sidebar

$(function() {
  var $window = $(window);
  var $sidebar = $(".sidebar");
  var $sidebarTop = $sidebar.position().top;
  var $sidebarHeight = $sidebar.height();
  var $footer = $('.footer');
  var $footerTop = $footer.position().top;

  $window.scroll(function(event) {
    $sidebar.addClass("fixed");
    var $scrollTop = $window.scrollTop();
    var $topPosition = Math.max(0, $sidebarTop - $scrollTop);
    
    if ($scrollTop + $sidebarHeight > $footerTop) {
      var $topPosition = Math.min($topPosition, $footerTop - $scrollTop - $sidebarHeight);
    }

    $sidebar.css("top", $topPosition);
  });
});