Why doesn't .includes() work with .classList?

Question

element.classList returns an array of classes, its my understanding .includes() is used with arrays, so I don\'t understand why this wont work, I k

Answer 1

Element.classList is a DOMTokenList object, though it prints an array-like in console. But if you try on Firefox, it'd return DOMTokenList["main-nav"]

And, includes is a method of Array instead of DOMTokenList.

Which is why it's expected to encounter li.classList.includes is not a function in your case.

You can use ES2015 spread operator to cast it to be an array.

[...li.classList].includes('main-nav')

Or alternatively, you can use DOMTokenList.contains method.

li.classList.contains('main-nav')

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Why is it declared as includes instead of has or contains? (thanks to @akinuri)

Quoting from the proposal

The web has classes like DOMStringList and DOMTokenList which are array-like, and have methods named contains with the same semantics as our includes. Unfortunately, meshing with those is not web-compatible.

Answer 2

The reason includes doesn't work is due to classList not being an array, but an array-like object. In this case it is a DOM Token List.

You can convert an array-like object to an array by using the following:

var liClasses = [].slice.apply(li.classList);

or

var liClasses = [...li.classList]; // es2015 syntax

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Otherwise, .includes() should be .contains(). See https://developer.mozilla.org/en-US/docs/Web/API/Element/classList

li.classList.contains('main-nav')

contains( String )

Checks if specified class value exists in class attribute of the element.