This section provides a quick description of how object inheriting properties and methods from its constructor's prototype object. A tutorial example and diagram is provided to show relations of an object, its constructor, and the constructor's prototype object.
From previous chapters in this book, we learned that:
A constructor is a function. But it is also an object.
Each constructor has built-in default property called "prototype".
The constructor's "prototype" is an object.
Properties and methods can be added to the constructor's prototype object.
Each object will inherit properties and methods from its constructor's prototype object.
To demonstrate those rules, I wrote the following tutorial example to allow "myBook"
to inherit the "price" property from the constructor's prototype object:
<html>
<!-- Inheritance_from_Prototype.html
Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
-->
<head>
<title>Inheritance from Prototype</title>
</head>
<body>
<pre>
<script type="text/javascript">
// Define the constructor: "Book"
function Book(title, author) {
this.title = title;
this.author = author;
}
// Adding a property to the "prototype"
Book.prototype.price = 9.99;
// Creating an object: "myBook"
var myBook = new Book("JavaScript Tutorials", "Herong Yang");
// Showing the object, constructor, and prototype
showObject(myBook, "myBook");
showObject(Book, "Book");
showObject(Book.prototype, "Book.prototype");
function showObject(object, name) {
document.writeln("\n\""+name+"\" properties:");
for (var item in object) {
document.writeln(" "+item+": "+object[item]);
}
}
</script>
</pre>
</body>
</html>
The output of this tutorial example confirms what we learned earlier:
