Scroll only in hovered div

.js

$('#menu').on('DOMMouseScroll mousewheel', function(ev) {
  var $this = $(this),
    scrollTop = this.scrollTop,
    scrollHeight = this.scrollHeight,
    height = $this.height(),
    delta = (ev.type == 'DOMMouseScroll' ?
      ev.originalEvent.detail * -40 :
      ev.originalEvent.wheelDelta),
    up = delta > 0;

    var prevent = function() {
      ev.stopPropagation();
      ev.preventDefault();
      ev.returnValue = false;
      return false;
    }

    if (!up && -delta > scrollHeight - height - scrollTop) {
      // Scrolling down, but this will take us past the bottom.
      $this.scrollTop(scrollHeight);
      return prevent();
    } else if (up && delta > scrollTop) {
      // Scrolling up, but this will take us past the top.
      $this.scrollTop(0);
      return prevent();
    }
});