jQuery.fn.goToTop = function(settings) {
        settings = jQuery.extend({
            min: 1,  //设置距离顶部的最小值为1
            fadeSpeed: 200,  //设置一个淡出淡入的速度
            ieOffset: 50  //处理IE的兼容问题
        },settings);
        return this.each(function(){
            //listen for scroll
            var el = $(this);
            el.css("display","none");//in case the user forgot
            $(window).scroll(function(){
                //stupid IE hack
                if(!jQuery.support.hrefNormalized) {  //设置这个按钮的css属性
                    el.css({
                        "position": "absolute",
                        "top" : $(window).scrollTop() + $(window).height() - settings.ieOffset -30
                    });
                }
               

                if($(window).scrollTop() >= settings.min) {
                    el.fadeIn(settings.fadeSpeed);
                } else {
                    el.fadeOut(settings.fadeSpeed);
                }
            });
        });
    };
    

    $(function(){
        var goToTopButton = "<div alt='Go To Top' title='Go To Top' id='goToTop'><a href='#'></a></div>";
        $(".pane").append(goToTopButton);  //插入按钮的html标签
        if($(window).scrollTop() < 400) {
            $("#goToTop").hide();//滚动条距离顶端的距离小于showDistance是隐藏goToTop按钮
        }
      

        $("#goToTop").goToTop({
            min:300,
            fadeSpeed: 500
        });
      

        $("#goToTop").click(function(e){
            e.preventDefault();
            $("html,body").animate({scrollTop:0},"slow");
        });
    });