Fade-in effect for a button when the user scrolls down and button comes into view and a fade-out effect when the user continue scrolls down and button goes out of view.

 var scrollButton = $('#scrollButton');
      
      scrollButton.css('opacity', '0');

      $(window).scroll(function() {
        var scrollPos = $(this).scrollTop();

        if (scrollPos > 100) {
          scrollButton.css('opacity', '1');
        } else {
          scrollButton.css('opacity', '0');
        }
      });