isomorphic.js

// I'll have a entry point for the server app, like server.js,
// and and entry for the client app, like client.js

// in server.js I do this:
IS_CLIENT = false;

// in client.js I do this:
IS_CLIENT = true;

// Then anywhere in your code you can do this:
if (IS_CLIENT) {
  doStuff();
}

// and then uglifyjs dead code eliminiation on client.js will see
// stuff like this

if (false) {
  doStuff();
}

// and wipe it out