当滚动条滑到一定距离自动隐藏某个div怎么实现?滑到一定距离会自动给这个div加上display:none
<script type="text/javascript">
$(window).scroll( function() {
if(window.document.body.scrollTop>=500){
document.getElementById("top_bar").style.display = "none";
}
else{
document.getElementById("top_bar").style.display = "block";
}
} );
</script>
<div class="top_bar" id="top_bar"></div>