Objective-C isn't a proprietary language, as this author annoyingly suggests several times. Regardless, the new JavaScriptCore framework is pretty impressive. It would be nice if there were some official Apple docs beyond the one WWDC video, though.
It's worth noting that there are some discrepancies between that codebase and the actual behavior of the public iOS 7 framework, but if you're not doing anything too crazy it probably won't be an issue.