20 实现ES6的extends

    function B(name){
      this.name = name;
    };
    function A(name,age){
      //1.将A的原型指向B
      Object.setPrototypeOf(A,B);
      //2.用A的实例作为this调用B,得到继承B之后的实例，这一步相当于调用super
      Object.getPrototypeOf(A).call(this, name)
      //3.将A原有的属性添加到新实例上
      this.age = age; 
      //4.返回新实例对象
      return this;
    };
    var a = new A('poetry',22);
    console.log(a);