The title is a tiny bit misleading. Indeed all of those are JavaScript emulators, but not all of them were written in JS. At least one on the list (mupen64plus)

The title is a tiny bit misleading. Indeed all of those are JavaScript emulators, but not all of them were written in JS. At least one on the list (mupen64plus) is transpiled from C++ using the (amazing) emscripten.