In this JavaScriptCore tutorial you'll learn how to build an iOS companion app for a web app, reusing parts of its existing JavaScript via JavaScriptCore.
This is a cool topic. Although I hate JavaScript, I enjoyed learning something new. 
What to do when (after months/years working this way) you find a bug in the VM like
https://bugs.webkit.org/show_bug.cgi?id=151495
and your application is crashing each time it enter garbage collection of young objects?
You see the bug was reported in Webkit for loooooong time and not resolved.
And when you try to download sources from the project (that it is said it is open source) you do not have the build instructions to build binary for iOS nor a good way to debug (Xcode debugger freeze when app crash).
Most people will say… it will be solved soon; but in practice it has been long time waiting and applications crashing one or more time per hour of work.
This tutorial is more than six months old so questions regarding it are no longer supported for the moment. We will update it as soon as possible. Thank you! :]