EntityTestMapField.php

<?php

namespace Drupal\entity_test\Entity;

use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;

/**
 * An entity used for testing map base field values.
 *
 * @ContentEntityType(
 *   id = "entity_test_map_field",
 *   label = @Translation("Entity Test map field"),
 *   base_table = "entity_test_map_field",
 *   entity_keys = {
 *     "uuid" = "uuid",
 *     "id" = "id",
 *     "label" = "name",
 *     "langcode" = "langcode",
 *   },
 *   admin_permission = "administer entity_test content",
 * )
 */
class EntityTestMapField extends EntityTest {
    
    /**
     * {@inheritdoc}
     */
    public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
        $fields = parent::baseFieldDefinitions($entity_type);
        $fields['data'] = BaseFieldDefinition::create('map')->setLabel(t('Data'))
            ->setDescription(t('A serialized array of additional data.'));
        return $fields;
    }

}