function person(){
   this.name = 'taobao';
   this.showMess = function(){
      return this.name;
   }
}
person.prototype.getVal = function(){
   return "hellow word";
}
function child(){
   person.call(this);
   this.num = 123;
}

//提供一个寄生函数
function _initJS(p,c){
   var val = Object(p.prototype);
   val.constructor = c;
   c.prototype = val;
}
//调用寄生
_initJS(person,child);

//这里记得放在后面定义child
child.prototype.showNum = function(){
   return this.num
}

(){
   return this.num
}
var c2 = new child()
undefined
c2.getVal()
"hellow word"
c2.showMess()
"taobao"
c2.showNum()
123