I think Haskell's been working really well for us. Strong types give a lot of confidence during refactoring. The language is performant too which has definitely helped us as we've been scaling.

Personally, I like to stay pretty close to the javascript when doing web work. Elm, Purescript, GHCJS all have their benefits but I find it just too difficult to integrate with the rest of the (disjointed) web development toolkit.