ActionScript 3, Undocumented Features?

You have been entrusted to be in on some secret features of ActionScript 3.  Only a handful of programmers around the world will see this information.  This is mostly due to the fact you have been able to hack your way into one of the most obscure blogs on the planet!

Actually, I don't know if I am breaking new ground here but I haven't been able to find any mention of this feature.  I really discovered it sometime ago while writing an earlier post but I didn't know the extent of the find.

And the find is fairly extensive but I've come up with only a few practical uses and one of those was already discussed in the mentioned post.  I don't know how far back these features go.  It may be they were around before ActionScript 2; before I ever used Flash.  Maybe veteran Flash developers find it so ho-hum they forget to pass it on to newbies.  I haven't tried this with AS2 because I don't want to fire up Flash again and have to re-learn anything.  BE WARNED: Be careful how you use undocumented features.  Also, "undocumented" means I haven't been able to find anything about them in the Adobe documentation.

Please let me know if you can find this someplace.  Someone needs to write a book about how to best use and find available resources for Flash, Flex and ActionScript.

The Secret Features:

You can insert multiple comma separated expressions when the syntax calls for an expression between parentheses.

For example:

do
{
    // do something
} while( a = b, a == 0 );

The while test will be performed on the rightmost expression, a == 0, but variable a will be set to the value of b prior to the test.  You are not limited in the number of expressions but you are limited in the type of expression.  For example, you cannot declare a variable with a var and you can't use an if statement.  The expression must resolve to a value.  The ? : expression can be used and you can call a function.

The do while statement, the while statement and the E4X filter predicate expression are the best candidates for this feature.  However it can also be done in the return and switch statements although I can't see any value in using it.  The return statement doesn't even require parentheses.

I was hoping to show a compelling example using do while or while but haven't been able to think of one.  I made this discovery when I was creating a loop and using IViewCursor to iterate over an XMLListCollection.  The where clause was getting too long to fit into a line and I wanted to break it up.  So I thought, what the heck, why not try it.

You can also do something similar with the for statement:

for( I = 0, N = 2; N < 5,I < 3; I++, N++ ){}

I think I recall seeing examples of the for statement before, but I don't remember if it was for ActionScript.

I guess when the ActionScript parser encounters the open parenthesis it handles it as though it is a function call with arguments without paying too much attention to the context.  It's okay, I won't tell Adobe if you won't.