网页导航条滚动固定

function Head_menu(menu_container) {
                    var _top = menu_container.offset().top;
                    this.scroll_div = function () {
                        var top = document.documentElement.scrollTop || document.body.scrollTop;
                        if (top > _top) {
                            menu_container.css({
                                'position': 'fixed',
                                'top': '0',
                            });
                        } else if (top < _top) {
                            menu_container.css({
                                'position': 'relative',
                                'top': '0',
                            });
                        }
                    }
                }
var menu_container = $('#menu_fixed');
                var head_menu = new Head_menu(menu_container);
                window.onscroll = function () {
                    head_menu.scroll_div();
                }