Where()
to filter out all values indata
for whichmatch
returnsfalse
. - Use
IEnumerable.Last()
to return only the last matching element.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static T FindLastBy<T>(IEnumerable<T> data, Predicate<T> match)
{
return data.Where(i => match(i)).Last();
}
}
int[] nums = {1, 2, 4, 5, 2, 2, 4};
_30s.FindLastBy(nums, x => x % 2 == 0); // 4
FindParityOutliers
- title: FindParityOutliers
- tags: array,list,math,advanced
Given a collection, returns the items that are parity outliers.
- Use
IEnumerable.GroupBy()
to create groups for each parity (0
and1
). - Use
IEnumerable.OrderBy()
in combination withIEnumerable.Count()
to order the two groups in ascending order based on frequency. - Use
IEnumerable.First()
to get the first element and return itsKey
property, which corresponds to the least common parity value. - Finally, use
IEnumerable.Where()
to get all elements with the least common parity value.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<int> FindParityOutliers(IEnumerable<int> items)
{
return items.Where(
i => i % 2 == items
.GroupBy(i => i % 2)
.OrderBy(i => i.Count())
.First()
.Key
);
}
}
int[] nums = {1, 2, 3, 4, 6};
_30s.FindParityOutliers(nums); // {1, 3}
Flatten
- title: Flatten
- tags: array,list,intermediate
Flattens a 2D collection into a single dimension.
- Use
IEnumerable.SelectMany()
to flatten the 2D enumerable object.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> Flatten<T>(IEnumerable<IEnumerable<T>> obj)
{
return obj.SelectMany(v => v);
}
}
int[][] x = {
new [] {1, 2, 3},
new [] {4, 5, 6}
};
_30s.Flatten(x); // {1, 2, 3, 4, 5, 6}
FormatDuration
- title: FormatDuration
- tags: date,utility,beginner
Returns the human readable format of the given number of seconds.
- Use
TimeSpan.FromSeconds()
to convert the number ofseconds
to aTimeSpan
object. - Use
TimeSpan.ToString()
with an appropriate format specifier to return a human readable string of the value.
public static partial class _30s
{
public static string FormatDuration(double seconds)
{
return TimeSpan.FromSeconds(seconds).ToString(@"d\.hh\:mm\:ss\.fff");
}
}
_30s.FormatDuration(34325055.574); // 397.06:44:15.574
Frequencies
- title: Frequencies
- tags: array,list,dictionary,intermediate
Returns a Dictionary
with the unique values of a collection as keys and their frequencies as the values.
- Use
IEnumerable.GroupBy()
to create groups for each distinct value in the collection. - Use
IEnumerable.ToDictionary()
to convert the result of the previous operation to aDictionary
.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static Dictionary<T,int> Frequencies<T>(IEnumerable<T> values)
{
return values
.GroupBy(v => v)
.ToDictionary(v => v.Key, v => v.Count());
}
}
char[] c = {'a', 'b', 'a', 'c', 'a', 'a', 'b'};
_30s.Frequencies(c); // { [a, 4], [b, 2], [c, 1] }
GCD
- title: GCD
- tags: math,recursion,overload,intermediate
Calculates the greatest common divisor of the given numbers.
- Define a
GCD()
function for two numbers, which uses recursion. - Base case is when
y
equals0
, which returnsx
. - Otherwise the GCD of
y
and the remainder of the divisionx/y
is returned. - Define an overload that accepts multiple numbers or an array and use
IEnumerable.Aggregate()
to applyGCD()
to them.
using System.Linq;
public static partial class _30s
{
public static int GCD(params int[] nums)
{
return nums.Aggregate(GCD);
}
public static int GCD(int x, int y)
{
return y == 0 ? x : GCD(y, x % y);
}
}
_30s.GCD(8, 36, 28); // 4
GetFirstN
- title: GetFirstN
- tags: array,list,intermediate
Returns the first n
elements in a collection.
- Use
IEnumerable.Count()
to check if the enumerable is non-empty. - Use
IEnumerable.Take(n)
to get the firstn
elements. - If the enumerable object is empty, return the
default()
value for the given enumerable. - Omit the second argument,
n
, to use a default value of1
.
public static partial class _30s
{
public static IEnumerable<T> GetFirstN<T>(IEnumerable<T> list, int n = 1)
{
return list.Count() != 0 ? list.Take(n) : default(IEnumerable<T>);
}
}
List<int> nums = new List<int> { 1, 2, 3, 4, 5 };
_30s.GetFirstN(nums); // { 1 }
_30s.GetFirstN(nums, 3); // { 1, 2, 3 }
GetLastN
- title: GetLastN
- tags: array,list,intermediate
Returns the last n
elements in a collection.
