Jquery当div超出页面时就固定到顶部否则跟随页面滚动

            $.fn.smartFloat = function () {
                var position = function (element) {
                    var top = element.position().top, pos = element.css("position");
                    $(window).scroll(function () {
                        var scrolls = $(this).scrollTop();
                        if (scrolls > top) {
                            if (window.XMLHttpRequest) {
                                element.css({
                                    position: "fixed",
                                    top: 0
                                });
                            } else {
                                element.css({
                                    top: scrolls
                                });
                            }
                        } else {
                            element.css({
                                position: pos,
                                top: top
                            });
                        }
                    });
                };
                return $(this).each(function () {
                    position($(this));
                });
            };

            $("#floatdiv").css("left", $("#rolltop").position().left - 1120).smartFloat();

<div id="rolltop">回到顶部</div>
<div id="floatdiv" style="position:absolute; top: 320px; width: 120px; height: 278px;background-color:#f00;"></div>

#rolltop {
    margin-right: -430px;
    position: fixed;
    right: 45%;
    top: 80%;
    width: 25px;
    background-color:#f00;
}