Why does yii2 sometimes return non-json format in ajax request?

Question

Yii

When getting return result of ajax request

$response  = [..];
Yii::$app->response->format = Response::FORMAT_JSON;
return $response;

returns

<![CDATA[YII-BLOCK-HEAD]]><![CDATA[YII-BLOCK-BODY-BEGIN]]>

then json The data
because of this, I can not display the answer through Jquery, since the answer is not in json format. how to get rid of this error?

Answer 1

This indicates that $response is not what is expected, and the application returns some kind of exception. Moreover, he is trying to give a template that is made a little against the rules. Most likely in /views/layouts/main.php the template is made without $this->beginPage(), $this->beginBody(), and the like.
I recommend using a controller inherited from \yii\rest\Controller for Ajax data, and not from \yii\web\Controller.