Queryselector and querySelectorAll vs getElementsByClassName and getElementById in JavaScript

javascript

performance

best-practices

dom-manipulation

byAlex Kataev·Jan 25, 2025

querySelector is your tool when you need CSS-selector style and expect one match:

const element = document.querySelector('.myClass'); // The classy one

When you're after multiple elements with the same selector, call for querySelectorAll and it delivers a static NodeList:

const elements = document.querySelectorAll('.myClass'); // The classy crowd

However, if your quest involves finding multiple members by their class, getElementsByClassName brings them to you live, in an ever-changing HTMLCollection:

const liveElements = document.getElementsByClassName('myClass'); // The changing class

Your swift ride to reach a unique element by its ID is getElementById:

const uniqueElement = document.getElementById('myId'); // One in a billion

When we talk about speed, getElementById and getElementsByClassName race ahead. Yet, querySelector/querySelectorAll pull out the win in cases requiring complex validation.

Performance chase and method chaining

When every millisecond matters, especially in larger DOMs, chaining getElementsByClassName after getElementById gains you a speed boost:

const myContainer = document.getElementById('myContainerId');
const itemsInMyContainer = myContainer.getElementsByClassName('items'); // The items in the container are fast and furious

This chaining method is a nimble tree navigator, descending from a well-known root, hence reducing search scope.

Keeping up with live changes using getElementsByClassName or getElementById ensures smooth performance, even when dealing with dynamic content or infinite scrolling.

The art of advanced querying

The party gets fancier when the guest list requires complex CSS selectors, querySelector becomes the VIP:

const coolInputs = document.querySelectorAll('input[type="checkbox"]:checked'); // Only the checked, no unchecked allowed

Got unique characters in your IDs? Worry not, querySelector comes with a cloak of invisibility using backslashes:

const myElement = document.querySelector('#unique\\:ID');  // Not so unique anymore

The trade-off game

Shifting gears between speed (getElement*) and swiss-army-knife flexibility (querySelector*)
Noting the live vs static collections duel: DOM changes do not affect NodeList from querySelectorAll
Shining some light on dynamic ID support: getElementById adopts newer IDs without a hitch
Sneak Peek: jQuery fans may find jQuery select commands zippier than querySelectorAll sometimes

NodeList vs HTMLCollection: The fine print

Let's reveal the secrets behind NodeList and HTMLCollection:

NodeList hearts forEach() method. HTMLCollection doesn't. You need to convert it to use Array methods.
querySelectorAll returns NodeList - a static snapshot, unlike the live HTMLCollection from getElementsByClassName.

Treading wisely with dynamic content

Handling dynamic IDs: the sturdy getElementById comfortably sails the sea of change
Conscious altering live collections: changing a member of a HTMLCollection might cause disruptive ripples, tread carefully

Dashing through special characters

Dealing with special characters in selectors: backslashes (\) for querySelector, they thing they are pretty dashing!
Consult Mozilla's documentation for an in-depth understanding of the NodeSelector interface. They do a fantastic job, yes Mozilla, we're fans!