Types

Type Casting

Type casting is done with either the as operator:

var chair:Chair = furniture as Chair;

Or by wrapping the value in Type():

var chair:Chair = Chair(furniture);

If the cast fails with as, the result of that cast is null. If the cast fails by wrapping in Type(), a TypeError is thrown.

The Function type

Functions are of the type Function:

function example():void { }
trace(example is Function); // true

They can be referenced by other variables with the type Function:

var ref:Function = example;
ref(); // ref.call(), ref.apply(), etc.

And they can be passed in as arguments for parameters whose type is Function:

function test(callback:Function):void {
    callback();
}

test(function() {
    trace('It works!');
}); // Output: It works!

The Class type

References to class declarations are typed Class:

var spriteClass:Class = Sprite;

You can use variables typed Class to instantiate instances of that class:

var sprite:Sprite = new spriteClass();

This can be useful for passing an argument of type Class to a function that might create and instance of the provided class:

function create(type:Class, x:int, y:int):* {
    var thing:* = new type();
    
    thing.x = x;
    thing.y = y;
    
    return thing;
}

var sprite:Sprite = create(Sprite, 100, 100);

Annotating types

You can tell the compiler the type of a value by annotating it with :Type:

var value:int = 10; // A property "value" of type "int".

Function parameters and return types can also be annotated:

// This function accepts two ints and returns an int.
function sum(a:int, b:int):int {
    return a + b;
}

Attempting to assign a value with a mismatching type will result in a TypeError:

var sprite:Sprite = 10; // 10 is not a Sprite.

Checking types

You can use the is operator to validate whether a value is of a certain type:

var sprite:Sprite = new Sprite();

trace(sprite is Sprite); // true
trace(sprite is DisplayObject); // true, Sprite inherits DisplayObject
trace(sprite is IBitmapDrawable); // true, DisplayObject implements IBitmapDrawable
trace(sprite is Number); // false
trace(sprite is Bitmap); // false, Bitmap inherits DisplayObject
                         // but is not inherited by Sprite.

There is also an instanceof operator (deprecated) which works almost identical to is except that it returns false when checking for implemented interfaces and int/uint types.

The as operator can also by used just like is operator. This is especially usefull if you use some smart IDE like FlashDevelop which will give you a list of all possible properties of explicit object type. Example:

for (var i:int = 0; i < a.length; i++){
    var d:DisplayObject = a[i] as DisplayObject;
    if (!d) continue;
    d.//get hints here
    stage.addChild(d);
}

To get the same effect with is you would write (sligthly less convenient):

for (var i:int = 0; i < a.length; i++){
    if (a[i] is DisplayObject != true) continue;
    var d:DisplayObject = a[i] as DisplayObject;
    stage.addChild(d);
}

Just keep in mind that when checking coditions with as operator, given value will be fist converted to specified type and then result of that operation will be checked if not false, so be careful when using it with possible false/NaN values:

if(false as Boolean) trace("This will not be executed");
if(false as Boolean != null) trace("But this will be");

Below table shows some basic values and types with result of type operators. Green cells will evaluate to true, red to false and greay will cause compile/runtime errors.

Typed arrays

Unfortunately ActionScript 3 does not have a concept of generics, so it follows that there is no way to define a typed array as Array<T>. There is however a special class Vector.<T> which works in a similar way except you must provide a reference to a concrete class when instantiating the vector. This means there is no way to build abstractions on top of the Vector.<T> type (e.g. extend it and add new functionality) which is a huge drawback.

A simpler way to look at is is as though every class you define automatically had a companion class named Vector.<NameOfYourClass>.

With that said, there are still huge advantages to the Vector.<T> type over a conventional array:

• There is a performance increase when working with Vector.<T> vs arrays¹.
• You receive compile-type TypeErrors if you attempt to insert non-T values into the collection.
• IDEs provide useful type hinting information for the objects inside a Vector.<T> instance.

Examples of creating a Vector.<T>:

var strings:Vector.<String> = new Vector.<String>(); // or
var numbers:Vector.<Number> = new <Number>[];

¹ Vectors actually only provide notable performance improvements over arrays when working with primitive types (String, int, uint, Number, etc).