打印DOM树，为什么这里的document的hasChildNodes()返回的是false？？

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<script type="text/javascript" >
function listNode(node, level){
printInfo(node, level);
level++;
var nodes = node.childNodes;
getNodeInfo(nodes[0])
alert(nodes[0].hasChildNodes());  //为什么document的hasChildNodes();是false啊；想把dom树打印出来，不行 var ss=nodes[x].hasChildNodes(); for(var x=0;x<=nodes.length-1;x++){
if(nodes[x].hasChildNodes())
 listnode(nodes[x],level);
 else
printInfo(nodes[x],level);

}
}
function getSpace(level){
var s='';
for(var x;x<level;x++)
s=+"|--";
return s;
}
var str='';
function printInfo(node,level){
str+=getSpace(level)+"name:"+node.nodeName+'....'+"type:"+node.nodeType+"....."+"value:"+node.nodeValue+"<br/>"

}
function getNodesInfoList(){
//var body1 =document.getElementById("bodyid");
//getNodeInfo(body1)
//alert(body1.childNodes.length);
listNode(document,0);
document.write(str);
}
</script>
</head><body id="bodyid">
<input type="button" value="yanshi"  onclick="getNodesInfoList()" />
<div  style="background-color:#96F" id="divid"> asdafaf</div> 
<span>sdvssvv</span>
<table>
   <tr>
   <td>单元格一</td>
   <td>单元格二</td>
   </tr>
   <tr>
   <td>单元格三</td>
   <td>单元格四</td>
   </tr>
</table> 
<table>
   <tr>
   <td>单元格一</td>
   <td>单元格二</td>
   </tr>
   <tr>
   <td>单元格三</td>
   <td>单元格四</td>
   </tr>
    <tr>
   <td>单元格一</td>
   <td>单元格二</td>
   </tr>
   <tr>
   <td>单元格三</td>
   <td>单元格四</td>
   </tr>
</table> 
<a href="javascript:void(0)"></a>
<span>sdvssvv</span>
<!--我是注释-->
</body>
</html>