<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> </head> <body> <!--使用 prototype 属性可以向对象添加属性：--> <script type="text/javascript"> function employee(name,job,born) { this.name=name; this.job=job; this.born=born; } var bill=new employee("Bill Gates","Engineer",1985); employee.prototype.salary=null; bill.salary=20000; employee.prototype = { testProperty:"zhangjie" }/*这样做不会删除原来的属性，原因在于prototype只是用来添加属性的，并不会删除属性*/ var newOne = new employee(); document.write(bill.salary);/*正常显示2000*/ document.write(bill.testProperty); /*undefined*/ /*testProperty属性是在bill之后才设定的，*/ document.write(newOne.name);/*undefined未定义*/ document.write(newOne.testProperty);/*zhangjie*/ /*正常显示*/ </script> </body> </html>