I think that Elm is a fundamentally good language, being catered towards JS developers will hopefully attract more of them to FP development. However, I feel that the language is too restricted compare to other FP languages (Haskell, Purescript, even Scala), as a result you often need to write too much boilerplate and the Elm architecture doesn't seem to fit nicely for all kinds of components.
Which is why for now I'm looking into Purescript and GHCJS which support features such as Rank N types, generics, typeclasses, functional dependencies/type families. Even though I don't use many of those features directly, I do benefit from libraries that depend on them.
Typescript is a superset of ES6 anyway, proper JS documentation(e.g MDN) does have the notion of types anyway, but basing the documentation on TS means that the types are sound and consistent along the library.
Which is why for now I'm looking into Purescript and GHCJS which support features such as Rank N types, generics, typeclasses, functional dependencies/type families. Even though I don't use many of those features directly, I do benefit from libraries that depend on them.