Drupal 8 Output JSON

customController.php

<?php

// ...

use Symfony\Component\HttpFoundation\Response;

// Then later on (inside your controller's class), you have a function used
// for the route_name (on a hook_menu() item in your .module file), this
// function can return a JSON response...

$response = new Response();
$response->setContent(json_encode(array('hello' => 'world', 'goodbye' => 'world')));
$response->headers->set('Content-Type', 'application/json');
return $response;

For JsonResponse you have to declare it before.
use Symfony\Component\HttpFoundation\JsonResponse;

https://api.symfony.com/4.0/Symfony/Component/HttpFoundation/JsonResponse.html

For anyone arriving here from Google: JsonResponse works well these days. Might not have back in 2016.