The replace() method of the DOMTokenList interface replaces an existing token with a new token. If the first token doesn't exist, replace() returns false immediately, without adding the new token to the token list.

oldToken

A string representing the token you want to replace.

newToken

A string representing the token you want to replace oldToken with.

A boolean value, which is true if oldToken was successfully replaced, or false if not.

In the following example we retrieve the list of classes set on a <span> element as a DOMTokenList using Element.classList. We then replace a token in 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;

const result = classes.replace("c", "z");

span.textContent = result ? classes : "token not replaced successfully";

The output looks like this: