导航跟随滚动条置顶移动

#topmenu{position:absolute;top:100px;background:#4b4a4a;width:100%;margin:0 auto;clear:both;overflow:hidden;z-index:998;left:0}

<script type="text/javascript">

    jQuery(document).ready(function() {
        var topmenu = jQuery("#topmenu");
        var topmenu_top = topmenu.offset().top;
        reset_topmenu_top(topmenu, topmenu_top);
        jQuery(window).scroll(function() {
            reset_topmenu_top(topmenu, topmenu_top);
        });
    });
function reset_topmenu_top(topmenu, topmenu_top) {
    var document_scroll_top = jQuery(document).scrollTop();
    if (document_scroll_top > topmenu_top) {
        topmenu.css('top', document_scroll_top);
    }
    if (document_scroll_top <= topmenu_top) {
        topmenu.css('top', topmenu_top);
    }
}

</script>

效果图如下：