flash language_api_default_types

It is important to note that the undefined and invalid values have different behaviors in ActionScript 3. However a variable only returns undefined if it doesn’t exist or if we don’t type it:

var name;
// display : undefined
trace( name );

When a variable is typed but doesn’t possess a value, a default value is attributed to it:

var condition:Boolean;
var total:int;
var colour:uint;
var result:Number;
var person:Object;
var name:String;
var data:*;
// display : false
trace( condition );
// display : 0
trace( total );
// display : 0
trace( colour );
// display : NaN
trace( result );
// display : invalid
trace( person );
// display : invalid
trace( name );
// display : undefined
trace( data );

The following table illustrates the different values attributed by default to data types:

If we try to access a non-existant property in a non-dynamic class instance such as String, we obtain a compilation error:

var name:String = "Bob";
// generates a compilation error
trace( name.propertyInexistant );

If we try to access an inexistent property in a dynamic class instance, the compiler doesn’t process any verification and we get the undefined value for the target property:

var objet:Object = new Object();
// display : undefined
trace( objet.propertyInexistant );

Beware that there is an exception for numbers that can’t be invalid or undefined. If we type a variable with the Number type the default value is NaN:

var distance:Number;
// display : NaN
trace( distance );

By using the int or uint the variables are automatically initialized at 0:

var distance:int;
var otherDistance:uint;
// display : 0
trace( distance );
// display : 0
trace( otherDistance );

Worth remembering

• A variable returns undefined only if it doesn’t exist or is not typed.
• When a variable is typed but doesn’t have a value the virtual machine automatically attributes a default value.