Sync iterables and [[asyncIterator]]

herby at herby at
Mon Aug 20 09:59:18 UTC 2018

On August 20, 2018 1:46:45 AM GMT+02:00, Gus Caplan <me at> wrote:
>If you use for-await syntax, the js implementation will internally use
>GetIterator[1] which will look for Symbol.asyncIterator and if it can't
>find it it will return an async wrapper around Symbol.iterator -Gus

Would be nice to have something like Reflect.getAsyncIteratorOf(...), then. Or AsyncIterator.of(...), if something like AsyncIterator exists.
That would do the wrapping of sync ones as well.

>[1]: ---- On Sun, 19
>Aug 2018 16:34:27 -0500 Herbert Vojčík <herby at> wrote ----
>Hello! Lately, I created this in one project: export async function*
>zip (...iterables) { for (const iterators = =>
>(each[Symbol.iterator] || each[Symbol.asyncIterator]).apply(each)); ;)
>{ const all = await Promise.all( each => await
>; if (all.some(each => each.done)) break; yield
> => 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 =
> => (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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