Built-in Symbol.asyncIterator in Array

Question

According to MDN Web Docs:

There are currently no built-in JavaScript objects that have the [Symbol.asyncIterator] key set by default.

My question is how come this is working:

function promises() {
    return [
        new Promise(x => setTimeout(() => x(1), 1000)),
        new Promise(x => setTimeout(() => x(2), 2000)),
        new Promise(x => setTimeout(() => x(3), 3000)),
    ]
}

for await (x of promises()) console.log(x)

for await falls back to a synchronous iterator

Bergi
– Bergi

2019-10-05 18:52:50 +00:00
Commented Oct 5, 2019 at 18:52 — Bergi
– Bergi, Commented Oct 5, 2019 at 18:52

Bravo · Accepted Answer · 2019-10-05 18:39:44Z

1

MDN also states that

The for await...of statement creates a loop iterating over async iterable objects as well as on sync iterables, including: built-in String, Array, Array-like objects (e.g., arguments or NodeList), TypedArray, Map, Set, and user-defined async/sync iterables - MDN (emphasis mine)

answered Oct 5, 2019 at 18:39

Bravo

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5 Comments

Rafi Henig Over a year ago

Stupid mistake, I didn't notice the words "by default"

Bravo Over a year ago

by default makes no difference

Rafi Henig Over a year ago

There's no object that comes with "asyncIterator" by default, but if you use "for await", "asyncIterator' is created.

Bravo Over a year ago

@RafiHenig - oh, I don't see where that is documented

Rafi Henig Over a year ago

Because "asyncIterator" must be used in order to run the code above, and if it's not there by default then it must be created some how.

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