A slow async/await version of promiseMap

promiseMap.js

async function promiseMap(array, mapper, concurrency) {
  const promises = [];
  const results = Array.from({ length: array.length });
 
  const drainOne = async () => {
    const [pj, { index, resolved, rejected }] = await Promise.race(promises.map((p, pi) => p.then((v) => [pi, v])));
    promises.splice(pj, 1);
    
    if (resolved) {
      results[index] = resolved[0];
    } else {
      throw rejected[0];
    }
  }
  
  for (const [i, item] of array.entries()) {
    if (promises.length == concurrency) {
      await drainOne();
    }
    
    promises.push(mapper(item, i, array).then(
      (resolved) => ({ index: i, resolved: [resolved] }),
      (rejected) => ({ index: i, rejected: [rejected] }),
    ));
  }
 
  while (promises.length) {
    await drainOne();
  }
  
  return results;
}