logo 30 seconds of code Curated collection of useful JavaScript snippets that you can understand in 30 seconds or less.



Converts the given array elements into <li> tags and appends them to the list of the given id.

Use Array.prototype.map(), document.querySelector(), and an anonymous inner closure to create a list of html tags.

const arrayToHtmlList = (arr, listID) =>
  (el => (
    (el = document.querySelector('#' + listID)),
    (el.innerHTML += arr.map(item => `<li>${item}</li>`).join(''))
arrayToHtmlList(['item 1', 'item 2'], 'myListID');


Returns true if the bottom of the page is visible, false otherwise.

Use scrollY, scrollHeight and clientHeight 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


⚠️ 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.

Copy 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).

const copyToClipboard = str => {
  const el = document.createElement('textarea');
  el.value = str;
  el.setAttribute('readonly', '');
  el.style.position = 'absolute';
  el.style.left = '-9999px';
  const selected =
    document.getSelection().rangeCount > 0 ? document.getSelection().getRangeAt(0) : false;
  if (selected) {
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.


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 with Math.abs() and Math.floor() to calculate the time between each new text draw. Use document.querySelector().innerHTML to update the value of the selected element. Omit the fourth parameter, step, to use a default step of 1. Omit the fifth parameter, duration, to use a default duration of 2000ms.

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"


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. Set its innerHTML 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">
console.log(el.className); // 'container'


Creates a pub/sub (publish–subscribe) event hub with emit, on, and off methods.

Use Object.create(null) to create an empty hub object that does not inherit properties from Object.prototype. For emit, resolve the array of handlers based on the event argument and then run each one with Array.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 use Array.prototype.push() to add the handler to the array. For off, use Array.prototype.findIndex() to find the index of the handler in the event array and remove it using Array.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] = [];
  off(event, handler) {
    const i = (this.hub[event] || []).findIndex(h => h === handler);
    if (i > -1) this.hub[event].splice(i, 1);
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);


Returns the current URL.

Use window.location.href to get current URL.

const currentURL = () => window.location.href;
currentURL(); // 'https://google.com'


Detects wether the website is being opened in a mobile device or a desktop/laptop.

Use a regular expression to test the navigator.userAgent property to figure out if the device is a mobile device or a desktop/laptop.

const detectDeviceType = () =>
  /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
    ? 'Mobile'
    : 'Desktop';
detectDeviceType(); // "Mobile" or "Desktop"


Returns true if the parent element contains the child element, false otherwise.

Check that parent is not the same element as child, use parent.contains(child) to check if the parent element contains the child 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


Returns true if the element specified is visible in the viewport, false otherwise.

Use Element.getBoundingClientRect() and the window.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)


Encode a set of form elements as an object.

Use the FormData constructor to convert the HTML form to FormData, Array.from() to convert to an array. Collect the object from the array, using Array.prototype.reduce().

const formToObject = form =>
  Array.from(new FormData(form)).reduce(
    (acc, [key, value]) => ({
      [key]: value
formToObject(document.querySelector('#form')); // { email: '[email protected]', name: 'Test Name' }


Fetches all images from within an element and puts them into an array

Use Element.prototype.getElementsByTagName() to fetch all <img> elements inside the provided element, Array.prototype.map() to map every src attribute of their respective <img> element, then create a Set to eliminate duplicates and return the array.

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', '...']


Returns the scroll position of the current page.

Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop. You can omit el to use a default value of window.

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}


Returns 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'


Returns true if the element has the specified class, false otherwise.

Use element.classList.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


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.

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'


Hides all the elements specified.

Use NodeList.prototype.forEach() to apply display: 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


Redirects the page to HTTPS if its currently in HTTP. Also, pressing the back button doesn't take it back to the HTTP page as its replaced in the history.

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 with String.prototype.split() and remove the protocol part of the URL.

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


Inserts an HTML string after the end of the specified element.

Use el.insertAdjacentHTML() with a position of 'afterend' to parse htmlString and insert it after the end of el.

const insertAfter = (el, htmlString) => el.insertAdjacentHTML('afterend', htmlString);
insertAfter(document.getElementById('myId'), '<p>after</p>'); // <div id="myId">...</div> <p>after</p>


Inserts an HTML string before the start of the specified element.

