There are several ways to do it: compile to C++, compile to the Neko VM, or use Node (there are official bindings: https://github.com/HaxeFoundation/hxnodejs) In development it's Haxe (which is a lovely language to work with) but in production it can be something you're familiar with.

Do you mean Neko https://en.wikipedia.org/wiki/Neko_(programming_language)? Haxe itself compiles into another language(https://haxe.org/manual/compiler-usage.html) that would run on the server.

Haxelib (the Haxe package manager) client and server are both written in Haxe. Also try.haxe.org and kodegarden.org clients+servers are also written in Haxe.