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<script>
        extra=0;
        
        $(window).scroll(function() {

           var scrolledLength = ( $(window).height() + extra ) + $(window).scrollTop(),
               documentHeight = $(document).height();

           if( scrolledLength >= documentHeight ) {

               $("footer").addClass("open");
           }
           else if ( scrolledLength <= documentHeight && $("footer").hasClass('open') ) {
                $("footer").removeClass("open");
                }
        });
    </script>