pure nodejs Promise queue

q.js

class TaskQueue {
  constructor (con) {
    this.con  = con;
    this.running = 0;
    this.queue = [];
  }
  pushTask (task) {
    this.queue.push(task);
    this.next();
  }
  next() {
    while (this.running < this.con && this.queue.length) {
      const task = this.queue.shift();
      console.log('started task');
      task().then(() => {
        this.running--;
        this.next();
      });
      this.running++;
    }
  }
}

(() => {
  const t1 = () => {
    return new Promise(resolve => {
      setTimeout(() => {
        console.log(1);
        resolve();
      }, Math.random() * (1000 - 50) + 50);
    });
  };
  const t2 = () => {
    return new Promise(resolve => {
      setTimeout(() => {
        console.log(2);
        resolve();
      }, Math.random() * (3000 - 500) + 500);
    });
  };
  const q = new TaskQueue(2);
  q.pushTask(t1);
  q.pushTask(t1);
  q.pushTask(t2);
  q.pushTask(t2);
  q.pushTask(t1);
  q.pushTask(t2);
  console.log('started');
})();