重新指向了constructor,p2怎么访问不到copy函数?

        function Person(name) {
            this.name = name;
        }

        Person.prototype.copy = function() {
            return new this.constructor(this.name);
        }
        var p1 = new Person('李四');
        //console.log(Person.prototype);
        Person.prototype = {
            show: function() {
                console.log('show');
            }
        }
        Person.prototype.constructor=Person;
        //console.log(Person.prototype);
        var p2 = new Person('张三');