JS - Constructor还可以这样用

JS中Constructor好用法：

即在只知道一个对象实例的情况下（不知道对象名），重新初始化一个新实例；

function Person( firstname, lastname, age ) {
　　this.firstname = firstname;
　　this.lastname = lastname;
 4　　 this.age = age;
}

var dave = new Person("Dave", "Smith", 28);

document.write( dave.constructor + "<br />");
// function Person(firstname, lastname, age) { this.firstname = firstname; this.lastname = lastname; this.age = age; }

// we can create a new object from another object's constructor:
var mike = new dave.constructor("Mike", "Fox", 22);

// Is mike an instance of the Person object?
document.write( mike.constructor == Person );    // true