AS3: cast or “as”?

Question

Is there any difference of use, efficiency or background technique between

var mc:MovieClip = MovieClip(getChildByName(\"mc\"));

and

<

Answer 1

It is best practice to use the as keyword.

as has the advantage of not throwing an RTE (run-time error). For example, say you have a class Dog that cannot be cast into a MovieClip; this code will throw an RTE:

var dog:Dog = new Dog();
var mc:MovieClip = MovieClip(Dog);

TypeError: Error #1034: Type Coercion failed: cannot convert Dog to MovieClip.

In order for you to make this code "safe" you would have to encompass the cast in a try/catch block.

On the other hand, as would be safer because it simply returns null if the conversion fails and then you can check for the errors yourself without using a try/catch block:

var dog:Dog = new Dog();
var mc:MovieClip = Dog as MovieClip;
if (mc) 
    //conversion succeeded
else
    //conversion failed