prototype屬性可以將屬性和方法新增到任何物件(Number, Boolean, String 和Date等)。
註：原型(Prototype)是一個全域性的屬性，它可以使用在幾乎所有的物件。
object.prototype.name = value
這裡有一個例子展示了如何使用原型(prototype)屬性的屬性新增到物件：
<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>
這將產生以下結果：
Book title is : Perl Book author is : Mohtashim Book price is : 100