I would strongly recommend Clojure, which is dynamically but strongly typed LISP on JVM. The best part is the REPL, which is only true if you use a well-tuned I

I would strongly recommend Clojure, which is dynamically but strongly typed LISP on JVM. The best part is the REPL, which is only true if you use a well-tuned IDE. Personally I still use NightCode but there are many others out there. For me the biggest selling points were immutable data structures by default, nice core library (reduce, reductions, iterate, map with multiple arguments, ...) and super simple syntax. Being a LISP after all. And to extend to the client side you can turn onto ClojureScript, which is almost the same language which compiles to JS instead of JVM bytecode.