concurrently.ts

export async function* concurrently<T>(...gs: AsyncGenerator<T, void>[]): AsyncGenerator<T, void> {
  const k = (i: number) => (res: IteratorResult<T>) => [i, res] as const;
  const running = new Set(gs);
  try {
    const ps = gs.map((g, i) => g.next().then(k(i)));
    const queue = new Set(ps);
    while (queue.size > 0) {
      const [i, res] = await Promise.race(queue);
      const g = gs[i]!;
      queue.delete(ps[i]!);
      if (res.done) {
        running.delete(g);
      } else {
        const p = ps[i] = g.next().then(k(i));
        queue.add(p);
        yield res.value;
      }
    }
  } finally {
    await Promise.all(Array.from(running, g => g.return()));
  }
}