Adding an Action Type Enum

reducer.ts

enum ActionType {
  ADD_TODO = 'add_todo',
  ADD_TODO_SUCCESS = 'add_todo_success',
  ADD_TODO_ERROR = 'add_todo_error',
}

interface TodoState {
  loading: boolean;
  error: string | null;
  data: string[];
}

interface AddTodoAction {
  type: ActionType.ADD_TODO;
}

interface AddTodoSuccessAction {
  type: ActionType.ADD_TODO_SUCCESS;
  payload: string[];
}

interface AddTodoErrorAction {
  type: ActionType.ADD_TODO_ERROR;
  payload: string;
}

type Action = AddTodoAction | AddTodoSuccessAction | AddTodoErrorAction;

const initialState = {
  loading: false,
  error: null,
  data: [],
}

const reducer = (
  state: TodoState = initialState,
  action: Action
): TodoState => {
  switch (action.type) {
    case ActionType.ADD_TODO:
      return { loading: true, error: null, data: [] };

    case ActionType.ADD_TODO_SUCCESS:
      return { loading: false, error: null, data: action.payload };

    case ActionType.ADD_TODO_ERROR:
      return { loading: false, error: action.payload, data: [] };

    default:
      return state;
  }
};

export default reducer;