DOMTokenList.keys()

The keys() method of the DOMTokenList interface returns an iterator allowing to go through all keys contained in this object. The keys are of type unsigned integer.

None.

Returns an iterator.

In the following example we retrieve the list of classes set on a <span> element as a DOMTokenList using Element.classList. We then retrieve an iterator containing the keys using keys(), then iterate through those keys using a for ... of loop, writing each one to the <span>'s Node.textContent.

First, the HTML:

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

Now the JavaScript:

var span = document.querySelector("span");
var classes = span.classList;
var iterator = classes.keys();

for(var value of iterator) {
  span.textContent += value + ' ++ ';
}

The output looks like this: