EntityTestMapField.php

<?php

declare (strict_types=1);
namespace Drupal\entity_test\Entity;

use Drupal\Core\Entity\Attribute\ContentEntityType;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;

/**
 * An entity used for testing map base field values.
 */
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;
    }

}