MACROMEDIA FLEX 2-MIGRATING APPLICATIONS TO FLEX 2 Bedienungsanleitung PDF herunterladen (Seite 31)

Initializing variables 31

About undefined

In general, ActionScript 2.0 allowed accessing undeclared variables whose value had not yet

been set. By default, their value was

undefined. In ActionScript 3.0, accessing undeclared

variables results in a ReferenceError. You can use the

hasOwnProperty() method to check if a

variable has been declared:

if (hasOwnProperty('b') == false)

b = 20;

if (someObj.hasOwnProperty('myDynamicProp') == false)

someObj.myDynamicProp = 22;

Alternately, you can use a try/catch block to catch the ReferenceError; for example:

try {

bb;

} catch(x:ReferenceError) {

print("no bb");

bb = 20;

}

Another alternative is to check if the typeof the variable is undefined.

if (typeof bb == undefined)

bb = 22;

The typeof keyword does not throw a ReferenceError in this case, but rather returns

undefined.

For a dynamic class such as Object, the compiler expects that instances will often have

dynamic properties added to them. The compiler does not throw a ReferenceError when

accessing an undeclared property of a variable if its value is an instance of a dynamic class; for

example:

var a:Object = new Object();

if (a.foo == undefined) // Does not throw a ReferenceError.

a.foo = 22;

About NaN

NaN (Not a Number) is a special instance of Number used to indicate a value which is outside

the range of valid Numbers. Any Boolean valued comparison involving

NaN (such as ==, ===,

=>, and >) always returns false because any Number operation involving NaN is bogus. This

is exactly how C++, Java, and other languages treat

NaN as well.

For comparisons with NaN, use the global isNaN() method:

var a:Number;

if (isNaN(a))

a = 22;

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MACROMEDIA FLEX 2-MIGRATING APPLICATIONS TO FLEX 2 Bedienungsanleitung Seite 31