AppleScript: The Definitive Guide, 2nd Edition

Reference as Parameter

You can pass a value that is a reference as a parameter to a handler, and it remains a reference. So, for example:

local x
tell application "Finder"
    set x to folder 1
end tell
on setName(theRef)
    set name of theRef to "Jack"
end setName
setName(x)

That code successfully changes the name of a folder in the Finder.

You can pass as a parameter a reference to anything you can usefully make a reference to. Thus, the reference to operator almost provides a solution to the problem of passing by reference (see Chapter 9):

on doubleRef(theRef)
    set contents of theRef to 2 * theRef
end doubleRef
script s
    property x : 5
    display dialog x
end script
doubleRef(a reference to s's x)
run s -- 10

But this solution is not completely general, and indeed no completely general solution is possible, because you can't usefully make a reference to a local. Also you'll notice that the example worked only because the handler doubleRef knew in advance that it was going to be handed a reference—it explicitly dereferenced the reference in order to change the thing referred to. So passing a reference is a way to allow a handler to change a value in place, but this is not the same thing as passing by reference, and it doesn't work if the value to be changed is a local variable.

The use of a reference as a parameter can permit a handler to perform dynamic targeting. As long as a handler doesn't use any vocabulary that depends on a specific target, it can target an application whose identity is not ...