Windows 8 game development is designed mainly for C++ (which I don't know), but can be done in JS (which I do know). Is it worth it to learn C++ or should I continue with JS?
That depends on a which type of a games you want to create. If you make JS games already and you are fine with it, I see no reason to learn C++ (by the way I would not say that other game development options on Win8 is limited solely to C++, see also).
However, if you want to make to make games which are faster and with more complex graphics, then learning C++ can be an option. However be aware that native C++ abstraction level is very much lower of JS one, even if using frameworks and libraries.
I would certainly consider using JS,
there are a lot of frameworks (http://phonegap.com/) around to create JS applications that can be run on multiple platforms like Android and IOS without to much extra code.
This way you do not have to limit your distribution to windows only.