Previous - next links for Drupal 8

MYMODULE-previous-next.html.twig

{% if next or previous %}
  <div class="previous-next">
    <div class="previous-next__previous">
      {% if previous %}
        <a href="{{ previous.url }}" class="previous-next__link previous-next__link--previous">
          <span>← {{ 'Previous post'|t }}</span>
          {{ previous.label }}
        </a>
      {% endif %}
    </div>
    <div class="previous-next__next">
      {% if next %}
        <a href="{{ next.url }}" class="previous-next__link previous-next__link--next">
          <span>{{ 'Next post'|t }} →</span>
          {{ next.label }}
        </a>
      {% endif %}
    </div>
  </div>
{% endif %}

MYMODULE.module

/**
 * Implements hook_theme().
 */
function niklan_theme($existing, $type, $theme, $path) {
  $file = 'MYMODULE.theme.inc';
  return [
    'MYMODULE_previous_next' => [
      'variables' => [
        'entity' => NULL,
      ],
      'file' => $file,
    ],
  ];
}

MYMODULE.theme.inc

/**
 * Implements template_preprocess_HOOK().
 */
function template_preprocess_MYMODULE_previous_next(&$variables) {
  /** @var \Drupal\Core\Entity\EntityInterface $entity */
  $entity = $variables['entity'];
  $variables['next'] = [];
  $variables['previous'] = [];

  $next_entity_id = \Drupal::entityQuery($entity->getEntityTypeId())
    ->condition('type', $entity->bundle())
    ->condition('created', $entity->getCreatedTime(), '>')
    ->range(0, 1)
    ->sort('created', 'ASC')
    ->execute();
  if (!empty($next_entity_id)) {
    $next_entity = \Drupal::entityTypeManager()
      ->getStorage($entity->getEntityTypeId())
      ->load(reset($next_entity_id));
    $variables['next']['label'] = $next_entity->label();
    $variables['next']['url'] = $next_entity->toUrl()->toString(TRUE)->getGeneratedUrl();
  }

  $previous_entity_id = \Drupal::entityQuery($entity->getEntityTypeId())
    ->condition('type', $entity->bundle())
    ->condition('created', $entity->getCreatedTime(), '<')
    ->range(0, 1)
    ->sort('created', 'DESC')
    ->execute();
  if (!empty($previous_entity_id)) {
    $previous_entity = \Drupal::entityTypeManager()
      ->getStorage($entity->getEntityTypeId())
      ->load(reset($previous_entity_id));
    $variables['previous']['label'] = $previous_entity->label();
    $variables['previous']['url'] = $previous_entity->toUrl()->toString(TRUE)->getGeneratedUrl();
  }
}