默认代码  

function get_previousSibling(n){
        var x=n.previousSibling;
        while (x && x.nodeType!=1){
            x=x.previousSibling;
        }
        return x;
    }
    
    var x=document.getElementsByTagName("li")[4];
    document.write(x.nodeName);
    document.write(" = ");
    document.write(x.innerHTML);
    
    var y=get_previousSibling(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");      
    }

我的代码

   function get_previousSibing(m){
        var z=m.previousSibing;
        while(z && z.nodeType!=1){
            z=z.previousSibing;
        }
        return z;
    }
    var z=document.getElementsByTagName("li")[2];
    document.write("<br><br>")
    document.write(z.nodeName+"="+z.innerHTML);
    
    var d=get_previousSibing(z);
    if(d!=null)
    {
    document.write("<br  />previoussibling:"+d.nodeName+" = "+d.innerHTML);
    }
    else {
    document.write("<br>已经是最后一个节点");
    }