It's a Coffee syntax thing.

  ws.on 'close', -> ps.kill()

That compiles to:

  ws.on('close', function() {
    return ps.kill();
  });

I have never done any Coffee but it looks like arguments are separated by commas, and, in this case, the second argument is a function of no argument (so the left hand side of the arrow is empty).

Edit: it was meant to be a reply to the parent.