- Use
IEnumerable.Count()
to check if the enumerable is non-empty. - Use
IEnumerable.Skip(list.Count() - n)
to get the lastn
elements. - If the enumerable object is empty, return the
default()
value for the given enumerable. - Omit the second argument,
n
, to use a default value of1
.
public static partial class _30s
{
public static IEnumerable<T> GetLastN<T>(IEnumerable<T> list, int n = 1)
{
return list.Count() != 0 ? list.Skip(list.Count() - n) : default(IEnumerable<T>);
}
}
List<int> nums = new List<int> { 1, 2, 3, 4, 5 };
_30s.GetLastN(nums); // { 5 }
_30s.GetLastN(nums, 3); // { 3, 4, 5 }
GetType
- title: GetType
- tags: utility,type,beginner
Returns the type of the given object.
- Use
typeof()
on the given object's type.
public static partial class _30s
{
public static Type GetType<T>(T obj)
{
return typeof(T);
}
}
string s = "fooBar";
List<string> list = new List<string> { "a", "b", "c" };
_30s.GetType(s); // System.String
_30s.GetType(list); // System.Collections.Generic.List`1[System.String]
HaveSameContents
- title: HaveSameContents
- tags: array,list,dictionary,advanced
Returns true
if two collections contain the same elements regardless of order, false
otherwise.
- Use
IEnumerable.GroupBy()
to create groups for each distinct value in each collection,IEnumerable.ToDictionary()
to convert the result to aDictionary
. - Use
IEnumerable.Union()
andIEnumerable.Distinct()
to find the distinct values from both collections and loop over them using aforeach
loop. - Use
Dictionary.ContainsKey()
to check that each distinct value exists in both collections and compare the count for each one. - Return
false
if any value is not found in either collection or if any count does not match,true
otherwise.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static bool HaveSameContents<T>(IEnumerable<T> a, IEnumerable<T> b)
{
Dictionary<T,int> dA = a.GroupBy(v => v).ToDictionary(v => v.Key, v => v.Count());
Dictionary<T,int> dB = b.GroupBy(v => v).ToDictionary(v => v.Key, v => v.Count());
foreach (T val in a.Union(b).Distinct()) {
if (!dA.ContainsKey(val) || !dB.ContainsKey(val)) return false;
if (dA[val] != dB[val]) return false;
}
return true;
}
}
int[] a = {1, 2, 4};
int[] b = {2, 4, 1};
_30s.HaveSameContents(a, b); // true
Head
- title: Head
- tags: array,list,intermediate
Returns the head of a collection.
- Use
IEnumerable.Count()
to check if the enumerable is non-empty. - Use
IEnumerable.Take(1)
to get the first element,IEnumerable.ToArray()[0]
to convert to array and return the element. - If the enumerable object is empty, return the
default()
value for the given type.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static T Head<T>(IEnumerable<T> list)
{
return list.Count() != 0 ? list.Take(1).ToArray()[0] : default(T);
}
}
List<int> nums = new List<int> { 1, 2, 3, 4, 5 };
List<int> empty = new List<int> { };
char[] chars = {'A','B','C'};
_30s.Head(nums); // 1
_30s.Head(empty); // 0
_30s.Head(chars); // 'A'
HexToByteArray
- title: HexToByteArray
- tags: string,utility,advanced
Converts a hexadecimal string to a byte
array.
- Use
Enumerable.Range()
in combination withstring.Length
to get the indices of the given string in an array. - Use
Enumerable.Where()
to get only the even indices in the previous range. - Use
Enumerable.Select()
in combination withConvert.ToByte()
andstring.Substring()
to convert each byte's hex code to abyte
. - Finally, use
Enumerable.ToArray()
to return abyte[]
.
using System.Linq;
public static partial class _30s
{
public static byte[] HexToByteArray(string hex)
{
return Enumerable.Range(0, hex.Length)
.Where(x => x % 2 == 0)
.Select(x => Convert.ToByte(hex.Substring(x, 2), 16))
.ToArray();
}
}
_30s.HexToByteArray("F15936"); // { 241, 89, 54 }
IndexOfAll
- title: IndexOfAll
- tags: array,list,intermediate
Returns all indices of n
in an IList
.
- Use
Enumerable.Range()
to iterate over all indices indata
. - Use
Enumerable.Where()
in combination withobject.Equals()
to compare each value indata
ton
and return only matching indices.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<int> IndexOfAll<T>(IList<T> data, T n)
{
return Enumerable
.Range(0, data.Count())
.Where(i => object.Equals(n, data[i]));
}
}
int[] nums = {1, 2, 4, 5, 2, 2, 4};
_30s.IndexOfAll(_30s.IndexOfAll(nums, 2)); // {1, 4, 5}
Initialize2DArray
- title: Initialize2DArray
- tags: array,utility,advanced
Initializes a 2D array of the given width, height and value.
