Build something in it. Personally, I love Javascript. Other than a few admittedly major annoyances, I like it better than Perl for many tasks. For one, it's a better prototyping language than Perl if you don't need access to a DBMS.
But, honestly, the best thing to do is to build something. A lot of programmers have a program they always write to learn a new language. I don't, but that's cause I don't collect languages (yet). :-)
My criteria for good software:
- Does it work?
- Can someone else come in, make a change, and be reasonably certain no bugs were introduced?