AppleScript: The Definitive Guide, 2nd Edition

Declaration and Definition of Variables

There is no requirement in AppleScript that variables be declared explicitly. The rule is basically that if you use a word that AppleScript doesn't understand, the word is assumed to be the name of a variable. The following code, as a complete script, will compile just fine:

set x to x + 1

Although it is not necessary to declare a variable, it is possible to declare a variable. I'll explain how you do this, and argue that there are good reasons for doing it, in Chapter 10.

Definition

The code in that last example, as a complete script, will compile, but it won't run; at runtime, it generates an error. The error message reads: "The variable x is not defined." The problem is not that the variable x has never been declared! There is no need to declare it. AppleScript understands (or assumes) that x is supposed to be a variable. Nor is the problem that you are trying to assign to it. The problem is that you are trying to fetch its value, and it has no value. That's because x has never been assigned a value. An AppleScript variable is not defined until you first explicitly give it a value. To continue our shoebox analogy, there is no "x" shoebox to fetch the contents of, because you've never put anything into it.

This code both compiles and runs:

set x to 5
set x to x + 1

During execution of the first line, AppleScript observes that you're putting something into the "x" shoebox, but there is no such shoebox as yet. No problem; AppleScript creates the ...