- Use
Enumerable.Repeat()
to repeatvalue
width
times, convert to an array and repeatheight
times using the same method. - Use
IEnumerable.Select()
andIEnumerable.First()
to convert the jagged array to a 2D array.
using System.Linq;
public static partial class _30s
{
public static T[,] Initialize2DArray<T>(int width, int height, T value)
{
return new [] { new T [height, width] }
.Select(_ => new { x = _, y = Enumerable.Repeat(
Enumerable.Repeat(value, width).ToArray(), height
)
.ToArray()
.Select((a, ia) => a.Select((b, ib) => _[ia, ib] = b).Count()).Count() }
)
.Select(_ => _.x)
.First();
}
}
_30s.Initialize2DArray(2, 3, 5); // { {5, 5}, {5, 5}, {5, 5} }
IsA
- title: IsA
- tags: utility,type,beginner
Returns true
if the given object is of the specified type, false
otherwise.
- Use the
is
operator to check ifobj
is of the given type,T
.
public static partial class _30s
{
public static bool IsA<T>(object obj)
{
return obj is T;
}
}
string s = "fooBar";
_30s.IsA<string>(s); // true
_30s.IsA<int>(s); // false
IsContainedIn
- title: IsContainedIn
- tags: array,list,dictionary,advanced
Returns true
if the elements of the first collection are contained in the second one regardless of order, false
otherwise.
- Use
IEnumerable.GroupBy()
to create groups for each distinct value in each collection,IEnumerable.ToDictionary()
to convert the result to aDictionary
. - Use
IEnumerable.Distinct()
to find the distinct values from the first collection and loop over them using aforeach
loop. - Use
Dictionary.ContainsKey()
to check that each distinct value exists in the second collection and compare the count for each one. - Return
false
if any value is not found in the second collection or if any count in it is lower than in the first one,true
otherwise.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static bool IsContainedIn<T>(IEnumerable<T> a, IEnumerable<T> b)
{
Dictionary<T,int> dA = a.GroupBy(v => v).ToDictionary(v => v.Key, v => v.Count());
Dictionary<T,int> dB = b.GroupBy(v => v).ToDictionary(v => v.Key, v => v.Count());
foreach(T val in a.Distinct()) {
if (!dB.ContainsKey(val)) return false;
if (dA[val] > dB[val]) return false;
}
return true;
}
}
int[] a = {1, 4};
int[] b = {2, 4, 1};
_30s.IsContainedIn(a, b); // true
IsDivisible
- title: IsDivisible
- tags: math,beginner
Checks if the first numeric argument is divisible by the second one.
- Use the modulo operator (
%
) to check if the remainder is equal to0
.
public static partial class _30s
{
public static bool IsDivisible(long dividend, long divisor)
{
return dividend % divisor == 0;
}
}
_30s.IsDivisible(6, 3); // true
IsDouble
- title: IsDouble
- tags: math,type,intermediate
Returns true
if the given string can be parsed into a double, false
otherwise.
- Return the result of calling
Double.TryParse()
withNymberStyles.Float
for the givennum
string.
using System.Globalization;
public static partial class _30s
{
public static bool IsDouble(string num)
{
Double _ = 0.0;
return Double.TryParse(num, NumberStyles.Float, NumberFormatInfo.CurrentInfo, out _);
}
}
_30s.IsDouble("2"); // true
_30s.IsDouble("hi"); // false
IsEven
- title: IsEven
- tags: math,beginner
Returns true
if the given number is even, false
otherwise.
- Check whether a number is odd or even using the modulo (
%
) operator. - Return
true
if the number is even,false
if the number is odd.
public static partial class _30s
{
public static bool IsEven(int n)
{
return n % 2 == 0;
}
}
_30s.IsEven(2); // true
_30s.IsEven(3); // false
IsInteger
- title: IsInteger
- tags: math,type,intermediate
Returns true
if the given string can be parsed into an integer, false
otherwise.
- Return the result of calling
Double.TryParse()
withNymberStyles.Integer
for the givennum
string. - Use
Double.TryParse()
to allow handling of values larger thanInt64
.
using System.Globalization;
public static partial class _30s
{
public static bool IsInteger(string num)
{
Double _ = 0.0;
return Double.TryParse(num, NumberStyles.Integer, NumberFormatInfo.CurrentInfo, out _);
}
}
_30s.IsInteger("2"); // true
_30s.IsInteger("3.1"); // false
IsLower
- title: IsLower
- tags: string,beginner
Checks if a string is lower case.
- Convert the given string to lower case, using
string.ToLower()
and compare it to the original.
public static partial class _30s
{
public static bool IsLower(string str)
{
return str.ToLower() == str;
}
}
string s1 = "abc";
string s2 = "cDe";
_30s.IsLower(s1); // true
_30s.IsLower(s2); // false
IsNotA
- title: IsNotA
- tags: utility,type,beginner
Returns true
if the given object is not of the specified type, false
otherwise.
