【JavaScript】动态原型模式创建对象 ||为何不能用字面量创建原型对象?

   var proto = "";
    function Person(name, age, job) {
        this.name = name;
        this.age = age;
        this.job = job;
        if (typeof this.sayname != ‘function‘) {
            //这里不能使用字面量创建原型对象
            //Person.prototype.sayname = function() {
            //    console.log("name = " + this.name);
            //   };
            proto = Person.prototype;//没有经过字面量重写的原型对象
            //假如使用字面量创建原型对象 
            Person.prototype =
            {
                sayname: function() {
                    console.log("Name:" + this.name);
                }
            }
            console.log(proto == Person.prototype)//false 说明Person原型对象已被重写改变
        }
    }
    var person = new Person("Jack", 25, ‘Programmer‘);
    console.log(person.__proto__ == proto);//true 实例对象的__proto__指向原先的原型对象,而不是被字面量重写的原型对象
    person.sayname();//undefined 没有新原型对象的方法

        function Person() { };
        var proto = Person.prototype;
        var person = new Person;//实例对象在前
        Person.prototype =
        {
            name: "Jack",
            sayname: function() {
                console.log("Name:" + this.name);
            }
        }
        console.log(Person.prototype.isPrototypeOf(person)); //false 新原型不在person对象原型链上
        console.log(proto.isPrototypeOf(person)); //true 原先的原型对象在person对象原型链上

        function Person() { }
        var person = new Person();//实例对象在前
        Person.prototype.name = ‘Jack‘;
        Person.prototype.sayName = function() {
            console.log(this.name);
        }
        person.sayName();//Jack