Type-safety with UnknownAction in TypeScript

interface KnownAction {
  type: 'ACTION_TYPE1' | 'ACTION_TYPE2';
  // Other properties specific to known actions
}

interface UnknownAction {
  type: 'UNKNOWN_ACTION';
}

type Action = KnownAction | UnknownAction;

function isKnownAction(action: Action): action is KnownAction {
  return 'type' in action && action.type !== 'UNKNOWN_ACTION';
}

function reducer(state: any = {}, action: Action) {
  if (isKnownAction(action)) {
    switch (action.type) {
      case 'ACTION_TYPE1':
        // Handle ACTION_TYPE1
        break;
      case 'ACTION_TYPE2':
        // Handle ACTION_TYPE2
        break;
      default:
        // Ensure exhaustiveness of switch
        const exhaustiveCheck: never = action;
        break;
    }
  } else {
    // Handle unknown action
  }
  return state;
}