- Use the
is
operator to check ifobj
is not of the given type,T
.
public static partial class _30s
{
public static bool IsNotA<T>(object obj)
{
return !(obj is T);
}
}
string s = "fooBar";
_30s.IsNotA<string>(s); // false
_30s.IsNotA<int>(s); // true
IsOdd
- title: IsOdd
- tags: math,beginner
Returns true
if the given number is odd, false
otherwise.
- Check whether a number is odd or even using the modulo (
%
) operator. - Return
true
if the number is odd,false
if the number is even.
public static partial class _30s
{
public static bool IsOdd(int n)
{
return n % 2 != 0;
}
}
_30s.IsOdd(3); // true
_30s.IsOdd(4); // false
IsPowerOfTwo
- title: IsPowerOfTwo
- tags: math,intermediate
Returns true
if the given number is a power of 2
, false
otherwise.
- Use the bitwise binary AND operator (
&
) to determine ifn
is a power of2
. - Additionally, check that
n
is different from0
.
public static partial class _30s
{
public static bool IsPowerOfTwo(ulong n)
{
return (n != 0) && ((n & (n - 1)) == 0);
}
}
_30s.IsPowerOfTwo(0); // false
_30s.IsPowerOfTwo(1); // true
_30s.IsPowerOfTwo(8); // true
IsUpper
- title: IsUpper
- tags: string,beginner
Checks if a string is upper case.
- Convert the given string to upper case, using
string.ToUpper()
and compare it to the original.
public static partial class _30s
{
public static bool IsUpper(string str)
{
return str.ToUpper() == str;
}
}
string s1 = "ABC";
string s2 = "cDe";
_30s.IsUpper(s1); // true
_30s.IsUpper(s2); // false
IsWeekday
- title: IsWeekday
- tags: date,utility,beginner
Returns true
if the given DateTime
is a weekday, false
otherwise.
- Use
DateTime.DayOfWeek
to check if the givenDateTime
is not a Saturday or Sunday.
public static partial class _30s
{
public static bool IsWeekday(DateTime date)
{
return date.DayOfWeek != DayOfWeek.Saturday && date.DayOfWeek != DayOfWeek.Sunday;
}
}
_30s.IsWeekday(new DateTime(2020, 1, 15)); // true
_30s.IsWeekday(new DateTime(2020, 1, 19)); // false
IsWeekend
- title: IsWeekend
- tags: date,utility,beginner
Returns true
if the given DateTime
is a not weekday, false
otherwise.
- Use
DateTime.DayOfWeek
to check if the givenDateTime
is a Saturday or Sunday.
public static partial class _30s
{
public static bool IsWeekend(DateTime date)
{
return date.DayOfWeek == DayOfWeek.Saturday || date.DayOfWeek == DayOfWeek.Sunday;
}
}
_30s.IsWeekend(new DateTime(2020, 1, 15)); // false
_30s.IsWeekend(new DateTime(2020, 1, 19)); // true
KeepUpToN
- title: KeepUpToN
- tags: array,list,intermediate
Filters a collection keeping up to n
occurences of each value.
- Use
IEnumerable.Distinct()
in combination withIEnumerable.ToDictionary()
to create a dictionary with an initial count of0
for each distinct value indata
. - Use
IEnumerable.Where()
to filter out occurences after then
th one for each element, using the previously created dictionary.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> KeepUpToN<T>(IEnumerable<T> data, int n)
{
var occurences = data.Distinct().ToDictionary(i => i, value => 0);
return data.Where(i => occurences[i]++ < n);
}
}
int[] nums = {1, 1, 2, 3, 3, 3, 1};
_30s.KeepUpToN(nums, 2); // {1, 1, 2, 3, 3}
LCM
- title: LCM
- tags: math,recursion,intermediate
Calculates the least common multiple of the given numbers.
- Define a
_GCD()
method that determines the greatest common divisor, using recursion. - Use
_GCD()
and the fact thatLCM(x, y) = x * y / GCD(x,y)
to determine the least common multiple. - Use
IEnumerable.Aggregate()
to applyLCM()
to all the given arguments.
using System.Linq;
public static partial class _30s
{
public static int LCM(params int[] nums)
{
return nums.Aggregate((x,y) => (x * y) / _GCD(x, y));
}
private static int _GCD(int x, int y)
{
return y == 0 ? x : _GCD(y, x % y);
}
}
_30s.LCM(1, 3, 4, 5); // 60
_30s.LCM(new [] {12, 7}); // 84
Mask
- title: Mask
- tags: string,utility,intermediate
Replaces all but the last n
characters in a string with the specified mask
character.
