AppleScript: The Definitive Guide

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

Definition

The code in that last example, as a complete script, will compile, but it won’t run; at runtime, it generates an error. That’s because x has never been assigned a value. 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. An AppleScript variable is not defined until you first give it a value explicitly. 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 shoebox, labels it “x”, and puts 5 into it. Now the second line runs fine, because there is a shoebox “x” from which to fetch a value.

Once a variable has been defined in the course of ...