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30 ActionScript 2.0 to 3.0

Default values

The following example shows the default values for variables of different types:

var dog:int; // defaults to 0

var dog:uint; // defaults to 0

var dog:Boolean;// defaults to false

var dog:Number; // defaults to NaN

var dog:Object; // defaults to undefined

var cat; // type defaults to Object, so default value is undefined

For all other classes (such as String, XML, MovieClip, Sprite, or any user-defined class), null

is the default value for uninitialized variables of that type.

var dog:String; // defaults to null

var dog:UserDefinedClass; // defaults to null

var dog:MovieClip; // defaults to null

With the exception of Object, the types with a non-null default value are the types which can

never hold

null as a value. They get what a typecast of null to that type would result in.

To check for a possibly uninitialized variable, compare against the defaults described in

“Default values” on page 30. Alternatively, assign an initial, impossible value to your variable’s

declaration which you can then test against; for example:

var length:int = -1;

if (someTest())

length = 2;

if (someOtherTest())

length = 3;

if (length == -1)

length = lastResort();

For instances of classes, use the following pattern:

if (o) // ActionScript 3.0

if (o != null) // ActionScript 2.0

and

if (!o) // ActionScript 3.0

if (o == null) // ActionScript 2.0

Because a Boolean can only be true or false, and never undefined, write the following when

checking if a Boolean is true:

if (b) // ActionScript 3.0

if (b == true) // ActionScript 2.0

if (b != false) // ActionScript 2.0

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MACROMEDIA FLEX 2-MIGRATING APPLICATIONS TO FLEX 2 Manuale Utente Pagina 30