方法的链式调用

es6写法：

class Man {
  constructor(state) {
    this.num = state;下·
  }

  add(number) {
    this.num += number
    return this;
  }
  minus(number) {
    this.num -= number
    return this;
  }
};

const man = new Man(123).add(10).minus(100);