JQuery实现导航菜单栏随滚动条上下滚动

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JQuery实现导航菜单栏随滚动条上下滚动 <span>- alleyloft.com</span>
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<script type="text/javascript">
$(function(){
$(window).scroll(function(){
$offset = $('.placeholder').offset();//不能用自身的div，不然滚动起来会很卡
if($(window).scrollTop()>$offset.top){
$('.nav').css({
'position':'fixed',
'top':'0px',
'left':$offset.left+'px',
'z-index':'999'
});
}else{
$('.nav').removeAttr('style');
}
});
})
</script>