Think of it more like C (really more like a hybrid of C and JS) compiling to JS. It runs on the web, but it compiles to a very stylized kind of JS: it allocates a large typed array that represents a C-like heap and does its memory access on that typed array. This is the same basic approach that Emscripten uses for running C/C++ programs.

The purpose of this project is to experiment with a dialect of JS that integrates well with regular JS, so you can write performance-sensitive parts of your application in this dialect, and they can interoperate smoothly with the other parts of your normal JS code.