File server.js is being edited (by root with nano 2.4.2, PID xxxx); continue?

Check with tools like ps and htop whether this other nano instance is still running. If it's not, there's most likely a hidden dotfile in the same folder which leads nano to believe that the other instance is still running (at least vim works this way, I don't use nano; try ls -lA and look for a file that begins with .server.js or something like that.

In addition, if you're the only user on that machine and you're sure that you don't have an open editor on that file in another window, you can just accept nano's offer to continue, it will remove the leftover swap file, saving you a few steps.