O(1) Reducer

UserReducer.js

const actionTypes = {
  USER_LOGIN_SUBMIT: 'USER_LOGIN_SUBMIT',
  USER_LOGIN_SUCCESS: 'USER_LOGIN_SUCCESS',
  USER_LOGIN_FAILURE: 'USER_LOGIN_FAILURE'
};

const createInitialState = (initialState = {}) => ({
  id: null,
  email: null,
  isLoading: false,
  error: undefined,
  ...initialState,
});

const reducerMap = {
  [actionTypes.USER_LOGIN_SUBMIT]: (state) => ({
    ...state,
    isLoading: true
  }),
  [actionTypes.USER_LOGIN_SUCCESS]: (state, action) => ({
    ...state,
    id: action.payload.id,
    email: action.payload.email,
    isLoading: false,
    error: undefined
  }),
  [actionTypes.USER_LOGIN_FAILURE]: (state, action) => ({
    ...state,
    isLoading: false,
    error: action.payload
  })
};

const userReducer = (state = loadInitialState(), action) => {
  const reducer = reducerMap[action.type];
  return reducer ? reducer(state, action) : state;  
};