Node.js, very simple work queue

workQueue.ts

export type Task = () => Promise<void>;

export function addTask(doTask: Task) {
  TASKS.push(doTask);
  kickOffWorkers(MAX_WORKERS);
}

const TASKS: Task[] = [];
const workerPool: number[] = []; // "number" but really doesn't matter, this is just to keep track of how many parallel workers we already have
const MAX_WORKERS = 3;

function kickOffWorkers(numWorkers: number = MAX_WORKERS) {
  const numCurrentWorkers = workerPool.length;
  if (numCurrentWorkers < numWorkers) {
    const takeTaskAndRecurse = async () => {
      const task = TASKS.pop();
      if (!task) {
        // delete
        workerPool.splice(numCurrentWorkers, 1);
      } else {
        queueMicrotask(async () => {
          await task();
          queueMicrotask(takeTaskAndRecurse);
        });
      }
    };

    queueMicrotask(takeTaskAndRecurse);
    workerPool.push(Date.now()); // like I said, contents really don't matter
    // try to fill the worker pool
    kickOffWorkers(numWorkers);
  }
}

// DEMO
(async function () {
  const sleep = (milliseconds: number) => new Promise((resolve) => setTimeout(resolve, milliseconds));
  const sleepAndLog = (milliseconds: number, log: string) =>
    sleep(milliseconds).then(() => console.log(log, new Date()));

  console.log(`${MAX_WORKERS} threads`);
  console.log("_", new Date());

  addTask(async () => await sleepAndLog(1000, "1"));
  addTask(async () => await sleepAndLog(1000, "2"));
  addTask(async () => await sleepAndLog(1000, "3"));

  await sleep(4000);
  console.log("_", new Date());

  addTask(async () => await sleepAndLog(1000, "4"));
  addTask(async () => await sleepAndLog(1000, "5"));
  addTask(async () => await sleepAndLog(1000, "6"));
})();

Output with MAX_THREADS going from 1 to 3:

$ npx tsx index.ts
1 threads
_ 2024-06-13T04:47:46.777Z
3 2024-06-13T04:47:47.780Z
2 2024-06-13T04:47:48.781Z
1 2024-06-13T04:47:49.783Z
_ 2024-06-13T04:47:50.778Z
6 2024-06-13T04:47:51.780Z
5 2024-06-13T04:47:52.781Z
4 2024-06-13T04:47:53.783Z

$ npx tsx index.ts
2 threads
_ 2024-06-13T04:47:55.682Z
3 2024-06-13T04:47:56.686Z
2 2024-06-13T04:47:56.687Z
1 2024-06-13T04:47:57.687Z
_ 2024-06-13T04:47:59.685Z
6 2024-06-13T04:48:00.686Z
5 2024-06-13T04:48:00.686Z
4 2024-06-13T04:48:01.688Z

$ npx tsx index.ts
3 threads
_ 2024-06-13T04:48:04.119Z
3 2024-06-13T04:48:05.123Z
2 2024-06-13T04:48:05.124Z
1 2024-06-13T04:48:05.124Z
_ 2024-06-13T04:48:08.122Z
6 2024-06-13T04:48:09.124Z
5 2024-06-13T04:48:09.126Z
4 2024-06-13T04:48:10.127Z

(Maybe see also https://gist.github.com/fasiha/7f20043a12ce93401d8473aee037d90a)