code: protected function performAjaxValidation($models)
      {
          if (\Yii::$app->request->isAjax) {
              if (is_array($models)) {
                  $result = [];
                  foreach ($models as $model) {
                      if ($model->load(\Yii::$app->request->post())) {
                          \Yii::$app->response->format = Response::FORMAT_JSON;
                          $result = array_merge($result, ActiveForm::validate($model));
                      }
                  }
                  echo json_encode($result);
                  \Yii::$app->end();
              } else {
                  if ($models->load(\Yii::$app->request->post())) {
                      \Yii::$app->response->format = Response::FORMAT_JSON;
                      echo json_encode(ActiveForm::validate($models));
                      \Yii::$app->end();
                  }
              }
          }
      }
docs: Performs AJAX validation.
  @param array|Model $models
  @throws \yii\base\ExitException