class Semaphore {
  constructor(max) {
    this.tasks = [];
    this.counter = max;
    this.dispatch = this.dispatch.bind(this);
  }

  dispatch() {
    if (this.counter > 0 && this.tasks.length > 0) {
      this.counter--;
      this.tasks.shift()();
    }
  }

  release() {
    this.counter++;
    this.dispatch();
  }

  acquire() {
    return new Promise(res => {
      this.tasks.push(res);
      setTimeout(this.dispatch, 100);
    });
  }
}

var semaphore = new Semaphore(2);

var run = (async() => {
  await semaphore.acquire();

  console.log('first runner');

  await semaphore.acquire();

  console.log('second runner');

  await semaphore.acquire();

  console.log('third runner');
})();