AppleScript: The Definitive Guide, 2nd Edition

Free Variables

A variable defined outside a handler or script object, but globally visible within it, and not overshadowed by a declaration of the same name, is called a free variable with respect to that handler or script object. A free variable in AppleScript is identified with a top-level entity or global through the downward effect of its declaration. For example:

global x, y, z
script myScript
    property x : 1
    local y
    yy
    z
end script

Within myScript, x is explicitly defined as a property and y is explicitly defined as a local, so neither is a free variable. The variable yy isn't explicitly defined within myScript, but it isn't defined outside it either, so it is an implicit local and not a free variable. But the variable z is globally visible within myScript (from the global declaration at the start of the code), and the name is not redeclared within myScript, so z within myScript is a free variable, and is identified with the global z declared in the first line.

A free variable takes its value within the handler or script object at the time the code runs, not at the time the handler or script object is defined. For example:

global x
set x to 5
on myHandler( )
    display dialog x -- x is a free variable
end myHandler
set x to 10
myHandler( ) -- 10(not 5)

The dialog displays 10, not 5. It doesn't matter that x had been set to 5 when myHandler was defined; it only matters what its value is when the code inside myHandler actually runs. By that time, x has been set to 10.

Free variables' ...