$form['node_items'] = array(
            '#type' => 'textfield',
            '#autocomplete_route_name' => 'mymodulenode.autocomplete',
            '#autocomplete_route_parameters' => array('skip_node_id_string' => $skip_node_id_string),
);

public function autocomplete(Request $request,$skip_node_id_string) {

    $skip_node_id = explode("&", $skip_node_id_string);

    $string = $request->query->get('q');
    if ($string) {
      $nids = Drupal::entityQuery('node')
        ->condition('status', 'NODE_PUBLISHED')
        ->condition('nid', $skip_node_id , 'NOT IN')
        ->condition('title', $string, 'CONTAINS')
        ->execute();
      $nodes = Node::loadMultiple($nids);
      foreach($nodes as $node){
        $path_alias = 'node/'.$node->get('nid')->value;
        $alias = Drupal::service('path.alias_manager')->getPathByAlias($path_alias);
        //$alias = drupal_get_path_alias('node/'.$type['nid']);
        $alias = (strlen($alias) > 23) ? substr($alias,0,20).'...' : $alias;
        $matches[] = array('label' => $node->get('title')->value.'['. $node->get('nid')->value .', '.$alias .']', 'value' => $node->get('title')->value . '[nid:' . $node->get('nid')->value . ']');
      }
    }
    return new JsonResponse($matches);
  }