prototype属性可以将属性和方法添加到任何对象(Number, Boolean, String 和Date等)。
注：原型(Prototype)是一个全局的属性，它可以使用在几乎所有的对象。
语法
object.prototype.name = value
实例:
这里有一个例子展示了如何使用原型(prototype)属性的属性添加到对象：
<html>
<head>
<title>User-defined objects</title>
<script type="text/javascript">
function book(title, author){
this.title = title;
this.author = author;
}
</script>
</head>
<body>
<script type="text/javascript">
var myBook = new book("Perl", "Mohtashim");
book.prototype.price = null;
myBook.price = 100;
document.write("Book title is : " + myBook.title + "<br>");
document.write("Book author is : " + myBook.author + "<br>");
document.write("Book price is : " + myBook.price + "<br>");
</script>
</body>
</html>
这将产生以下结果：
Book title is : Perl Book author is : Mohtashim Book price is : 100