limit concurrency of promises

limit.ts

async function limitConcurrency<T, U>(
  arr: T[],
  fn: (t: T) => Promise<U>,
  limit: number,
): Promise<U[]> {
  const results: Promise<U>[] = [];
  const pool: Promise<void>[] = [];

  for (const elem of arr) {
    // add promise to the pool
    const promise = fn(elem);
    const running = promise.then(() => {
      const index = pool.indexOf(running);
      pool.splice(index, 1);
    });
    pool.push(running);
    results.push(promise);
    // if the pool is not full continue
    if (pool.length < limit) {
      continue;
    }
    // wait for one promise in the pool to resolve
    await Promise.race(pool);
  }

  return Promise.all(results);
}

const timeout = (i: number) =>
  new Promise(resolve => {
    setTimeout(() => resolve(i), i);
  });

async function main(): Promise<void> {
  await limitConcurrency(
    [100, 500, 300, 200, 1000, 200, 300, 50, 100, 120, 1232, 500],
    timeout,
    4,
  );
}

main();