public static function js_files(array $files) {
        if (empty($files)) {
            return '';
        }

        $compressed = array();
        foreach ($files as $file) {
            $content = file_get_contents($file);
            if ($content === false) {
                $compressed[] = "\n\n// Cannot read JS file ".basename(dirname(dirname($file))).'/'.basename(dirname($file)).'/'.basename($file)."\n\n";
                continue;
            }
            $compressed[] = self::js($content);
        }

        return implode(";\n", $compressed);
    }