TypeScript Union Types

typescript-union-types.ts

type FeedbackAction = 'FEEDBACK_SUBMISSION_INITIATION' | 'FEEDBACK_SUBMISSION_MESSAGE' | 'FEEDBACK_SUBMISSION_ERROR'; // etc. etc.

interface IAction<P> {
  type: FeedbackAction;
  payload: P;
};

interface IFeedbackState {
  error: Error;
  message: string;
  isSubmitting: boolean;
};

// we could even get fancy here and define our own Maybe type
// instead of using those pesky nulls.
export const initialState: IAppState = {
  error = null,
  message = null,
  isSubmitting =  false,
};

const feedbackReducer = (state: IFeedbackState = initialState, action: IAction<any>): IFeedbackState => {
  switch (action.type) {
    // oops we forgot to declare this type.
    // Not to worry, the compiler has our back!
    // The compiler warns us that FEEDBACK_CLEAR_ALERTS is not a valid FeedbackAction.
    case 'FEEDBACK_CLEAR_ALERTS':
      return {
        ...state,
        error: null,
        message: null,
      };
    case 'FEEDBACK_SUBMISSION_INITIATION':
      return {
        ...state,
        isSubmitting: true,
      };
    case 'FEEDBACK_SUBMISSION_ERROR':
      return {
        ...state,
        isSubmitting: false,
        error: action.payload.error,
      };
    case 'FEEDBACK_SUBMISSION_MESSAGE':
      return {
        ...state,
        isSubmitting: false,
        message: action.payload.message,
      };
    default:
      return state;
  }
};