AppleScript: The Definitive Guide, 2nd Edition

The Top Level

The top level of all is the script as a whole. (Okay, I lied. There are actually some secret levels even higher than that. But just pretend, for now, that there aren't.) What's more, the script as a whole is itself a script object, even though it has no name and is not defined by a block.

This strange-sounding fact is actually extremely cool. It gives your script a sort of overall uniformity. For example, a handler definition can't occur anywhere except at the top level of a script object. So why is it possible for this to be a script?

on sayHowdy( )
    display dialog "howdy"
end sayHowdy
sayHowdy( )

There's a handler definition sitting there, not nested in a script block. But that doesn't break the rule; it accords with it, because the script as a whole is itself a script object, so this handler definition is in fact at the top level of a script object, precisely where it should be.