Bali aims to be a fully fledged JS engine written in Nim, primarily for Ferus, but easily embeddable in any Nim program. It's in very early development right now and does not have a real API just yet, so things are very much subject to change. It has a very basic parser, AST interpreter and JS Value implementation. And yes, the reason why I've hardly touched ferus in a while is because of this.
JavaScript sucks.