BaseFieldDefinition::create

public static BaseFieldDefinition::create($type)

public static function create($type) {
  $field_definition = new static(array());
  $field_definition->type = $type;
  $field_definition->itemDefinition = FieldItemDataDefinition::create($field_definition);
  // Create a definition for the items, and initialize it with the default
  // settings for the field type.
  // @todo Cleanup in https://www.drupal.org/node/2116341.
  $field_type_manager = \Drupal::service('plugin.manager.field.field_type');
  $default_settings = $field_type_manager->getDefaultStorageSettings($type) + $field_type_manager->getDefaultFieldSettings($type);
  $field_definition->itemDefinition->setSettings($default_settings);
  return $field_definition;
}