I have a lot of JavaScript I need to write. I can do that, but I have some Python already which does something similar. I'd much prefer to adapt that and have it run within the browser.

There's the C-based reference implementation, and there's Jython in Java. Is there a JavaScript implementation along the same lines?

I have looked, but it may be that my google-fu has failed me. And it seems like something that ought to exist. Have I missed it?