Being a practicing Ruby developer and avid TDDer, there are two things I like at my side when learning something new: an interactive shell and a testing framework.

JS Test Driver may suit my needs for testing (still working on that one). And lo and behold, an interactive shell does exist for JS: Mozilla's Javascript Shell. Despite not being exceptionally convenient to install (as it requires a full build of SpiderMonkey), the shell looks promising so far, particularly to someone who is still exploring the nuances of javascript.