text: function A() {
      this.abc = 12;
  }
  A.prototype.show = function() {
      console.log(this.abc);
  };
  function B() {
      A.call(this);
  }
  B.prototype = A.prototype; //把A 的原型指給B，但B.prototype被改， A也會被改變
  var obj = new B();
  console.log(obj.abc);
  obj.show();