mergeAsyncIterators.mjs

export async function * mergeAsyncIterators(iterables) {
  const iterators = iterables.map(it => it[Symbol.asyncIterator]());
  const promises = new Map(iterators.map((it, i) => [i, it.next()]));

  while (promises.size > 0) {
    const next = Array.from(promises.entries(), async ([i, p]) => [i, await p]);
    const [index, result] = await Promise.race(next);

    if (result.done) {
      promises.delete(index);
    } else {
      yield result.value;
      promises.set(index, iterators[index].next());
    }
  }
}