[JS] Browser get scroll direction

var lastScrollTop = 0;
$(window).scroll(function(event){
   var st = $(this).scrollTop() || window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0;

출처: https://seunggabi.tistory.com/entry/JS-Browser-scrollTop [배우기를 멈추는 사람은 20세건 80세건 늙은 것이다.]
   if (st > lastScrollTop){
       // downscroll code
   } else {
      // upscroll code
   }
   lastScrollTop = st;
});

[출처] https://stackoverflow.com/questions/4326845/how-can-i-determine-the-direction-of-a-jquery-scroll-event