本文转载自掘金，原文章地址 https://juejin.cn/post/7011400717843824654

          
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          /**
 * 编写一个异步任务调度器
 */
class Scheduler {
  list = []; //用来承载还未执行的异步

  count = 0; //用来计数

  constructor(num) {
    this.num = num; //允许同时运行的异步函数的最大个数
  }

  async add(fn) {
    // 锁
    if (this.count >= this.num) {
      await new Promise((resolve) => {
        this.list.push(resolve);
      });
    }

    this.count++;

    console.log('fn before executed');

    const result = await fn();

    console.log('fn executed');

    this.count--;

    console.log('%c this.list.length %s', 'color: #bfffc8', this.list.length);

    // 如果有任务完成 则后面排队接上
    if (this.list.length > 0) {
      console.log('this.list.shift()();');
      this.list.shift()();
    }

    return result;
  }
}

const schedule = new Scheduler(3); //最多同一时间让它执行3个异步函数

const asyncFactory = (n, time) => {
  return () => {
    return new Promise((resolve) => {
      setTimeout(() => {
        resolve(n);
      }, time);
    });
  };
};

schedule.add(asyncFactory(1, 2000)).then((n) => {
  console.log(`异步任务:${n}`);
});
schedule.add(asyncFactory(2, 2000)).then((n) => {
  console.log(`异步任务:${n}`);
});
schedule.add(asyncFactory(3, 2000)).then((n) => {
  console.log(`异步任务:${n}`);
});
schedule.add(asyncFactory(4, 2000)).then((n) => {
  console.log(`异步任务:${n}`);
});
schedule.add(asyncFactory(5, 2000)).then((n) => {
  console.log(`异步任务:${n}`);
});
schedule.add(asyncFactory(6, 2000)).then((n) => {
  console.log(`异步任务:${n}`);
});