明明有上一个节点的，结果却是“已经是第一个节点了”
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>nextSibling</title>
</head>
<body>
<ul id="u1">
<li id="a">javascript</li>
<li id="b">jquery</li>
<li id="c">html</li>
</ul>
<ul id="u2">
<li id="d">css3</li>
<li id="e">php</li>
<li id="f">java</li>
</ul>
<script type="text/javascript">
function get_nextSibling(n){
var x=n.nextSibling;
while (x && x.nodeType!=1){
x=x.nextSibling;
}
return x;
}
var x=document.getElementsByTagName("li")[0];
document.write(x.nodeName);
document.write(" = ");
document.write(x.innerHTML);
var y=get_nextSibling(x);
if(y!=null){
document.write("<br />nextsibling: ");
document.write(y.nodeName);
document.write(" = ");
document.write(y.innerHTML);
}else{
document.write("<br>已经是最后一个节点");
}
// 获取上一个节点代码开始
function get_previousSibling(n){
var x=n.previousSibling;
while (x && x.nodeType != 1){
x=x.previousSibling;
}
return x;
}
var li5 = document.getElementsByTagName("li")[4];
document.write("<br/><br/>" + li5.nodeName);
document.write(" = ");
document.write(li5.innerHTML);
var node = get_previousSibling(a);
if(node != null){
document.write("<br/>previousSibling：");
document.write(node.nodeName);
document.write(" = ");
document.write(node.innerHTML);
}else{
document.write("<br/>已经是第一个节点了");
}
</script>
</body>
</html>为什么明明上一个节点是css的，结果却是“已经是第一个节点了”？问题出在哪里？请大神指教
