JavaScript has the best interoperability with JavaScript. What exactly are you asking? What server language is the best for writing backbends to single page apps? Use node.js, you don’t have to learn another language. What other languages can you use in the backend to avoid the pain of using JavaScript in the frontend? All, having a REST API and delivering JSON makes your life more enjoyable (as a programmer, bets are off for everything personal). Personally, I would recommend something like Ruby or Python or if you are into something more exotic Elixir or maybe even give Perl 6 a try.
If you are thinking about running JavaScript code from inside another language, just don’t. You don’t want to go there.
If none of these answers suffice you might want to flesh out your question to be more precise.