Promise.all with concurrency limit

promise-all-concurrent.md

Create n parallel runners to run Promises, to avoiding running too many Promises in the mean time with Promise.all()

promise-all-concurrent.ts

const promiseAllConcurrent = async <T>(
  concurrent: number,
  values: readonly (() => T | PromiseLike<T>)[]
): Promise<T[]> => {
  // store the resolved result
  const result: T[] = [];

  // next index of Promise to resolve
  let nextIndex = 0;

  // runner to resolve Promise of `nextIndex`
  const run = async (): Promise<void> => {
    if (nextIndex < values.length) {
      const currentIndex = nextIndex;
      nextIndex += 1;
      result[currentIndex] = await values[currentIndex]();
      return run();
    }
  };

  await Promise.all(new Array(concurrent).fill(undefined).map(() => run()));

  return result;
};

const timeout = (n) =>
  new Promise((resolve) =>
    setTimeout(() => {
      console.log(`${n} resolved!`);
      resolve(n);
    }, n)
  );

promiseAllConcurrent(2, [
  () => timeout(3000),
  () => timeout(1000),
  () => timeout(2000),
  () => timeout(4000),
  () => timeout(6000),
  () => timeout(5000),
]).then(console.log);