That is something I was worried about, because afaik, ghcjs is still fairly experimental. But ability to use ghc's magic to produce javascript sounds aluring :-)

I think, that if you could integrate source-map capabilities (haven't tried http://hackage.haskell.org/package/sourcemap yet) it might make the debug process more bearable.

But if you would be looking for haskell-like-language with more comprehensible to-javascript output, http://www.purescript.org/ pleasantly suprised me (even though I had time to barely get past the hello world :-)