I think that would depend on what I already knew, what I was interested in, and what I hoped to accomplish during this time (whether it be a hobby thing or professional development). Perhaps you could fill us in if you're looking for suggestions?

Personally I would probably sit down and learn the core of javascript, getting as deep as possible working toward a better understanding of what's going on under the hood. This is because in my daily work I really just scratch the surface superficially using it to write programs. I would want to understand it to the point where I could start to write something like Node.js from scratch (right now I don't even know where I would begin with this).

The reason for this is because I know C this way, so every time I write a line of code in C, I instinctively visualize what's going on under the hood.

I would like to be able to visualize this same thing with js.