Day 24 of #javascript30 is making a navigation bar that sticks to the top of the window.
This is something I have done more than once before. With some frustration, though. The same effect can be achieved with CSS's position: fixed, but you still need JS to slap on the class or property at the right time.
scrollX and scrollY
Keep in mind that that IE does not supprt these two at all. Edge does, but to be safe, use pageYOffset or pageXOffset - these are aliases for scrollX and scrollY. See the docs for scrollY for an explanation.