Javascript/jquery

Can someone help me on my project. How to set up scroll tracking for mobile devices. And if a user scrolls up 200 pixels (or any other arbitrary number), POPUP will ACTIVATE. And the scroll tracking only applies for mobile devices. Here is my code but not what I need, this only popup when I reach on the top screen not when I scrolls up to 200 pixels.

let prevY = 0
$(document).scroll(function() {
prevY = $(this).scrollTop();
if (0 <= 200) {
// (!sessionStorage.getItem(“visited”))
$(“#off–wrapper”).fadeIn();
sessionStorage.setItem(“visited”, “true”);
console.log(‘Going down’)
} else {

        console.log(‘Going up’)
}
});

@programmingwithmosh please Mosh I need help.