Question

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

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

and

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

?

The choice is just matter of convention, preference or are there cases where you can’t use one?

Answer 1

This article describes the differences well:

A key difference between casting and the as operator is the behavior
on failure. When a cast fails in ActionScript 2, null is returned.
When a cast fails in ActionScript 3, a TypeError is thrown. With the
as operator in ActionScript 3, whenever a cast fails the default value
for the datatype is returned.

as also allows you to cast to Array, which wasn’t possible before since the conversion function Array() took precedence.

EDIT: concerning performance, using as is reported to be faster than the function call style casting in various articles: [1] [2] [3]. The first article cited looks at the performance differences in depth and reports that as is 4x-4.5x faster.

EDIT 2: Not only is as 4x-4.5x faster in the normal best case, but when you wrap the (cast) style conversion in a try-catch block, and an error actually ends up being thrown, it’s more like 30x – 230x faster. In AS3, if you think you’re going to do something exceptional (in that it could throw an error) then it’s clear that you should always look before you leap. Never use try/catch unless forced to by the API, and indeed that means to never (cast) It also is instructive to look at the performance implications of try/catch even when no exception is thrown. There’s a performance penalty to setting up a try/catch block even in the happy case that nothing goes wrong.