I found following project that generates Qt bindings for Qt Script. I haven't tried it though.
https://code.google.com/p/qtscriptgenerator/
It is recommended that Javascript code should be kept to a minimum. Since it is bad for performance as you mentioned, especially when it is run during animation. I don't know what you are up to, but it would be better to implement UI with QML(minimum Javascript) and logic with Qt (C++).