                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /JavaScript进阶篇

大神，帮帮忙，为啥没有输出

<!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>已经是最后一个节点");
    }
//任务
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/>"+"这已经是第一个节点了")；
    }
</script>
</body>
</html>