$(function(){  
    $('#return-top').hide();  
    $(function(){  
        $(window).scroll(function(){  
            if($(window).scrollTop()>10){  
                $('#return-top').fadeIn(300);  
            }else{
                $('#return-top').fadeOut(200);
            }  
    });  
    $('#return-top').click(function(){   
        $('body,html').animate({scrollTop:0},300);  
        return false;  
          
        })  
      
    })

     //获取要定位元素距离浏览器顶部的距离
     var navH = $(".nav").offset().top;
     //滚动条事件
     $(window).scroll(function(){
      //获取滚动条的滑动距离
      var scroH = $(this).scrollTop();
      //滚动条的滑动距离大于等于定位元素距离浏览器顶部的距离，就固定，反之就不固定
      if(scroH>=navH){
       $(".nav").css({"position":"fixed","top":0,"left":"0","margin-left":"0","right": 0,"z-index": 99999});
      }else if(scroH<navH){
       $(".nav").css({"position":"static","margin":"0 auto"});
      }
     })  
})