If you want to go this route, I strongly recommend you focus on javascript. There's enough going on in the frontend that it'll take you a year or two of solid effort before you're good in a number of them. By focusing heavily on one area, you'll get yourself marketable much quicker.

Now, the context in which you learn javascript and demonstrate it is up to you. If mapping/gis is your passion, then go with that. There are plenty of good mapping projects in the browser on github that could use help. If you would prefer a personal project you could, say, do some d3 visualizations using the geo facilities and have a great demo piece.