重写原型对象之后，为什么最初的原型对象还会存在

Question

        function Person(){}        var friend=new Person();        console.log(Person.prototype);            
        Person.prototype={                        
            constructor:Person,            name:'ytf',            age:'20',            job:'student',            sayName:function(){                console.log(this.name);
            }
        }        console.log(Person.prototype);        console.log(friend.sayName());