Javascript interacting with the DOM

Bottom visible

This snippet checks whether the bottom of a page is visible.

const bottomVisible = () =>
  document.documentElement.clientHeight + window.scrollY >=
  (document.documentElement.scrollHeight ||

bottomVisible() // true
Element contains

This snippet checks whether the parent element contains the child.

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
Get style

This snippet can be used to get the value of a CSS rule for a particular element.

const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName]

getStyle(document.querySelector('p'), 'font-size') // '16px'

This snippet can be used to hide all elements specified.

const hide = (...el) => [...el].forEach((e) => ( = 'none'))

hide(document.querySelectorAll('img')) // Hides all <img> elements on the page
Insert after

This snippet can be used to insert an HTML string after the end of a particular element.

const insertAfter = (el, htmlString) =>
  el.insertAdjacentHTML('afterend', htmlString)

insertAfter(document.getElementById('myId'), '<p>after</p>') // <div id="myId">...</div> <p>after</p>
Insert before

This snippet can be used to insert an HTML string before a particular element.

const insertBefore = (el, htmlString) =>
  el.insertAdjacentHTML('beforebegin', htmlString)

insertBefore(document.getElementById('myId'), '<p>before</p>') // <p>before</p> <div id="myId">...</div>
Is browser tab focused

This snippet can be used to determine whether the browser tab is focused.

const isBrowserTabFocused = () => !document.hidden

isBrowserTabFocused() // true
Node list to array

This snippet can be used to convert a nodeList to an array.

const nodeListToArray = (nodeList) => [...nodeList]

nodeListToArray(document.childNodes) // [ <!DOCTYPE html>, html ]
scroll to top

This snippet can be used to do a smooth scroll to the top of the current page.

const scrollToTop = () => {
  const c = document.documentElement.scrollTop || document.body.scrollTop
  if (c > 0) {
    window.scrollTo(0, c - c / 8)


This snippet can be used to serialize a cookie name-value pair into a Set-Cookie header string.

const serializeCookie = (name, val) =>

serializeCookie('foo', 'bar') // 'foo=bar'
Set style

This snippet can be used to set the value of a CSS rule for a particular element.

const setStyle = (el, ruleName, val) => ([ruleName] = val)

setStyle(document.querySelector('p'), 'font-size', '20px')
// The first <p> element on the page will have a font-size of 20px

This snippet can be used to show all the elements specified.

const show = (...el) => [...el].forEach((e) => ( = ''))

show(...document.querySelectorAll('img')) // Shows all <img> elements on the page
Smooth scroll

This snippet can be used to smoothly scroll the element on which it is called into the visible area of the browser window.

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
Start a css transition with js fairly synchronosly.

But watchout, the new web animation api put this snippet in the legacy bin. Read my intro article.

function setTransitionStylesOnElement(element, startStyles, endStyles) {
  return new Promise((resolve) => {
    Object.assign(, startStyles)
    setTimeout(() => Object.assign(, endStyles), 100)
    element.addEventListener('transitionend', postAnimation)
    function postAnimation() {
      element.removeEventListener('transitionend', postAnimation)
// Useage.
const hiddenStyles = { opacity: 0, transition: 'all 0.5s' }
const visibleStyles = { opacity: 1, transition: 'all 0.5s' }

setStylesOnElement(overlayElement, visibleStyles, hiddenStyles).then((el) =>
Lazy-loading stylesheet on scroll
const loadStyle = (src) => {
  if (document.createStylesheet) {
  } else {
    const stylesheet = document.createElement('link')
    stylesheet.href = src
    stylesheet.type = 'text/css'
    stylesheet.rel = 'stylesheet'
let scrollFlag = false
window.onscroll = () => {
  if (!scrollFlag) {
    console.log('Started Scrolling!')
    setTimeout(() => {
    }, 1000)
    scrollFlag = true
Mutation observer example
// Select the node that will be observed for mutations
const targetNode = document.getElementById('some-id')

// Options for the observer (which mutations to observe)
const config = { attributes: true, childList: true, subtree: true }

// Callback function to execute when mutations are observed
const callback = function (mutationsList, observer) {
  // Use traditional 'for loops' for IE 11
  for (const mutation of mutationsList) {
    if (mutation.type === 'childList') {
      console.log('A child node has been added or removed.')
    } else if (mutation.type === 'attributes') {
      console.log('The ' + mutation.attributeName + ' attribute was modified.')

// Create an observer instance linked to the callback function
const observer = new MutationObserver(callback)

// Start observing the target node for configured mutations
observer.observe(targetNode, config)

// Later, you can stop observing