My choice of IDE … perhaps it is the lightest one around. It is a bookmarklet called Jsenv.
Its not all rosy though …
the Pros are
1. ideal for a “just-do-it” approach to trying something out.
2. very light weight.
3. all the advantages of js
the cons are
1. cannot save the file.
2. no syntax highlighing, intellisense etc. ( you can use shell tool for autocompletion. see the jsenv link)