AppleScript: The Definitive Guide, 2nd Edition

Scope of Undeclared Variables

In AppleScript, you do not have to declare variables. When you use a name that, by the preceding rules of scope, is not an existing variable, AppleScript does not complain; rather, it creates the variable for you. How it does this depends upon the location of the code that uses the nonexistent variable name:

Code at the top level

The variable is created as a global. There is no explicit global declaration, so there is no downward effect, but other scopes can see this variable through a global declaration. I call this an implicit global.

Code not at the top level

The variable is created as a local. I call this an implicit local .

Let's illustrate an implicit global first:

set x to 5
on getX( )
    global x
    display dialog x
end getX
getX( ) -- 5

The first line never said explicitly that x should be a global. But it clearly is one, since when getX comes along and asks to see a global called x, the x created in the first line is what it sees. (Incidentally, you can move the "set x to 5" line to after the getX handler definition and the script will still work; the important thing is that the global x be defined by the time getX runs, not necessarily before getX itself is defined.)

Incidentally, a variable created implicitly in a script's top-level explicit run handler is an implicit global as well, just as if you'd declared it at the absolute top level:

on run
    set howdy to "Howdy"
    sayHowdy( ) -- Howdy end run on sayHowdy( ) global howdy display dialog howdy end ...