For server and client side I'd go with a strongly typed language. This to help me maintaining the code base, mainly by making refactoring cheap. Server side th

For server and client side I'd go with a strongly typed language. This to help me maintaining the code base, mainly by making refactoring cheap. Server side they are: Haskell, OCaml (or Facebook's Reason), F#, Rust. Client side: PureScript and Elm; and to a lesser extend: TypeScript and Flow. Now what counts is maturity of the "web" ecosystem. For server side I think Haskell and OCaml are more mature then F# and Rust in this regard. Client side non of the contenders are _very_ mature, where TypeScript and Flow are more of an extension on vanilla JS, so they will have good interop with existing libs. For me Rust is a little too low-level for web dev, but it depends on the performance characteristics you are looking for.