Sync iterables and [[asyncIterator]]

[Isiah Meadows]

There isn't currently a means to trivially convert them, but sync iterables
can still be used in `for await`. They're implicitly wrapped into an async
iterable\*, and then used that way.

\* That's supposed to be a spec implementation detail, but was accidentally
exposed at one point. I recall reading about it somewhere, but I can't
easily find it.

On Sun, Aug 19, 2018 at 14:38 Herbert Vojčík <herby at mailbox.sk> wrote:

> Hello!
>
>
> Lately, I created this in one project:
>
> export async function* zip (...iterables) {
>      for (const iterators = iterables.map(each => (each[Symbol.iterator]
> || each[Symbol.asyncIterator]).apply(each)); ;) {
>          const all = await Promise.all(iterators.map(async each => await
> each.next()));
>          if (all.some(each => each.done)) break;
>          yield all.map(each => each.value);
>      }
> }
>
> It was sync generator before, but once one of the entries became async
> generator, I changed the zip itself to be async.
>
> My question is to the pretty ugly
>
>    const iterators = iterables.map(each => (each[Symbol.iterator] ||
> each[Symbol.asyncIterator]).apply(each))
>
> Is there some more idiomatic way to find "any iterator; sync or async" /
> alternatively, could sync iterator return respective [[asyncIterator]]
> as well, just promisifying the result (as a sort of proto-proposal of
> sort)?
>
> Or is there some easy way to change sync iterable to async iterable, so
> I could change the other ones at call site?
>
>
> Thanks, Herby
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