Language overview

Objects lie at the heart of the ActionScript 3.0 language—they are its fundamental building blocks. Every variable you declare, every function you write, and every class instance you create is an object. You can think of an ActionScript 3.0 program as a group of objects that carry out tasks, respond to events, and communicate with one another.

Programmers familiar with object-oriented programming (OOP) in Java or C++ may think of objects as modules that contain two kinds of members: data stored in member variables or properties, and behavior accessible through methods. ActionScript 3.0 defines objects in a similar but slightly different way. In ActionScript 3.0, objects are simply collections of properties. These properties are containers that can hold not only data, but also functions or other objects. If a function is attached to an object in this way, it is called a method.

While the ActionScript 3.0 definition may seem a little odd to programmers with a Java or C++ background, in practice, defining object types with ActionScript 3.0 classes is very similar to the way classes are defined in Java or C++. The distinction between the two definitions of object is important when discussing the ActionScript object model and other advanced topics, but in most other situations the term properties means class member variables as opposed to methods. ActionScript 3.0 Reference for the Adobe Flash Platform, for example, uses the term properties to mean variables or getter-setter properties. It uses the term methods to mean functions that are part of a class.

One subtle difference between classes in ActionScript and classes in Java or C++ is that in ActionScript, classes are not just abstract entities. ActionScript classes are represented by class objects that store the class’s properties and methods. This allows for techniques that may seem alien to Java and C++ programmers, such as including statements or executable code at the top level of a class or package.

Another difference between ActionScript classes and Java or C++ classes is that every ActionScript class has something called a prototypeobject. In previous versions of ActionScript, prototype objects, linked together into prototype chains, served collectively as the foundation of the entire class inheritance hierarchy. In ActionScript 3.0, however, prototype objects play only a small role in the inheritance system. The prototype object can still be useful, however, as an alternative to static properties and methods if you want to share a property and its value among all the instances of a class.

In the past, advanced ActionScript programmers could directly manipulate the prototype chain with special built-in language elements. Now that the language provides a more mature implementation of a class-based programming interface, many of these special language elements, such as __proto__ and __resolve, are no longer part of the language. Moreover, optimizations of the internal inheritance mechanism that provide significant performance improvements preclude direct access to the inheritance mechanism.