AppleScript: The Definitive Guide, 2nd Edition

Scripting Addition

AppleScript is a little language, and at a very early stage it was felt to be a bit too little. An architecture was therefore devised whereby Apple, as well as third-party developers, could extend the language by means of scripting additions. A scripting addition is a code library, typically written in a compiled lower-level language such as C, whose purpose is usually to endow AppleScript with some functionality that can be implemented in this lower-level language (possibly by calling into the Macintosh Toolbox) but is otherwise missing from AppleScript itself. On Mac OS 9 and before, a scripting addition is a resource file of type 'osax'; on Mac OS X it can also be a bundle with extension .osax. Therefore it is common parlance to refer to a scripting addition as an osax (official plural, osaxen).

When an instance of the AppleScript scripting component comes into existence, it loads any scripting additions found in any of several locations , namely /System/Library/ScriptingAdditions and the corresponding folders in /Library and ~/Library. (On Mac OS 9 and before, there is just one location, the Scripting Additions folder of the System Folder. Observe the lack of a space in the Mac OS X folder name ScriptingAdditions; the tale of how that happened is gory and not to be recounted here, and in any case it's too late now to change it.) If a script depends upon a scripting addition, it is up to the end user to install that scripting addition first on any machine where ...