--获取后缀名，结果 .jpg

var extName = "/upload/head_img/20150902102539.jpg";
var ta = extName.substring(extName.indexOf("."));

--判断是否是焦点

if (document.activeElement.id == "txtAutoC") {
//判断是否是焦点
}

http://www.cnblogs.com/yejiurui/p/3413796.html

http://blog.csdn.net/xyxjn/article/details/41805365