ActionScript for Flash MX: The Definitive Guide, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

The Almighty Prototype Chain

|

305

The __proto__ Property

We saw earlier in this chapter that the instanceof operator tells us whether an

instance belongs to a particular class. But how does the interpreter know an individ-

ual object’s class? Or, more specifically, how does the interpreter know which con-

structor’s

prototype to access when searching for an object property? While that kind

of internal information normally is hidden in object-oriented languages, Action-

Script exposes it to the developer.

When any object is created, the interpreter automatically assigns it a special prop-

erty called

__proto__ (note the two underscores on either side of the word proto).

Then, into

__proto__, the interpreter copies a reference to the object’s class’s

prototype property. For example, when we create an instance of Book called asGuide,

asGuide.__proto__ is set to Book.prototype:

var asGuide = new Book("Colin Moock", "A book about ActionScript");

trace(asGuide.__proto__ = = Book.prototype); // Displays: true

Primarily, the __proto__ property is used internally by the ActionScript interpreter.

However, we can use it to determine the class of an object (as shown earlier in

Example 12-5) or to access overridden properties. Although

__proto__ is writable, it

should not be tampered with in most situations. ...