javascript控制DIV显示

<script>
function getPosition() {
var top = document.documentElement.scrollTop;
var left = document.documentElement.scrollLeft;
var height = document.documentElement.clientHeight;
var width = document.documentElement.clientWidth;
return {top:top,left:left,height:height,width:width};
}

function showPop(){
var width = 300; //弹出框的宽度
var height = 160; //弹出框的高度
var obj = document.getElementById("div_pop");

obj.style.display = "block";
obj.style.position = "absolute";
obj.style.zindex = "999";
obj.style.width = width + "px";
obj.style.height = height + "px";

var Position = getPosition();
leftadd = (Position.width-width)/2;
topadd = (Position.height-height)/2;
obj.style.top = (Position.top + topadd) + "px";
obj.style.left = (Position.left + leftadd) + "px";

//滚动条移动时候改变DIV位置
window.onscroll = function (){
var Position = getPosition();
obj.style.top = (Position.top + topadd) +"px";
obj.style.left = (Position.left + leftadd) +"px";
};
}

function hidePop(){
document.getElementById("div_pop").style.display = "none";
}
</script>