jquery vertical mousewheel smooth scrolling

I know it’s a really old post, but here is a good solution I made :

function handle(delta) {
    var animationInterval = 20; //lower is faster
  var scrollSpeed = 20; //lower is faster

    if (end == null) {
    end = $(window).scrollTop();
  }
  end -= 20 * delta;
  goUp = delta > 0;

  if (interval == null) {
    interval = setInterval(function () {
      var scrollTop = $(window).scrollTop();
      var step = Math.round((end - scrollTop) / scrollSpeed);
      if (scrollTop <= 0 || 
          scrollTop >= $(window).prop("scrollHeight") - $(window).height() ||
          goUp && step > -1 || 
          !goUp && step < 1 ) {
        clearInterval(interval);
        interval = null;
        end = null;
      }
      $(window).scrollTop(scrollTop + step );
    }, animationInterval);
  }
}

Test it : http://jsfiddle.net/y4swj2ts/3/