- Use
string.Substring()
to get the lastn
characters of the passed string,str
. - Use
string.PadLeft()
to add as manymask
characters as necessary to the start of the string to return a string of the same length. - Omit the third argument,
mask
, to use a default character of'*'
. - Omit the second argument,
n
, to keep a default of4
characters unmasked.
public static partial class _30s
{
public static string Mask(string str, int n = 4, char mask = '*')
{
return str.Substring(str.Length - n).PadLeft(str.Length, mask);
}
}
string s = "1234567890";
_30s.Mask(s); // "******7890"
_30s.Mask(s, 3); // "*******890"
_30s.Mask(s, 2, '/$'); // "$$$$$$$$90"
MaxBy
- title: MaxBy
- tags: math,list,array,lambda,intermediate
Returns the maximum of a collection, after mapping each element to a value using the provided function.
- Use
IEnumerable.Select()
to map each element to the value returned by the provided selector function,fn
. - Use
IEnumerable.Max()
to get the maximum of the resulting values.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static double MaxBy<T>(IEnumerable<T> values, Func<T,int> fn)
{
return values.Select(fn).Max();
}
}
var p = new [] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
_30s.MaxBy(p, v => v.a); // 3
_30s.MaxBy(p, v => v.b); // 2
MaxDateTime
- title: MaxDateTime
- tags: date,beginner
Returns the maximum of two DateTime
values.
- Use the conditional operator (
?:
) to return the maximum of the two values.
public static partial class _30s
{
public static DateTime MaxDateTime(DateTime d1, DateTime d2)
{
return (d1 > d2) ? d1 : d2;
}
}
DateTime d1 = new DateTime(DateTime.MaxValue.Ticks);
DateTime d2 = new DateTime(DateTime.MinValue.Ticks);
_30s.MaxDateTime(d1, d2); // 12/31/9999 11:59:59 PM
Median
- title: Median
- tags: math,intermediate
Finds the median of a list of numbers.
- Use the
params
keyword to accept either an array or a variable number of arguments. - Sort the array using
Array.sort()
and find the median. - Which is either the middle element of the list, if the list length is odd or the average of the two middle elements, if the list length is even.
public static partial class _30s
{
public static double Median(params double[] values)
{
Array.Sort(values);
if (values.Length % 2 == 0)
return (values[values.Length / 2 - 1] + values[values.Length / 2]) / 2;
return (double)values[values.Length / 2];
}
}
double[] nums = { 5, 6, 7, 8 };
_30s.Median(4, 8, 1); // 4
_30s.Median(nums); // 6.5
MinBy
- title: MinBy
- tags: math,list,array,lambda,intermediate
Returns the minimum of a collection, after mapping each element to a value using the provided function.
- Use
IEnumerable.Select()
to map each element to the value returned by the provided selector function,fn
. - Use
IEnumerable.Min()
to get the minimum of the resulting values.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static double MinBy<T>(IEnumerable<T> values, Func<T,int> fn)
{
return values.Select(fn).Min();
}
}
var p = new [] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
_30s.MinBy(p, v => v.a); // 2
_30s.MinBy(p, v => v.b); // 1
MinDateTime
- title: MinDateTime
- tags: date,beginner
Returns the minimum of two DateTime
values.
- Use the conditional operator (
?:
) to return the minimum of the two values.
public static partial class _30s
{
public static DateTime MinDateTime(DateTime d1, DateTime d2)
{
return (d1 < d2) ? d1 : d2;
}
}
DateTime d1 = new DateTime(DateTime.MaxValue.Ticks);
DateTime d2 = new DateTime(DateTime.MinValue.Ticks);
_30s.MinDateTime(d1, d2); // 1/1/0001 12:00:00 AM
MostFrequent
- title: MostFrequent
- tags: array,list,intermediate
Returns the most frequent element of a collection.
- Use
IEnumerable.GroupBy()
to groupvalues
by value. - Use
IEnumerable.OrderByDescending()
in combination withIEnumerable.Count()
to order the results in descending order based on frequency. - Use
IEnumerable.First()
to get the first element and return itsKey
property, which corresponds to the element's value.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static T MostFrequent<T>(IEnumerable<T> values)
{
return values
.GroupBy(v => v)
.OrderByDescending(v => v.Count())
.First()
.Key;
}
}
int[] nums = { 1, 2, 3, 3, 2, 3 };
List<string> str = new List<string> { "a", "b", "b", "c" };
_30s.MostFrequent(nums); // 3
_30s.MostFrequent(str); // "b"
None
- title: None
- tags: array,list,lambda,overload,intermediate
Returns true
if the provided predicate function returns false
for all elements in a collection, false
otherwise.
