The add() method of the DOMTokenList interface adds the given tokens to the list, omitting any that are already present.

tokenN

A string representing a token (or tokens) to add to the DOMTokenList.

None.

SyntaxError DOMException

Thrown if one of the arguments is an empty string

InvalidCharacterError DOMException

Thrown if a token contains ASCII whitespace.

In the following example, we retrieve the list of classes set on a <span> element as a DOMTokenList, using Element.classList. We then add a new token to the list, and write the list into 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;
classes.add("d");
span.textContent = classes;

The output looks like this:

You can add multiple tokens as well:

span.classList.add("d", "e", "f");