ActionScript: The Definitive Guide

Object Properties

Properties are named data containers associated with an object. They are defined by an object’s class and then set individually for each object instance. Like variables, object properties can contain any kind of data—strings, numbers, Booleans, null, undefined, functions, arrays, movie clips, or even other objects.

Referring to Properties

The familiar dot operator gives us access to an object’s properties. We separate the name of the property from the object it belongs to using a dot (a period), as follows:

                  objectName.propertyName

where objectName is the name of our object and propertyName must be a legal identifier that matches the name of some property of objectName.

For example, if we have a ball object instance with a radius property, we can access radius using:

ball.radius

Alternatively, we may refer to a property using the [] operator, as follows:

                  objectName[propertyName]

The [] operator allows us to compose a property name using any expression that resolves to a string. For example:

trace(ball["radius"]);

var prop = "radius";
trace(ball[prop]);  // prop resolves to "radius"

Built-in ActionScript properties are accessed in exactly the same way. Recall the syntax for retrieving the value of pi:

Math.PI

In that expression, we’re accessing the built-in PI property of the Math object. However, in pure OOP, we’ll nearly never access an object’s properties directly; instead, we’ll use methods to access property values. For example, to check the volume property of an instance ...