Javascript's secret weapon[1] -- especially for beginning programmers -- is its dev tools. If you learn JS' debugging toolkit early on, it will accelerate your learning in general. I recommend Code School's "Discover DevTools" course [2] for beginning devs, but I'm not aware of a high-quality introduction for experienced devs.
Shameless plug: I teach at a school for JS devs named Hack Reactor. If rewriting Underscore as a learning tool sounds like fun, you should apply. http://hackreactor.com
Shameless plug: I teach at a school for JS devs named Hack Reactor. If rewriting Underscore as a learning tool sounds like fun, you should apply. http://hackreactor.com
[1] until now. [2] Chapters 3-4. http://discover-devtools.codeschool.com/chapters/3