switch-away-from-switch-2.js

const createReducer = (initialState, reducerMap) => (
  state = initialState,
  action,
) => {
  // Pick a reducer from the object by key
  const reducer = state[action.type]
  if (!reducer) {
    return state
  }
  // Run the reducer if present
  return reducer(state, action)
}

const reducerLoadingMap = (actionInit, actionSuccess, actionError) => ({
  [actionInit.type]: () => true,
  [actionSuccess.type]: () => false,
  [actionError.type]: () => false,
})

class CatsGetInit extends ActionStandard {}
class CatsGetSuccess extends ActionStandard {}
class CatsGetError extends ActionStandard {}

const reducerCatsLoading = createReducer(
  false,
  reducerLoadingMap(CatsGetInit, CatsGetSuccess, CatsGetError),
)
/*  Now we can easily extend it like this:
    const reducerCatsLoading = createReducer(
      false,
      {
        ...reducerLoadingMap(CatsGetInit, CatsGetSuccess, CatsGetError),
        ... some custom stuff
      }
    )
*/
const reducerCatsData = createReducer(undefined, {
  [CatsGetSuccess.type]: () => action.payload,
})
const reducerCatsError = createReducer(undefined, {
  [CatsGetError.type]: () => action.payload,
})

const reducerCats = combineReducers({
  data: reducerCatsData,
  loading: reducerCatsLoading),
  error: reducerCatsError,
})