Using the "as" operator vs casting?

Question

I've recently been reading the docs regarding data types in
AS3 and am still not clear on a few things. If anyone has blogged
on this, please point me there.
One problem is that "casting" syntax is the same as top-level
function syntax. Compare String(objString) to XML(event.result).
The latter is a top-level function, but the former is a "cast". Or
is it?
An option to the String cast is objString.toString(). What is
the difference? I am not so much interested in the underlying
details of the language, as in understanding when to use which.
The same question exists for the top-level function
XML(event.result), which can be done with the "as" operator:
event.result as XML. When to use which? Are there performance
issues? The "as operator" documentation is unsatisfying.
Any discussion of this will be appreciated.
Tracy

Answer 1

ActionScript 3.0 has the
as operator because it circumvents a deficiency with the
cast operation () in ActionScript. Consider this:
var a:Object = new Array(1,2,3,4);
var b:Array = Array(a);
What is b? It is an Array of 1 element: [ [1,2,3,4] ];
The
as operator takes care of this:
var b:Array = a as Array;
The actual difference in the operators is that if the
variable being cast is not of the proper type, the
as operator returns null while the () operator throws an
exception:
var j:Number = Number(a); // exception!
var k:Number = a as Number; // NaN in this case, but null
normally
The feeling here at Adobe is to use the
as operator since it is not ambiguous and just check for
null. If you prefer to catch exceptions and know when X(y) is a
cast vs a conversion, then use that. More of a preference but at
times only
as will do what you need.