Our magic weapon is Erlang, which is a wonderful language for working with protocols, and a wonderful runtime for working with many things happening at the same time.
The other magic weapons are the Cowboy webserver and the Gun http client -- both written by Loïc Hoguin. Loïc doesn't work for us, but we've been sponsoring him financially for over two years (https://medium.com/@abs/what-we-learned-from-sponsoring-an-o...).
Our frontend doesn't really matter as much, since it's just a dashboard, but we use TypeScript with Flux and React (it's been great).