The Element method getElementsByClassName() returns a live HTMLCollection which contains every descendant element which has the specified class name or names.

The method getElementsByClassName() on the Document interface works essentially the same way, except it acts on the entire document, starting at the document root.

names

A string containing one or more class names to match on, separated by whitespace.

An HTMLCollection providing a live-updating list of every element which is a member of every class in names.

As always, the returned collection is live, meaning that it always reflects the current state of the DOM tree rooted at the element on which the function was called. As new elements that match names are added to the subtree, they immediately appear in the collection. Similarly, if an existing element that doesn't match names has its set of classes adjusted so that it matches, it immediately appears in the collection.

The opposite is also true; as elements no longer match the set of names, they are immediately removed from the collection.

To look for elements that include among their classes a single specified class, we just provide that class name when calling getElementsByClassName():

element.getElementsByClassName("test");

This example finds all elements that have a class of test, which are also a descendant of the element that has the id of main:

document.getElementById("main").getElementsByClassName("test");

To find elements whose class lists include both the red and test classes:

element.getElementsByClassName("red test");

You can use either the item() method on the returned HTMLCollection or standard array syntax to examine individual elements in the collection. However, the following code will not work as one might expect because "matches" will change as soon as any "colorbox" class is removed.

const matches = element.getElementsByClassName("colorbox");

for (let i = 0; i < matches.length; i++) {
  matches[i].classList.remove("colorbox");
  matches.item(i).classList.add("hueframe");
}

Instead, use another method, such as:

const matches = element.getElementsByClassName("colorbox");

while (matches.length > 0) {
  matches.item(0).classList.add("hueframe");
  matches[0].classList.remove("colorbox");
}

This code finds descendant elements with the "colorbox" class, adds the class "hueframe", by calling item(0), then removes "colorbox" (using array notation). Another element (if any are left) will then become item(0).

We can also use Array methods on any HTMLCollection by passing the HTMLCollection as the method's this value. Here we'll find all <div> elements that have a class of test:

const testElements = document.getElementsByClassName("test");
const testDivs = Array.prototype.filter.call(
  testElements,
  (testElement) => testElement.nodeName === "DIV",
);