asyncCombine.ts

export async function* asyncCombine<S, T>(
  a: AsyncIterable<S>,
  b: AsyncIterable<T>,
): AsyncIterable<[S | undefined, T | undefined]> {
  let aVal: S | undefined;
  let bVal: T | undefined;
  let aDone = false;
  let bDone = false;

  const aIter = a[Symbol.asyncIterator]();
  const bIter = b[Symbol.asyncIterator]();
  let aPromise: Promise<void> | undefined;
  let bPromise: Promise<void> | undefined;

  const aRequestNext = () =>
    aPromise = aIter.next().then((res) => {
      if (res.done) aDone = true;
      aVal = res.value;
      if (!aDone) aRequestNext();
    });
  const bRequestNext = () =>
    bPromise = bIter.next().then((res) => {
      if (res.done) bDone = true;
      bVal = res.value;
      if (!bDone) bRequestNext();
    });

  aRequestNext();
  bRequestNext();

  while (true) {
    if (!aDone && !bDone) await Promise.any([aPromise, bPromise]);
    else if (aDone) await bPromise;
    else if (bDone) await aPromise;

    if (aDone && bDone) return;
    yield [aVal, bVal];
  }
}