@thi.ng/transducers-async/sidechain.js

Async versions of various highly composable transducers, reducers and iterators

async function* sidechain(src, side, opts) {
  const { lastOnly = true } = opts || {};
  const $iter = src[Symbol.asyncIterator]();
  const $side = side[Symbol.asyncIterator]();
  const promises = [
    $iter.next().then((res) => [res]),
    $side.next().then((res) => [res, true])
  ];
  let buf = [];
  while (true) {
    const [res, side2] = await Promise.race(promises);
    if (res.done) return;
    if (side2) {
      promises[1] = $side.next().then((res2) => [res2, true]);
      if (!buf.length) continue;
      if (lastOnly) {
        yield buf[0];
        buf.length = 0;
      } else {
        yield buf;
        buf = [];
      }
    } else {
      promises[0] = $iter.next().then((res2) => [res2]);
      if (lastOnly) buf[0] = res.value;
      else buf.push(res.value);
    }
  }
}
export {
  sidechain
};