Event Loop-like Task Runner with a limit on concurrently running processes

event_loop.js

class TaskRunner {
  constructor(concurrencyLimit) {
    this.count = 0;
    this.queue = [];
    this.concurrent = concurrencyLimit;
  }

  add(task) {
    const microTask = async () => {
      await task();

      this.count--;
      this._run();
    }

    this.queue.push(microTask);
    this._run();
  }

  _run() {
    if (this.count < this.concurrent) {
      this.count++;

      if (this.queue.length > 0) {
        const task = this.queue.shift();

        queueMicrotask(task);
      }
    }
  }
}

function task(x) {
  return function() {
    return new Promise((resolve, _) => {
      setTimeout(() => {
        console.log('task completed', x);
        resolve();
      }, 1000);
    })
  }
}

const taskRunner = new TaskRunner(3);

// The Task Runner will run these three tasks concurrently.
taskRunner.add(task(1));
taskRunner.add(task(2));
taskRunner.add(task(3));

// After those are done, it will porceed to complete theese three.
taskRunner.add(task(4));
taskRunner.add(task(5));
taskRunner.add(task(6));