为什么previousSibling的这些不显示？

    function get_nextSibling(n)
    {
        var x=n.nextSibling;
        while(x.nodeType!=1)
        {
            x=x.nextSibling;
        }
        return x;
    }
    function get_previousSibling(n)
    {
        var x=n.previousSibling;
        while(x.nodeType!=1)
        {
        x=x.previousSibling;
        }
        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");      
    }
    var z=get_previousSibling(x);
    
    if(z!=null)
    {
        document.write(z.nodeName);
        document.write(" = ");
        document.write(z.innerHTML);
    }