- Screen dimensions - (screen.width, screen.height)

- Window dimensions - (window.innerWidth, window.innerHeight)

- Window X/Y position - (window.screenX, window.screenY)

...or whichever calculation works best for you.

The original work by Bjorn Staal https://twitter.com/_nonfigurativ_/status/172732259457002734 used localStorage, but I found sockets more fun, because if tweaked a bit, this can be shared with friends.

Here's a demo of how it works and the codebase: https://github.com/Momciloo/fun-with-sockets/