Queue 的那一题我是这么写的，不过没有用promise，不知道是不是必须要用。         class Queue {             constructor () {                 this.queue = [];                 this.timer;             }             next () {                 if (this.queue.length) {                     let curr_fn = this.queue.shift();                     this.timer = curr_fn();                 }             }             start () {                 this.next();             }             stop () {                 if (this.timer) {                     clearTimeout(this.timer);                 }             }             task (time, fn) {                 this.queue.push(() => {                     let timer = setTimeout(() => {                         fn();                         this.next();                     }, time);                     return timer;                 });                 return this;             }         }