Use el.insertAdjacentHTML() with a position of 'beforebegin' to parse htmlString and insert it before the start of el.

const insertBefore = (el, htmlString) => el.insertAdjacentHTML('beforebegin', htmlString);
insertBefore(document.getElementById('myId'), '<p>before</p>'); // <p>before</p> <div id="myId">...</div>

Recommended Resource - JavaScript: The Hard Parts

Take your JavaScript to the next level. Gain an understanding of callbacks, higher order functions, closure, asynchronous and object-oriented JavaScript!


Returns true if the browser tab of the page is focused, false otherwise.

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


Converts a NodeList to an array.

Use spread operator inside new array to convert a NodeList to an array.

const nodeListToArray = nodeList => [...nodeList];
nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]


Returns 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 (all true).

const observeMutations = (element, callback, options) => {
  const observer = new MutationObserver(mutations => mutations.forEach(m => callback(m)));
        childList: true,
        attributes: true,
        attributeOldValue: true,
        characterData: true,
        characterDataOldValue: true,
        subtree: true
  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


Removes an event listener from an element.

Use EventTarget.removeEventListener() to remove an event listener from an element. Omit the fourth argument opts to use false 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


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 correct this context. Returns a reference to the custom delegator function, in order to be possible to use with off. Omit opts to default to non-delegation behavior and event bubbling.

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


Run the callback whenever the user input type changes (mouse or touch). Useful for enabling/disabling code depending on the input device. This process is dynamic and works with hybrid devices (e.g. touchscreen laptops).

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, using performance.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.');


Returns the prefixed version (if necessary) of a CSS property that the browser supports.

Use Array.prototype.findIndex() on an array of vendor prefix strings to test if document.body has one of them defined in its CSSStyleDeclaration object, otherwise return null. Use String.prototype.charAt() and String.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'


Invokes the provided callback on each animation frame.

Use recursion. Provided that running is true, continue invoking window.requestAnimationFrame() which invokes the provided callback. Return an object with two methods start and stop to allow manual control of the recording. Omit the second argument, autoStart, to implicitly call start when the function is invoked.

const recordAnimationFrames = (callback, autoStart = true) => {
  let running = true,
  const stop = () => {
    running = false;
  const start = () => {
    running = true;
  const run = () => {
    raf = requestAnimationFrame(() => {
      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


Redirects to a specified URL.

Use window.location.href or window.location.replace() to redirect to url. 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);


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 a Blob 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 promise, listening for onmessage and onerror 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'


Smooth-scrolls to the top of the page.

Get distance from top using document.documentElement.scrollTop or document.body.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.scrollTo(0, c - c / 8);


Encode a set of form elements as a query string.

Use the FormData constructor to convert the HTML form to FormData, Array.from() to convert to an array, passing a map function as the second argument. Use Array.prototype.map() and window.encodeURIComponent() to encode each field's value. Use Array.prototype.join() with appropriate argumens to produce an appropriate query string.

const serializeForm = form =>
  Array.from(new FormData(form), field => field.map(encodeURIComponent).join('=')).join('&');
serializeForm(document.querySelector('#form')); // email=test%40email.com&name=Test%20Name


Sets the value of a CSS rule for the specified element.

Use element.style to set the value of the CSS rule for the specified element to val.

const setStyle = (el, ruleName, val) => (el.style[ruleName] = val);
setStyle(document.querySelector('p'), 'font-size', '20px'); // The first <p> element on the page will have a font-size of 20px


Shows all the elements specified.

Use the spread operator (...) and Array.prototype.forEach() to clear the display 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


Smoothly scrolls the element on which it's called into the visible area of the browser window.

Use .scrollIntoView method to scroll the element. Pass { behavior: 'smooth' } to .scrollIntoView so it scrolls smoothly.

const smoothScroll = element =>
    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


Toggle a class for an element.

Use element.classList.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


Triggers a specific event on a given element, optionally passing custom data.

Use new CustomEvent() to create an event from the specified eventType and details. Use el.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' });


Generates a UUID in a browser.

Use crypto API to generate a UUID, compliant with RFC4122 version 4.

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'