The read-only length property of the DOMTokenList interface is an integer representing the number of objects stored in the object.

An positive integer, or 0 if the list is empty.

In the following example we retrieve the list of classes set on a <span> element as a DOMTokenList using Element.classList, then write the length of the list to the <span>'s Node.textContent.

First, the HTML:

<span class="a b c"></span>

Now the JavaScript:

const span = document.querySelector("span");
const classes = span.classList;
const length = classes.length;

span.textContent = `classList length = ${length}`;

The output looks like this: