purpose of jquery.scrolly code example

Example 1: jquery on scroll down

Check current scrollTop vs previous scrollTop

var lastScrollTop = 0;
$(window).scroll(function(event){
   var st = $(this).scrollTop();
   if (st > lastScrollTop){
       // downscroll code
   } else {
      // upscroll code
   }
   lastScrollTop = st;
});

Example 2: jquery on wheel event

$("#main-div" ).on( 'wheel', function() {
   alert("Hello! I am an alert box!!");
});