public static function save($filename='excel', $format='Xlsx')
    {
        $objPhpSpreadsheet = self::validExcelObj();

        // Create Writer first
        $objWriter = \PhpOffice\PhpSpreadsheet\IOFactory::createWriter($objPhpSpreadsheet, $format);

        /**
         * Type Mapping
         */
        $inTypeList = isset(self::$_writerTypeInfo[$format]) ? true : false;
        $extension = ($inTypeList) ? self::$_writerTypeInfo[$format]['extension'] : '';
        
        // Check if filename contained extension
        $filepath = (stripos($filename, $extension)===(strlen($filename)-strlen($extension)))
            ? $filename 
            : "{$filename}{$extension}";

        // Save file
        $objWriter->save($filepath);
        return $filepath;
    }