Emscripten is an LLVM to JavaScript compiler. It takes LLVM bitcode (which can be generated from C/C++ using Clang, or any other language that can be converted into LLVM bitcode) and compiles that into JavaScript, which can be run on the web.

Using Emscripten, you can

• Compile C and C++ code into JavaScript;
• Run code in languages like Python as well, by compiling CPython from C to JavaScript and interpreting code in that on the web.

https://github.com/kripken/emscripten/wiki