I just realized that I totally misunderstood your question (it's late here)

you want what's called untabify in emacs?

Theoretically one could do a regex to s/^\t/    /g (roughly), but what if multi line code incidentally starts with a tab?

The edge cases are so hairy that I doubt that our dev will try, especially because it needs to be configurable per user.

Of course you could extend my wiki solution with such js code...

See Good Intentions: Wikisyntax for the Monastery