Is a JavaScript array iterator itself iterable according to ES6?

Question

The following works on the JavaScript runtimes I've checked (Node.js and Firefox):

let xs = [0, 1, 2]
let it = xs[Symbol.iterator]()
for (let x of it) console.log(x)

This works because the it iterator is itself iterable, i.e. it has a Symbol.iterator property. One could presume this is because Array.prototype[Symbol.iterator] is implemented as a generator.

My question is, can I rely on it being iterable? That is, is it required by the spec?

I've looked (this is the relevant part, I think) and I can't find anything that says that an array iterator should be iterable, or that it must be implemented with a generator.

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

You can! All standard iterators for core objects extend %IteratorPrototype% which specifically defines:

25.1.2.1 %IteratorPrototype% [ @@iterator ] ( )

The following steps are taken:

Return the this value.

The value of the name property of this function is [Symbol.iterator].

So every builtin iterator will automatically return itself when used as an iterable, so using it in a for...of or any other iteration context is safe.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 15, 2017 at 1:19

loganfsmyth

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Is a JavaScript array iterator itself iterable according to ES6?

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25.1.2.1 %IteratorPrototype% [ @@iterator ] ( )

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