前一个兄弟节点封装

<div id="box">

        <p>段落A</p>

        <!-- 注释 -->   

        <p id="para">段落B</p>

        <span></span>

        <p>段落C</p>

    </div>

--------------------------------------------------

​var para=document.getElementById('para');

        function getElementPrevSibling(node){

            if(node.previousSibling.nodeType==1){

                return node.previousSibling;

            }else{

                getElementPrevSibling(node.previousSibling);

            }

        }

        console.log(getElementPrevSibling(para));

为什么得到的是undefined