获取目录树递归，为啥得到的结果为undefined

获取目录树递归，为什么得到的结果为undefined？
获取目录树递归，为什么得到的结果为undefined？

HTML code

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/mootools/1.4.5/mootools-yui-compressed.js"></script><script type="text/javascript">    var drres={        categories:[            {categories_id:'4',parent_id:'0'},            {categories_id:'6',parent_id:'4'},            {categories_id:'7',parent_id:'6'}        ]    }        //获取目录树数组，如['4','6','7']    function get_upper_categories(cid,arr){        var arr=arr||[];        var a=drres.categories.filter(function (n,i){            return n.categories_id==cid;        });        if(a.length>0){            arr.unshift(cid);            get_upper_categories(a[0]['parent_id'],arr);        }else{            return arr;        }    }        var tree=get_upper_categories('7');    alert(tree);</script>