public function minifyJS() {
      $js_dir = './js';
      if (is_dir($js_dir)) {
         foreach (glob("$js_dir/*.js") as $js_file) {
            if (!$this->endsWith($js_file, 'min.js')) {
               $this->taskMinify($js_file)
                  ->to(str_replace('.js', '.min.js', $js_file))
                  ->type('js')
                  ->run();
            }
         }
      }
      return $this;
   }