Emscripten C/C++-to-JavaScript project uses a back-end derived from LLVM for greater speed and a more dedicated compiler.

However, JavaScript compilers can optimize your JavaScript code and generate improved code. The compiled JavaScript files are smaller than your original JavaScript files, reducing download times ...

Roslyn features open source C# and Visual Basic compilers offering code analysis APIs. DuoCode is another tool for compiling to JavaScript without actually writing JavaScript.

Sparkplug is a new JavaScript compiler that fills the gap between needing to start executing quickly and optimizing the code for maximum performance.