[iLemming]

Elm is great and Purescript even better. I haven't though heard anyone building apps using Elm on both back-end and front-end.

I have enourmous respect for Anders Hejlsberg and his work but Typescript is not an answer. Typescript is limited in sense that from the beginning it was doomed to be just a superset of Javascript. And we all know about deficiencies and "bad parts" of Javascript.

Languages do matter and those with strong functional emphasis are better, because they enforce you to pursue specific discipline that allows you to write better code.

Of all functional languages (more or less popular today) in my opinion the best option is Clojure and Clojurescript. Clojurescript is simply amazing. It's robust, easy to learn, fast, adaptable, pragmatic, can cover your needs on all major platforms including iOS and Android. For a long time advocates of statically typed languages would point to one "weakness" of Clojure - its dynamically typed nature. But to begin with dynamism of Clojure types always considered its strength. Besides, Clojure today carries an instrument far more powerful than static types - http://clojure.org/about/spec. Clojure community is growing, language is gaining popularity. Because behind seemingly innocuous, and even arguably (at the first sight) not very appealing lispy syntax, there is an incredible elegance, power and true astonishment.