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为什么代码输出，一直显示else语句的内容？麻烦帮我看一下哪里出错了，谢谢

<!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");      
    // }
 //任务部分   
    function get_previousSibling(a){
        var b =a.previousSibling;
        while (b && b != 1){
            b = b.previousSibling;
        }
        return b;
    }
    var b = document.getElementsByTagName("li")[4];
    document.write(b.nodeName+"="+b.innerHTML+"<br>");
     var z =get_previousSibling(b);
     if(z!=null){
         document.write("previousSibling:"+z.nodeName+"="+z.innerHTML);
     }
     else {
         document.write("这是第一个节点了");
     }
</script>
</body>
</html>