Cakephp3: How can I return json data?

Instead of returning the json_encode result, set the response body with that result and return it back.

public function add()
{
    $this->autoRender = false; // avoid to render view

    $location = $this->Locations->newEntity();
    if ($this->request->is('post')) {
        $location = $this->Locations->patchEntity($location, $this->request->data);
        if ($this->Locations->save($location)) {
            //$this->Flash->success(__('The location has been saved.'));
            //return $this->redirect(['action' => 'index']);
            $resultJ = json_encode(array('result' => 'success'));
            $this->response->type('json');
            $this->response->body($resultJ);
            return $this->response;
        } else {
            //$this->Flash->error(__('The location could not be saved. Please, try again.'));
            $resultJ = json_encode(array('result' => 'error', 'errors' => $location->errors()));

            $this->response->type('json');
            $this->response->body($resultJ);
            return $this->response;
        }
    }
    $this->set(compact('location'));
    $this->set('_serialize', ['location']);
}

Edit (credit to @Warren Sergent)

Since CakePHP 3.4, we should use

return $this->response->withType("application/json")->withStringBody(json_encode($result));

Instead of :

$this->response->type('json');
$this->response->body($resultJ);
return $this->response;

CakePHP Documentation

Most answers I've seen here are either outdated, overloaded with unnecessary information, or rely on withBody(), which feels workaround-ish and not a CakePHP way.

Here's what worked for me instead:

$my_results = ['foo'=>'bar'];

$this->set([
    'my_response' => $my_results,
    '_serialize' => 'my_response',
]);
$this->RequestHandler->renderAs($this, 'json');

More info on RequestHandler. Seemingly it's not getting deprecated anytime soon.

UPDATE: CakePHP 4

$this->set(['my_response' => $my_results]);
$this->viewBuilder()->setOption('serialize', true);
$this->RequestHandler->renderAs($this, 'json');

More info