AppleScript: The Definitive Guide, 2nd Edition

Script Properties

A script property (often just called a property) is a variable defined and initialized at the top level of a script object through a special statement called a property declaration . The syntax is:

property propertyName : initialValue

For example:

script s
    property x : 5
end script

The abbreviation for property is prop.

A property is a variable, so its value can be set and fetched in the normal way. For example:

script s
    property x : 10
    display dialog x -- 10
    set x to 5
    display dialog x -- 5
end script

A property declaration can appear only at the top level of a script object. But the script as a whole is a script object. Therefore a property declaration can appear at the top level of the script, and the script object that owns it is the script as a whole. This is a legal script:

property x : 5
display dialog x

A script property declaration is like set, not like copy. When a script property is initialized to a value where this makes a difference (a list, a record, a date, or a script object) it is set by reference (see "Set by Reference" in Chapter 7). Here's proof:

property L : {1, 2, 3}
script s
    property LL : L
    set end of LL to 4
end script
run s
L -- {1, 2, 3, 4}