Hello and welcome!
Interoperability with C is very good since Nimrod compiles to C. Wrapping C++ code is a bit more complex but is also possible by compiling to C++. For examples of wrapping C code take a look at any of the wrappers in Nimrod's stdlib: https://github.com/Araq/Nimrod/tree/master/lib/wrappers and here is an example of wrapping C++ code: https://github.com/Araq/Nimrod/blob/master/examples/c%2B%2Biface/irrlichtex.nim.
Nimrod has its own javascript backend so emscripten is unnecessary. It's as simple as 'nimrod js file.nim'. Example JS project here.
Hope this helps!