javascript parallel async promise races in batches

promiseBatch.js

const promiseBatch = async function* (concurrency, list, fn) {
  for (let p = [], i = 0, l = list.length; i < l || p.length > 0;) {
    if (i < l) {
      let _p;
      _p = fn(list[i]).then(r => [_p.__id, r]);
      _p.__id = i++;
      if (p.push(_p) < concurrency) {
        continue;
      }
    }
    const [id, res] = await Promise.race(p);
    p = p.filter(x => x.__id !== id);
    yield res;
  }
};

const compileTranslationUnit = async (unit) => {
  // example async work function ...
};

for await (const r of promiseBatch(CONCURRENCY, unit_files, compileTranslationUnit)) {
   // r is whatever the fn returns (optional), in the order of completion
}