Add mapIntoIterable function

[aleclarson]

This is like Radashi's map function, but meant for use with for…await to allow handling of results as soon as possible. Unlike map, all items are processed in parallel, so perhaps a better name is needed. Once withSemaphore (#222) is introduced, you can easily limit how many items are processed at one time, if required.

Example:

// Fetch user details for a list of IDs
const userIds = ['id1', 'id2', 'id3'];
const fetchUser = async (id: string) => {
  const response = await fetch(`/api/users/${id}`);
  return response.json();
};

for await (const user of mapIntoIterable(userIds, fetchUser)) {
  console.log(user); // Users appear in original order as requests complete
}

Proposed implementation

async function* mapIntoIterable<T, U>(
  array: readonly T[],
  fn: (item: T, index: number) => Promise<U>
): AsyncIterableIterator<U> {
  const results = new Array<Promise<U>>(array.length);
  for (let i = 0; i < array.length; i++) {
    results[i] = fn(array[i], i);
  }
  for (const result of results) {
    yield await result;
  }
}

[aleclarson]

I ended up not using this, so it's probably not the right abstraction.

Closing for now, but if you find value in this proposal, please speak up. 👍