typed-reducer.js

type TAction<T: $Subtype<string>, P> = {
  type: T,
  payload: P,
};

const ActionTypes = {
  STRING: 'STRING',
  NUMBER: 'NUMBER',
  LOGIN: 'LOGIN',
  LOGIN_PAGE: 'LOGIN_PAGE',
  LOGIN_ASYNC: 'LOGIN_ASYNC',
};

type TUser = {
  +id: string,
  +name: string,
  +age: number,
};

type TString = TAction<typeof ActionTypes.STRING, string>;
type TNumber = TAction<typeof ActionTypes.NUMBER, number>;
type TLogin = TAction<typeof ActionTypes.LOGIN, TUser>;

type TActions =
  | TString
  | TNumber
  | TLogin
  ;

/*:: const exhaustive = (action: empty) => null; */
const reducer = (state: boolean, action: TActions): boolean => {
  switch (action.type) {
    case ActionTypes.STRING:
      return action.payload;
    case ActionTypes.NUMBER:
      return action.payload;
    case ActionTypes.LOGIN:
      return action.payload;
    default:
      /*:: exhaustive(action) */
      return state;
  }
};