.AS File Format – Type checking and conversion

Type annotations

A colon and a type follow the name of a variable, constant or parameter, and the parameter list of a function for its return type.

var count:int = 0;
const LABEL:String = "Score";
var anything:*;                       // untyped, may hold undefined

function scale(value:Number, by:Number = 2):Number {
    return value * by;
}

function reset():void {
    count = 0;
}

var items:Vector.<String> = new Vector.<String>();

The type is a class or interface name, *, or for a return type void. A missing annotation means *, but in strict mode the compiler warns about it.

Strict mode and standard mode

The compiler has two modes. Strict mode is the default in Flash authoring, Flash Builder and the Flex and AIR compilers (-strict=true); standard mode is -strict=false, or turning off strict mode in the publish settings. The run-time checks are the same in both.

Strict mode treats untyped values (*, or a value read with []) as unchecked: the compiler lets them through and the runtime checks them. Standard mode exists for code written in a looser style, such as code moved from ActionScript 2.0.

Implicit conversion

A value is converted when it is assigned, passed as an argument, returned or used with an operator such as + or <. The rules depend on the target type:

• Numeric types – int, uint and Number convert to one another silently, even in strict mode. The result can wrap: var u:uint = -3 stores 4294967293.
• Boolean – any value converts, by the rule in the table below, in both modes.
• String, Number from an unrelated type – strict mode refuses it at compile time; in standard mode, or when the value is untyped, the runtime converts it by the tables below.
• A class or interface type – the value must be an instance of that class, a subclass or an implementing class, or null. Otherwise the runtime throws TypeError 1034, “Type Coercion failed”.

Explicit conversion and casts

Writing the type as a function, Type(value), converts or casts the value on purpose, which strict mode accepts.

Testing types

For the exact class name, flash.utils.getQualifiedClassName(x) gives a string such as "flash.display::Sprite", and flash.utils.describeType(x) gives the whole type as XML.

Conversion to Number, int and uint

Octal is not read: Number("044") is 44, where ActionScript 2.0 gave 36. uint("-5") is 4294967291. parseInt and parseFloat read a number from the start of a string and stop at the first character that does not fit.

Conversion to Boolean

Conversion to String