I've been trying to do this for a while. No clue if it is possible. Very short question. (like Javascript to snap-blocks)
It'd be tricky to implement that, just because there are about 2.7 bazillion features in JS, and you'd have to implement a translation for each of them.
The other direction is doable; open the Codification project in File>Open>Examples. Here the problem is that if your Snap! code does anything graphical, JS probably doesn't have an easy way to support it unless you have Snap! running.