Ladies and gentlemen, JS-ception.
https://github.com/jterrace/js.js/ (https://github.com/jterrace/js.js/)
SpiderMonkey compiled via LLVM, then run through Emscripten. Reportedly REALLY slow, but the fact is it exists.
Hmm, I wonder if this could be sped up by using EM_ASM on the embedded JS in SpiderMonkey...
...Wow.
We need to go deeper.