AppleScript: The Definitive Guide, 2nd Edition

Declare Your Variables

Sounds like "eat your vegetables," doesn't it? Well, it should. Each motto is good advice, no matter how unpalatable it may seem at first. I strongly advise you to declare all your variables—even (especially!) your locals. Unfortunately, AppleScript gives you no help with this; you can't make it warn you when a variable is undeclared. Rather, it silently declares it for you. This behavior, doubtless intended to make programming in AppleScript easier, is in fact the source of more mistakes and confusion among AppleScript programmers than any other feature of the language.

The trouble is that if you are lulled into a false sense of security because there's no need to declare your variables, then you can be surprised when some other scope tramples them. For example, imagine that your script starts like this:

set x to 5

That's top-level code, so you've just implicitly declared x a global. This means that any other handler or script object anywhere in this script can access your x and change its value, just by saying this:

global x

This other code may not have intended to trample on your x; perhaps it was trying to establish a global of its own, possibly in order to communicate its value to code at a lower level, or to function as persistent storage between calls. But the damage is done, just because of the upward effects of a global declaration. And to think that you could have prevented this by declaring your top-level x local to start with.

The converse is also true. ...