Should it await `next.value` when not passing a mapper function?

[nicolo-ribaudo]

Consider this async iterable:

const it = {
  [Symbol.asyncIterator]() {
    return {
      async next() {
        if (i > 2) return { done: true };
        i++;
        return { value: Promise.resolve(i), done: false }
      }
    }
  }
}

Unless I'm reading the spec wrong, await Array.fromAsync(it) returns [Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)] while await Array.fromAsync(it, x => x) returns [1, 2, 3].

Currently, when using Array.from, x => x is the "default mapper function" (even if it's not specified as such): Array.from(something) is always equivalent to Array.from(something, x => x). However, there is no "default function" that can be passed to Array.fromAsync.

If we changed Array.fromAsync(something) to await the next.value values, await Array.fromAsync(it) would always return the same result as await Array.fromAsync(it, x => x). In practice, it means making it behave like

const arr = [];
for await (const nextValue of it) arr.push(await nextValue);

[nicolo-ribaudo]

I just noticed that the readme says (emphasis is mine)

mapfn is a mapping callback, which is called on each value yielded from the input – the result of which is awaited then added to the array. Unlike Array.from, mapfn may be an async function.) By default, this is essentially an identity function.

[ljharb]

If the default is identity, then explicitly passing identity must of course produce the same values. My intuition is that if the explicit identity awaits, then the implicit one also should.

[js-choi]

Yes, good catch; I agree. The value of each iteration should be awaited when no mapping function is given. This is a spec bug, and I will fix it soon.

[zloirock]

Iteration helpers do not await in similar cases.

In for-await-of, nextValue is a promise, required explicit awaiting.

I think that it should be agreed upon between both proposals.

[zloirock]

tc39/proposal-iterator-helpers#168

[zloirock]

@js-choi it's better to leave it open until aligning it with the iterator helpers proposal.