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       $(document).ready(function () {
            function slide_menu() {
                scroll_value = $(window).scrollTop();
                if (scroll_value > 500) {
                    $('#userNav').show('slide', { direction: 'up' }, 750);
                };
                if (scroll_value < 500) {
                    $('#userNav').hide('slide', { direction: 'up' }, 750);
                };
            }
                setInterval(slide_menu, 5);
                slide_menu();
       }); 

1. This code runs fine in jsfiddle. There are also no errors in WebMatrix but the code doesn't work in web browser. Firefox Inspector shows:
   
2. f.easing[e.animatedProperties[this.prop]] is not a function

When I change the lines with show/hide on something simple like "alert" it works. I think there is a problem but I don't know how to fix it, I'm beginner.