promise-concurrency-limit.ts: Execute concurrently the asynchronous functions passed, with a limit in concurrency

promise-concurrency-limit.ts

/**
 * Execute concurrently the asynchronous functions `fns` passed, but
 * not more than `maxConcurrency` at the same time. All functions
 * return the same type T, an a promise with T[] inside is returned.
 */
const execConcurrent = async<T> (
  fns: (() => Promise<T>)[],
  maxConcurrency: number,
): Promise<T[]> => {
  const res: T[] = [];
  while (fns.length) {
    const results = await Promise.all( fns.splice(0, maxConcurrency).map(f => f()) );
    res.push(...results);
  }
  return res;
};