怎样使一个div永远定位在浏览器的底部？页面滚动时这个div不动？

总算一位仁兄帮着搞定了
散分啦！
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{
document.getElementById("bottomdiv").style.top=document.body.clientHeight-30+document.body.scrollTop;
document.getElementById("bottomdiv").style.width=document.body.clientWidth;
document.getElementById("bodydiv").style.height=document.body.clientHeight;
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</body>