Incidentally, I'd still mostly agree with your comment, as the underlying script runtime isn't really designed with application development in mind. For example, I'm pretty sure Xcode debugging support for AppleScript doesn't even include breakpoints or related features like single-stepping (Apple's Script Editor certainly doesn't), and that Script Debugger had to pretty much implement its own AppleScript runtime to support these features.

On another side note, while Yosemite is the first time Apple has supplied or supported JavaScript as an OSA language, third parties have integrated the Mozilla JavaScript engine into OSA in the past in what I believe is an abandoned project (Google "JavaScript OSA").