js获取文件大小

var  uploadImgSizeLimit=function(target){        alert(target);            //以下校验图片大小            var agent = window.navigator.userAgent;            var isIE=/msie/i.test(agent) && !window.opera;            var fileSize = 0;            if (isIE && !target.files) {             //如果是ie6以下的版本，直接fileSize可以获取到大小                if (agent.indexOf("MSIE 5.0") > 0 || agent.indexOf("MSIE 6.0") > 0) {                    var img = new Image();                    var fname = target.value;                    img.src = fname;                    fileSize = img.fileSize;                }else {                    //ie7版本以上需要打开安全设置                     try {                           var fso, f, fname, fsize;                            var flength = 1000; //设置上传的文件最大值（单位：kb），超过此值则不上传。                            var finame = target.value;                            fso = new ActiveXObject("Scripting.FileSystemObject");                            f = fso.GetFile(finame);//文件的物理路径                            fname = fso.GetFileName(finame);//文件名（包括扩展名）                            fileSize = f.Size; //文件大小（bit）                        }                     catch(e){                                alert(e " 跳出此消息框，是由于你的activex控件没有设置好,"                                 "你可以在浏览器菜单栏上依次选择"                                 "工具->internet选项->"安全"选项卡->自定义级别,"                                 "打开"安全设置"对话框，把"对没有标记为安全的"                                "ActiveX控件进行初始化和脚本运行"，改为"启动"即可");                            return false;                     }                }            }else {        //    火狐版本下的文件大小获取                fileSize = target.files[0].size;            }               var size = fileSize / 1024;             alert(size);                                     return false;        };