- Use
IEnumerable.ToArray()
,Array.Exists()
to test if all elements in the collection returnfalse
based on the predicate function,match
. - Omit the predicate function,
match
, to use the overload that checks if each value isnull
by default.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static bool None<T>(IEnumerable<T> data, Predicate<T> match)
{
return !Array.Exists(data.ToArray(), match);
}
public static bool None<T>(IEnumerable<T> data)
{
return Array.Exists(data.ToArray(), val => val == null);
}
}
int[] nums = { 4, 2, 3 };
_30s.None(nums, x => x < 0); // true
_30s.None(nums); // false
PadNumber
- title: PadNumber
- tags: string,utility,beginner
Pads a given number to the specified length
.
- Use
Int32.ToString()
with an appropriate format specifier, produced using string interpolation.
public static partial class _30s
{
public static string PadNumber(int n, int length)
{
return n.ToString($"D{length}");
}
}
_30s.PadNumber(1234,6); // "001234"
RandoubleInRange
- title: RandoubleInRange
- tags: math,utility,random,beginner
Returns a random double in the specified range.
- Use
Random.NextDouble()
to generate a random value and map it to the desired range using multiplication.
public static partial class _30s
{
public static double RandoubleInRange(double min, double max)
{
return (new Random().NextDouble() * (max - min)) + min;
}
}
_30s.RandoubleInRange(0.5, 5); // 2.20486941011849
RandomIntegerInRange
- title: RandomIntegerInRange
- tags: math,utility,random,beginner
Returns a random integer in the specified range.
- Use
Random.Next()
to generate an integer in the desired range.
public static partial class _30s
{
public static int RandomIntegerInRange(int min, int max)
{
return new Random().Next(min, max);
}
}
_30s.RandomIntegerInRange(0, 5); // 2
Repeat
- title: Repeat
- tags: string,beginner
Creates a new string by repeating the given string n
times.
- Use
Enumerable.Repeat()
to repeats
n
times,string.Concat()
to convert the result to astring
.
using System.Linq;
public static partial class _30s
{
public static string Repeat(string s, int n)
{
return string.Concat(Enumerable.Repeat(s, n));
}
}
_30s.Repeat("Ha",5); // "HaHaHaHaHa"
Reverse
- title: Reverse
- tags: string,beginner
Reverses a string.
- Use
string.ToCharArray()
to convert the string to an array ofchar
,Array.Reverse()
to reverse the array. - Use
IEnumerable.ToArray()
to create an array ofchar
and pass it to anew string()
.
using System.Linq;
public static partial class _30s
{
public static void Reverse(string s)
{
return new string(s.ToCharArray().Reverse().ToArray());
}
}
string s = "Hello World";
_30s.Reverse(s); // "dlroW olleH"
Shuffle
- title: Shuffle
- tags: array,list,random,intermediate
Randomizes the order of the values of an IList
, updating the original IList
object.
- Use the Fisher-Yates algorithm to reorder the elements of the given
IList
object.
using System.Collections.Generic;
public static partial class _30s
{
public static void Shuffle<T>(IList<T> list)
{
Random rand = new Random();
for (int n = list.Count() - 1 ; n > 0 ; n--)
{
int k = rand.Next(n + 1);
T value = list[k];
list[k] = list[n];
list[n] = value;
}
}
}
List<int> nums = new List<int> { 1, 2, 3, 4, 5, 6 };
int[] arr = { 1, 2, 3, 4, 5, 6 };
_30s.Shuffle(nums); // nums = { 3, 5, 2, 1, 4, 6 }
_30s.Shuffle(arr); // arr = { 6, 2, 5, 1, 4, 3 }
SplitLines
- title: SplitLines
- tags: string,beginner
Splits a multiline string into an array of lines.
- Use
string.Split()
with all forms of the newline separator to split the string into an array of strings.
using System.Collections.Generic;
public static partial class _30s
{
public static string[] SplitLines(string s)
{
return s.Split(new [] { "\r\n", "\r", "\n" }, StringSplitOptions.None);
}
}
string s = "This\nis a\nmultiline\nstring.\n";
_30s.SplitLines(s); // {"This", "is a", "multiline", "string." , ""}
SplitStringBy
- title: SplitStringBy
- tags: string,utility,beginner
Splits a string into an array of strings using a multicharacter (string) separator.
- Use
string.Split()
with the givenseparator
to split the string into an array of strings.
using System.Collections.Generic;
public static partial class _30s
{
public static string[] SplitStringBy(string s, string separator)
{
return s.Split(new [] {separator}, StringSplitOptions.None);
}
}
string s = "Apples--oranges--pears";
_30s.SplitStringBy(s,"--"); // {Apples, oranges, pears}
Stringify
- title: Stringify
- tags: utility,array,list,string,beginner
Combines the elements of an enumerable object into a string.
