理解prototype—原型链

eg：

//类   构造函数
function Person(name){
    this.name = name;
}

//实例化

var p1 = new Person3();

//创建原型对象
Person3.prototype.age = "23";
Person3.prototype.say = function(){
    alert(this.name+":"+this.age)
}
p1.say();