drupal 8 API route controller

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Response;

/**
 * Implements hook_node_presave().
 */
function abc_node_presave($node)
{
  if($enable_field_full_json_string && ($node->bundle() == "dynamic_api")){
    $json_response_arr = abc($node);
$ref_link_entities = $node->get('field_api_key_value_pairs')->referencedEntities();
$key_value = $ref_link_node->get('field_pair_api_key')->value;
  foreach ($ref_link_entities as $ref_link_node){
    $json_response = json_encode($json_response_arr, JSON_PRETTY_PRINT | JSON_HEX_TAG | JSON_HEX_APOS | JSON_HEX_AMP | JSON_HEX_QUOT);
    $node->set('field_full_json_string', $json_response);
  }
}

$json_value = $node->get('field_json_api_content')->value;
    $response = new Response();
    $response->setContent($json_value);
    $response->headers->set('Content-Type', 'application/json');
    return $response;

$query = \Drupal::entityQuery('node')
    ->condition('status', 1)
    ->condition('field_content_api_id.value', $node_api_id);

    $nids = $query->execute();
    $node = \Drupal\node\Entity\Node::load(reset($nids));
    if(empty($node)){
      $response = "Not found";
      return new JsonResponse($response);
    }
    $response = abc($node);
    if ($functioncall) {
      return $response;
    } else {
      return new JsonResponse($response);
    }