The question could be interpreted different ways...
If you have already coded a part of your application in C++ and need to add modules or scripts, that you want to add in the C++ code eventually for improved performance, you could use Qt which is a solid, very complete multi-platform framework.
It includes support for scripts (ECMAScript, the standardized version of Javascript), from which you can access your C++ objects if you need to. It also provides support for plug-ins.
With that approach, you can even proceed in three steps,
- first do some tests in Javascript,
- then code the base of your application in C++ and still use a part in scripts (for example if you need to try different algorithms),
- and finally port everything, or at least the critical parts, in C++.
Another option is to embed a Python interpreter in your code, although that will probably require more work.