Since you don't seem set on a language, let me throw my hump in for OCaml. It's supports a functional style without forcing you to use it for everything; js_of_

Since you don't seem set on a language, let me throw my hump in for OCaml. It's supports a functional style without forcing you to use it for everything; js_of_ocaml compiles OCaml's dense, high-performance bytecode to Javascript that apparently integrates well with web APIs.