For a javscript editor within pythonista or editorial, check out
https://github.com/jsbain/EditAreaForPythonista
This has a primitive way to open, browse and save files, and the editor has syntax highlighting, etc. there are some keyboard quirks, and things like the search dialog have issues.
I nearly have a codemirror instance working, which I think will be a lot better than editarea.
But seriously, develop all you can off tablet, the lack of any real javascript debug capabilities is a killer.