i'm wondering what browser property i can use to monitor where the user is in terms of page height?

let's say the page is 1356 pixels in height. the user is going to have to scroll up and down to view page elements. right now i need to write a sort of utility function that monitors where the user currently is.

what browser property exists that i can use for this? is there some sort of scrollTop or scrollPageHeight property?

one reason i want to do this is because i've employed a parallax js library to use for navigation and animation both and need some sort of readout to track where i currently am on the page

any help would be much appreciated, thanks!