Flanagan - JavaScript: the Definitive Guide: Master the Worlds Most-Used Programming Language

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With Early Release ebooks, you get books in their earliest formthe authors raw and unedited content as he or she writesso you can take advantage of these technologies long before the official release of these titles. The following will be Chapter 9 in the final release of the book.

JavaScript objects were covered in Chapter 6. That chapter treatedeach object as a unique set of properties, different from every otherobject. It is often useful, however, to define a class of objectsthat share certain properties. Members, or instances, of the classhave their own properties to hold or define their state, but they alsohave methods that define their behavior. These methods are defined bythe class and shared by all instances. Imagine a class named Complexthat represents and performs arithmetic on complex numbers, forexample. A Complex instance would have properties to hold the real andimaginary parts (the state) of the complex number. And the Complexclass would define methods to perform addition and multiplication (thebehavior) of those numbers.

In JavaScript, classes use prototype-based inheritance: if two objectsinherit properties (generally function-valued properties, or methods)from the same prototype, then we say that those objects are instancesof the same class. That, in a nutshell, is how JavaScript classeswork. JavaScript prototypes and inheritance were coveredearlier, and you will need to be familiarwith that material to understand this chapter. Thischapter covers prototypes in .

If two objects inherit from the same prototype, this typically (butnot necessarily) means that they were created and initialized by thesame constructor function or factory function.