如何高效获取尾部字符串

<script type="text/javascript">
        var  getString=function(fileName){
                return fileName.split(".").pop();
        }
        //例1
        var fileName1 = "photo.jpg";
        var getString1 = getString(fileName1);    // 获取值 jpg
        alert(getString1);

        //例2
        var fileName2 = "main.js";
        var getString2 = getString(fileName2);   // 获取值 js
        alert(getString2);
    </script>