Views formatter that concatenates the names of referenced entities

ReferencedEntityNamesFieldFormatter.php

<?php
/**
 * @file
 */

namespace Drupal\my_module\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Field\FieldItemListInterface;
/**
 * Plugin implementation of the 'Referenced entity names' views field formatter.
 *
 * @FieldFormatter(
 *   id = "referenced_entity_names_field_formatter",
 *   label = @Translation("Referenced entity names"),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class ReferencedEntityNamesFieldFormatter extends FormatterBase {
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $element = [];
    foreach ($items as $delta => $item) {
      if (is_object($item->entity->target_id->entity)) {
        $element[$delta] = ['#markup' => $item->entity->target_id->entity->name->value];
      }
    }
    return $element;
  }

}