42Implement asyncMap() preserving result order

Implement asyncMap(items, mapper). It calls the async mapper(item, index) for every item in parallel and returns a promise of the results in input order, however long each call takes.

const users = await asyncMap([3, 1, 2], async (id) => {
  const res = await fetch(`/api/users/${id}`);
  return res.json();
});
// users[0] is user 3, even if user 1 arrived first

Requirements

• Start every mapper call right away (do not wait for one before starting the next).
• Results follow the input order, not completion order.
• Reject as soon as any call rejects, with that reason.
• If the mapper throws synchronously, reject instead of throwing.
• Accept any iterable; an empty input resolves to [].