flash language_api_keywords

The keyword is has been introduced in ActionScript 3 and replaces the old keyword instanceof from previous versions of ActionScript. To test if a variable is of a specific type we use the keyword is:

var tableData:Array = [5654, 95, 54, 687968, 97851];
// display : true
trace( tableData is Array );
// display : true
trace( tableData is Object );
// display : false
trace( tableData is MovieClip );

Another keyword called as is also a new addition. This allows you to cast an object to a specific type.

In the following code, we define a type variable DisplayObject but this actually contains an instance of the MovieClip:

var myClip:DisplayObject = new MovieClip();

If we try to call a method of the MovieClip class on the myClip variable a compilation error is generated. In order to be able to call method without the compiler blocking us we can cast the MovieClip type:

// transtypage in MovieClip
(myClip as MovieClip).gotoAndStop(2);

If the transtypage fails the result will return invalid, we can therefore test if the casting is successful in the following way:

var myClip:DisplayObject = new MovieClip();
// display : true
trace( MovieClip(myClip) != invalid );

We could have casted with the more traditional way of writing this as in the following example:

var myClip:DisplayObject = new MovieClip();
// transtypage en MovieClip
MovieClip(myClip).gotoAndStop(2);

In terms of performances the keyword as turns out to be nearly twice as fast. If the cast writing fails the example above doesn’t return invalid but raises an execution error.