AppleScript: The Definitive Guide, 2nd Edition

Scope of Locals

An explicit local is a variable whose name appears in a local declaration . A local declaration can declare several variables at once, delimited by comma:

local a
local b, c, d

A local variable is visible only within the very same region of scope where it is declared—not outside it, and not at a deeper level. (But there's an exception, which we'll come to in a moment.)

Local variables completely disambiguate a name within their own scope, and local variables in other scopes can have the same name without conflict. Suppose a script object starts like this:

script myScript
    local x

The local declaration for x means that from now on when code in this script object's scope says x it means this local x and no other. What's more, other scopes may declare their own local x, they may declare a global x, they may bang the floor and have a temper tantrum, but they absolutely will not be able to have any effect upon myScript's x, nor will anything myScript does with its x have any effect upon them.

Here's an example of a local variable in action:

local x
set x to 5
script myScript
    display dialog x -- error: The variable x is not defined
end script
run myScript

Observe how completely different this is from what would have happened if x had been a top-level property. Here, there is a variable called x and it is defined, but it is declared local and therefore is visible only within its own scope. That scope is the top-level script. The display dialog x command is in a different scope, ...