                        課程
                    
                        /前端開發
                        
                            /JavaScript
                        
                        /JavaScript進階篇

用document.getElementsByTagName("li")獲得的Li元素出現以下現象的原因？

<!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;
????}
????function?get_previousSibling(n){
????????var?x=n.previousSibling;
????????while(x?&&?x.nodeType!=1){
????????????x=x.previousSibling;
????????}
????????return?x;
????}
?
????var?x=document.getElementsByTagName("li")[1];
????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");??????
????}
????
????var?z=get_previousSibling(x);
????
????if(z!=null){
????????document.write("<br?/>previoussibling:?");
????????document.write(z.nodeName);
????????document.write("?=?");
????????document.write(z.innerHTML+"<br?/>");
????}else{
??????document.write("<br>已經是第一個節點");??????
????}
?
</script>
</body>
</html>

當?var x=document.getElementsByTagName("li")[2];時：