节点的值为什么是null
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>节点属性</title>
</head>
<body>
<ul>
<li>javascript</li>
<li>HTML/CSS</li>
<li>jQuery</li>
</ul>
<script type="text/javascript">
var aa=document.getElementsByTagName("li");
for(var i=0;i<aa.length;i++){
var bb=aa[i].nodeName;
var cc=aa[i].nodeValue;
var dd=aa[i].nodeType;
document.write("第"+(i+1)+"个节点名称:"+bb);
document.write("<br>");
document.write("第"+(i+1)+"个节点的值:"+cc);
document.write("<br>");
document.write("第"+(i+1)+"个节点类型:"+dd);
document.write("<br>");
}
</script>
</body>
</html>
