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Emscripten is an open source LLVM to JavaScript compiler. It lets you take code written in C or C++ and run it on the web. It can also be used to compile entire language runtimes, like CPython, letting you run code in those languages on the web as well.

Currently a major focus of work in Emscripten is connecting with 'normal' JavaScript code. In other words, making it easy to compile some existing C++ library, then use it from your handwritten JavaScript on the web.