用Promise实现：带延时功能的链式调用

 // 1) 调用方式
 new People('whr').sleep(3).eat('apple').sleep(5).eat('durian');

 // 2) 打印结果
 'hello, whr' -(等待3s)--> 'whr eat apple' -(等待5s)--> 'whr eat durian'

 // 3) 以下是代码实现
 class People {
   constructor(name) {
     this.name = name;
     this.sayHello();
     this.queue = Promise.resolve();
   }
   sayHello() {
     console.log(`hello, ${this.name}`);
   }
   sleep(time) {
     this.queue = this.queue.then(() => {
       return new Promise(res => {
         setTimeout(() => {
           res();
         }, time * 1000)
       })
     })
     return this;
   }
   eat(food) {
     this.queue = this.queue.then(() => {
       console.log(`${this.name} eat ${food}`);
     })
     return this;
   }
 }