- Use
string.Join()
to combine all elements in theIEnumerable
into astring
, usingdelimiter
. - Omit the second argument,
delimiter
, to use the default delimiter of","
.
using System.Collections.Generic;
public static partial class _30s
{
public static string Stringify<T>(IEnumerable<T> elements, string delimiter = ",")
{
return string.Join(delimiter, elements);
}
}
IList<string> s = new List<string> {"a", "b", "c"};
int[] n = {1, 2, 3};
_30s.Stringify(s); // "a,b,c"
_30s.Stringify(n, " "); // "1 2 3"
Subarray
- title: Subarray
- tags: array,intermediate
Returns a subarray of the given array starting at the given index and having the specified length.
- Use
ArraySegment()
with the given array,arr
,start
andlength
to get the subarray. - Convert the result to an array, using
ArraySegment.ToArray()
.
using System.Collections.Generic;
public static partial class _30s
{
public static T[] Subarray<T>(T[] arr, int start, int length)
{
return new ArraySegment<T>( arr, start, length ).ToArray();
}
}
int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
_30s.Subarray(nums,3,6); // {3, 4, 5, 6, 7, 8}
SumBy
- title: SumBy
- tags: math,array,list,lambda,intermediate
Returns the sum of a collection, after mapping each element to a value using the provided function.
- Use
IEnumerable.Select()
to map each element of either collection to adouble
,IEnumerable.Sum()
to sum the values.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static double SumBy<T>(IEnumerable<T> data, Func<T,double> map)
{
return data.Select(map).Sum();
}
}
var p = new[] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
_30s.SumBy(p, x => x.a); // 5
Swap
- title: Swap
- tags: utility,intermediate
Swaps the values of two variables of the same type.
- Pass both values by reference using the
ref
keyword, then use atemp
variable to swap their values.
public static partial class _30s
{
public static void Swap<T>(ref T val1, ref T val2)
{
var temp = val1;
val1 = val2;
val2 = temp;
}
}
string a = "Ipsum";
string b = "Lorem";
_30s.Swap(ref a, ref b); // a = "Lorem", b = "Ipsum"
SymmetricDifference
- title: SymmetricDifference
- tags: array,list,beginner
Returns the symmetric difference betweend two collections.
- Use
IEnumerable.Except()
to only return elements in one enumerable object and not the other. - Use
IEnumerable.Union()
to combine the result of applying that to each object.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> SymmetricDifference<T>(IEnumerable<T> a, IEnumerable<T> b)
{
return a.Except(b).Union(b.Except(a));
}
}
int[] a = { 1, 2, 3, 5 };
int[] b = { 1, 2, 4 };
_30s.SymmetricDifference(a, b); // { 3, 5, 4 }
SymmetricDifferenceBy
- title: SymmetricDifferenceBy
- tags: array,list,lambda,advanced
Returns the symmetric difference betweend two collections, after applying the provided function to each element of both.
- Use
IEnumerable.Select()
to map each element of either collection to the desired type. - Use
IEnumerable.Except()
to only return elements in one enumerable object and not the other. - Use
IEnumerable.Union()
to combine the result of applying that to each object.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<R> SymmetricDifferenceBy<T,R>(IEnumerable<T> a, IEnumerable<T> b, Func<T,R> map)
{
IEnumerable<R> mapA = a.Select(map);
IEnumerable<R> mapB = b.Select(map);
return mapA.Except(mapB).Union(mapB.Except(mapA));
}
}
var p = new[] {
new { a = 3, b = 2},
new { a = 2, b = 1}
};
var q = new[] {
new { a = 6, b = 2},
new { a = 6, b = 3}
};
_30s.SymmetricDifferenceBy(p, q, x => x.b); // { 1, 3 }
Tail
- title: Tail
- tags: array,list,beginner
Returns the tail of a collection.
- Use
IEnumerable.Count()
to check if the enumerable is non-empty. - Use
IEnumerable.Skip(1)
to get the whole object except for the first element. - If the enumerable object is empty, return the
default()
value for the given enumerable.
using System.Collections.Generic;
using System.Linq;
public static partial class _30s
{
public static IEnumerable<T> Tail<T>(IEnumerable<T> list)
{
return list.Count() != 0 ? list.Skip(1) : default(IEnumerable<T>);
}
}
List<int> nums = new List<int> { 1, 2, 3, 4, 5 };
char[] chars = {'A','B','C'};
_30s.Tail(nums); // { 2, 3, 4, 5 }
_30s.Tail(chars); // {'B','C'}
ToCamelCase
- title: ToCamelCase
- tags: string,regex,advanced
Converts a string to camel case.
