JavaScript实现元素定位与坐标获取

<!DOCTYPE html> 
<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 
<title>每天一个JavaScript实例-操作元素定位元素</title> 
<style> 
div#a{ 
width:500px; 
} 
div{ 
border:1px solid #000; 
padding:10px; 
} 
#cursor{ 
position:absolute; 
background-color:#ff0; 
width:20px; 
height:20px; 
left:50px; 
top:300px; 
} 
</style> 
<script> 
function positionObject(obj){ 
var rect = obj.getBoundingClientRect(); 
return [rect.left,rect.top]; 
} 
window.onload = function(){ 
var tst = document.documentElement.getBoundingClientRect(); 
alert(tst.top); 
var cont = 'A'; 
var cursor = document.getElementById("cursor"); 
while(cont){ 
cont = prompt("where do you want to move the cursor block?","A"); 
if(cont){ 
cont = cont.toLowerCase(); 
if(cont == "a"||cont=="b"||cont=="c"){ 
var elem = document.getElementById(cont); 
var pos = positionObject(elem); 
console.log(pos); 
cursor.setAttribute("style","top:"+pos[1]+"px;"+"left:"+pos[0]+"px"); 
} 
} 

} 
} 
</script> 
</head> 

<body> 

<div id = "a"> 
<p>A</p> 
<div id ="b"> 
<p>B</p> 
<div id="c"> 
<p>C</p> 
</div> 
</div> 
</div> 
<div id="cursor"> 

</div> 
</body> 
</html>