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;
  }
}