ActionScript for Flash MX: The Definitive Guide, Second Edition

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Chapter 12: Objects and Classes

In this case, the [] operator is less concise than the dot operator and offers no appar-

ent benefit. The

[] operator is useful when the property name is a complex expres-

sion. Next, rather than using a string literal, we specify the property name with an

expression that yields the string “radius”:

var prop = "radius"; // Set a variable that contains the property name.

trace(redBall[prop]); // The variable prop contains the

// property name "radius".

Built-in ActionScript properties are also accessed via the dot and [] operators. Here

we access the value of the Math object’s PI property, first using the dot operator,

then using the

[] operator:

trace(Math.PI);

trace(Math["PI"]);

However, in pure OOP, we’ll almost 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); // INCORRECT!

Remember that the dot operator requires us to specify a property as an

identifier, not a string, whereas the [ ] operator requires a string, not

an identifier.

Using a for-in Loop to Iterate over an Object’s Properties

In Chapter 8, you learned that a for-in loop can be used to enumerate ...