                        課程
                    
                        /前端開發
                        
                            /JavaScript
                        
                        /JavaScript進階篇

大神，幫幫忙，為啥沒有輸出

<!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>已經是最后一個節點");???? ?
??? }
//任務
function get_previousSibling(a)
??? {
??????? var b=a.previousSibling;
??????? while( b.nodeType!=1)
??????????? {
??????????????? b=b.previousSibling;
??????????? }
??????????? return b;
??? }
??? var b=document.getElementsTagName("li")[2];
??? document.write(b.nodeName);
??? document.write("=");
??? document.write(b.innerHTML);
?? ?
??? var c=get_previousSibling(b);
??? if(c!=null)
??? {
??????? document.write("<br/>previousSibling"+c.nodeName+"="+c.innerHTML);
??? }
??? else
??? {
??????? document.write("<br/>"+"這已經是第一個節點了")；
??? }
</script>
</body>
</html>