超过一定高度对象自动悬浮

<script type="text/javascript">
var navH = document.getElementById('id').offsetTop;//获取要定位元素距离浏览器顶部的距离
window.onscroll = function(){
var scroH = document.documentElement.scrollTop || document.body.scrollTop;//获取滚动条的滑动距离
var bar = document.getElementById('id');
if(scroH>=navH){//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
bar.style.position  = "fixed";
bar.style.top  = "0px";
}else if(scroH<navH){
bar.style.position  = "static";
}
}
</script>