12.3.1. Referring to Properties

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

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 of the built-in Sound class, we use:

trace(mySound.getVolume( ));

not:

trace(mySound.volume);

12.3.2. Using a for-in Loop to Access an Object's Properties

Like all loops, the for-in statement includes a header and a body. The body of a for-in statement is automatically executed once for each property in the specified object. We don't need to know the number of properties or their names, because as each cycle of the loop executes, our "iterator" variable automatically becomes the name of the next property. We can therefore access the properties of the object, like this:

// List all the properties of the ball object
for (var prop in ball) {
  trace("Property " + prop + " has the value " + ball[prop]);
}