light light - 1 year ago 110
Javascript Question

Are HTMLCollection and NodeList iterables?

In ES6, an iterable is an object that allows

for... of
, and has a Symbol.iterator key.

Arrays are iterables, as are Sets and Maps. The question is: are HTMLCollection and NodeList iterables? Are they supposed to be?

MDN documentation seems to suggest a
is an iterable.

loops will loop over NodeList objects correctly, in browsers that support
(like Firefox 13 and later)

This appears to corroborate Firefox's behaviour.

I tested the following code in both Chrome and Firefox, and was surprised to find that Firefox seem to think they are iterables, but Chrome does not. In addition, Firefox thinks that the iterators returned by
are one and the same.

var col = document.getElementsByClassName('test'); // Should get HTMLCollection of 2 elems
var nod = document.querySelectorAll('.test'); // Should get NodeList of 2 elems
var arr = []; // Should get Array of 2 elems

console.log(col[Symbol.iterator]); // Firefox: iterator function, Chrome: undefined
console.log(nod[Symbol.iterator]); // Firefox: iterator function, Chrome: undefined
console.log(arr[Symbol.iterator]); // Firefox & Chrome: iterator function
console.log(col[Symbol.iterator] === nod[Symbol.iterator]); // Firefox: true
console.log(col[Symbol.iterator] === arr[Symbol.iterator]); // Firefox: false

<div class="test">1</div>
<div class="test">2</div>

One really weird, confusing thing: running the code snippet produces a different result from copying it and running in an actual file/console in Firefox (particularly last comparison). Any enlightenment on this weird behaviour here would be appreciated too.

Answer Source

Symbol.iterator support for NodeList, HTMLCollection, DOMTokenList, and DOMSettableTokenList was discussed and added to the WHATWG's DOM spec last year.