User Guide

Terminology 11
Terminology
As with all scripting languages, ActionScript uses its own terminology. The following list provides
an introduction to important ActionScript terms:
Boolean is a true or false value.
Classes are data types that you can create to define a new type of object. To define a class,
you use the
class keyword in a script file.
Constants are elements that don’t change. For example, the constant Key.TAB always has the
same meaning: it indicates the Tab key on a keyboard. Constants are useful for comparing values.
Constructors are functions that you use to define (initialize) the properties and methods of a
class. By definition, constructors are functions within a class definition that have the same name
as the class. For example, the following code defines a Circle class and implements a
constructor function:
// file Circle.as
class Circle {
private var circumference:Number;
// constructor
function Circle(radius:Number){
this.circumference = 2 * Math.PI * radius;
}
}
The term constructor is also used when you create (instantiate) an object based on a particular
class. The following statements are calls to the constructor functions for the built-in Array class
and the custom Circle class:
var my_array:Array = new Array();
var my_circle:Circle = new Circle(9);
Data types
describe the kind of information a variable or ActionScript element can contain. The
built-in ActionScript data types are String, Number, Boolean, Object, MovieClip, Function, null,
and undefined. For more information, see “About data types” on page 18.
Events are actions that occur while a SWF file is playing. For example, different events are
generated when a movie clip loads, the user clicks a button or movie clip, or the user types on
the keyboard.
Event handlers are special actions that manage events such as mouseDown or load. There are two
kinds of ActionScript event handlers: event handler methods and event listeners. Some
commands can be used both as event handlers and as event listeners and are included in both
subcategories. For more information on event management, see Developing Flex Applications.
Expressions are any legal combination of ActionScript symbols that represent a value. An
expression consists of operators and operands. For example, in the expression
x + 2, x and 2 are
operands and
+ is an operator.
Functions are blocks of reusable code that can be passed parameters and can return a value. For
more information, see “Creating functions” on page 41.