- Use
Regex.Matches()
with an appropriate regular expression to break the string into words. - Use
string.Join()
andstring.ToLower()
to convert the words to lowercase and combine them adding - Use
CultureInfo.TextInfo.ToTitleCase()
on the result to convert it to title case,string.Replace()
with a regular expression to remove spaces afterwards. - Finally, use
IEnumerable.Select()
on the result to convert the first character to lowercase and return a string from the result.
using System.Globalization;
using System.Text.RegularExpressions;
using System.Linq;
public static partial class _30s
{
public static string ToCamelCase(string str)
{
Regex pattern = new Regex(@"[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+");
return new string(
new CultureInfo("en-US", false)
.TextInfo
.ToTitleCase(
string.Join(" ", pattern.Matches(str)).ToLower()
)
.Replace(@" ", "")
.Select((x, i) => i == 0 ? char.ToLower(x) : x)
.ToArray()
);
}
}
_30s.ToCamelCase("some_database_field_name"); // "someDatabaseFieldName"
_30s.ToCamelCase("Some label that needs to be title-cased"); // "someLabelThatNeedsToBeCamelized"
_30s.ToCamelCase("some-package-name"); // "somePackageName"
_30s.ToCamelCase("some-mixed_string with spaces_underscores-and-hyphens"); // "someMixedStringWithSpacesUnderscoresAndHyphens"
ToKebabCase
- title: ToKebabCase
- tags: string,regex,intermediate
Converts a string to kebab case.
- Use
Regex.Matches()
with an appropriate regular expression to break the string into words. - Use
string.Join()
andstring.ToLower()
to convert the words to lowercase and combine them adding-
as a separator.
using System.Text.RegularExpressions;
public static partial class _30s
{
public static string ToKebabCase(string str)
{
Regex pattern = new Regex(@"[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+");
return string.Join("-", pattern.Matches(str)).ToLower();
}
}
_30s.ToKebabCase("camelCase"); // "camel-case"
_30s.ToKebabCase("some text"); // "some-text"
_30s.ToKebabCase("some-mixed_string With spaces_underscores-and-hyphens"); // "some-mixed-string-with-spaces-underscores-and-hyphens"
_30s.ToKebabCase("AllThe-small Things"); // "all-the-small-things"
ToSnakeCase
- title: ToSnakeCase
- tags: string,regex,intermediate
Converts a string to snake case.
- Use
Regex.Matches()
with an appropriate regular expression to break the string into words. - Use
string.Join()
andstring.ToLower()
to convert the words to lowercase and combine them adding_
as a separator.
using System.Text.RegularExpressions;
public static partial class _30s
{
public static string ToSnakeCase(string str)
{
Regex pattern = new Regex(@"[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+");
return string.Join("_", pattern.Matches(str)).ToLower();
}
}
_30s.ToSnakeCase("camelCase"); // "camel_case"
_30s.ToSnakeCase("some text"); // "some_text"
_30s.ToSnakeCase("some-mixed_string With spaces_underscores-and-hyphens"); // "some_mixed_string_with_spaces_underscores_and_hyphens"
_30s.ToSnakeCase("AllThe-small Things"); // "all_the_small_things"
ToTitleCase
- title: ToTitleCase
- tags: string,regex,intermediate
Converts a string to title case.
- Use
Regex.Matches()
with an appropriate regular expression to break the string into words. - Use
string.Join()
andstring.ToLower()
to convert the words to lowercase and combine them adding - Use
CultureInfo.TextInfo.ToTitleCase()
on the result to convert it to title case.
using System.Globalization;
using System.Text.RegularExpressions;
public static partial class _30s
{
public static string ToTitleCase(string str)
{
Regex pattern = new Regex(@"[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+");
return new CultureInfo("en-US", false)
.TextInfo
.ToTitleCase(
string.Join(" ", pattern.Matches(str)).ToLower()
);
}
}
_30s.ToTitleCase("some_database_field_name"); // "Some Database Field Name"
_30s.ToTitleCase("Some label that needs to be title-cased"); // "Some Label That Needs To Be Title Cased"
_30s.ToTitleCase("some-package-name"); // "Some Package Name"
_30s.ToTitleCase("some-mixed_string with spaces_underscores-and-hyphens"); // "Some Mixed String With Spaces Underscores And Hyphens"
Tomorrow
- title: Tomorrow
- tags: date,beginner
Returns tomorrow's DateTime
value.
- Use
DateTime.Now
to get the current date, then useDateTime.AddDays(1)
to increment by1
.
public static partial class _30s
{
public static DateTime Tomorrow()
{
return DateTime.Now.AddDays(1);
}
}
_30s.Tomorrow(); // 12/22/2019 11:00:49 AM (if it's 12/21/2019 11:00:49 AM)
Yesterday
- title: Yesterday
- tags: date,beginner
Returns yesterday's DateTime
value.
- Use
DateTime.Now
to get the current date, then useDateTime.AddDays(-1)
to decrement by1
.
public static partial class _30s
{
public static DateTime Yesterday()
{
return DateTime.Now.AddDays(-1);
}
}
_30s.Yesterday(); // 12/20/2019 11:00:49 AM (if it's 12/21/2019 11:00:49 AM)