Parallel execution of multiple async functions

parallel-by.ts

export async function parallelBy(
  fns: Array<() => Promise<any>>,
  limit: number
) {
  let resolved = 0;
  let flying: Array<Promise<any>> = [];
  const results = Array(fns.length);
  let done = false;

  if (limit >= fns.length) {
    return Promise.all(fns.map((fn) => fn()));
  }

  function exec(index: number) {
    const fn = fns[index];
    const promise = fn();
    flying.push(promise);

    promise
      .then((result) => {
        results[index] = result;
        flying = flying.filter((p) => p !== promise);
        resolved += 1;
        done = resolved === fns.length;
      })
      .catch((reason) => {
        throw reason;
      });
  }

  let index = 0;
  while (index < limit) {
    exec(index);
    index++;
  }

  while (!done && index < fns.length) {
    await Promise.race(flying);
    exec(index);
    index++;
  }

  await Promise.all(flying);
  return results;
}