js判断鼠标向上滚动并浮动导航

<div id="Jnav">
    <ul class="nav">
        <li><a href="#">WEB前端开发</a></li>
        <li><a href="#">前端开发博客</a></li>
        <li><a href="#">前端开发</a></li>
        <li><a href="#">前端开发</a></li>
    </ul>
</div>

.nav{width:980px; margin:0 auto;}
.nav li{display:inline-block; *display:inline; *zoom:1; margin:0 10px;}
.nav li a{display:block; padding:5px 10px;}
.fixed-nav{
    position: fixed;
    width:100%;
    top:-40px; 
    -webkit-transition: top .5s;
    -moz-transition: top .5s;
    -o-transition: top .5s;
    transition: top .5s;
    -webkit-box-shadow: 0 2px 2px rgba(0,0,0,.1);
    -moz-box-shadow: 0 2px 2px rgba(0,0,0,.1);
    box-shadow: 0 2px 2px rgba(0,0,0,.1);
}
.fixed-nav-appear{top:0;}

var $nav = $('#Jnav'), navTop = $nav.offset().top, navH = $nav.outerHeight(),winTop_1=0,winWidth=$(window).width(), holder=jQuery('<div>');
$(window).on('scroll',function(){
    var winTop_2 = $(window).scrollTop();
    holder.css('height',navH);
    //开始浮动，不过不显示
    if(winTop_2>navTop && winWidth>980){
        holder.show().insertBefore($nav);
        $nav.addClass('fixed-nav');
    }else{
        holder.hide();
        $nav.removeClass('fixed-nav');
    }
    //判断鼠标向上滚动，显示出来
    if(winTop_2>winTop_1 && winWidth>980){
        $nav.removeClass('fixed-nav-appear');
    }else if(winTop_2<winTop_1){
        $nav.addClass('fixed-nav-appear');
    }
    winTop_1 = $(window).scrollTop();
})