How to use AJAX in CakePHP 3.5?

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Just a suggestion, use the new Cake\Http\Response functions format withType() withBody() etc:
https://book.cakephp.org/3.0/en/controllers/request-response.html#setting-the-body

$json_data = json_encode($data);
$response = $this->response->withType('json')->withStringBody($json_data);
return $response;