Combine redux reducers without adding nesting

Ok, decided to do it for fun, not too much code… This will wrap a reducer and only provide it with keys that it has returned itself.

// don't provide keys to reducers that don't supply them
const filterReducer = (reducer) => {
  let lastState = undefined;
  return (state, action) => {
    if (lastState === undefined || state == undefined) {
      lastState = reducer(state, action);
      return lastState;
    }
    var filteredState = {};
    Object.keys(lastState).forEach( (key) => {
      filteredState[key] = state[key];
    });
    var newState = reducer(filteredState, action);
    lastState = newState;
    return newState;
  };
}

In your tests:

const reducerA = filterReducer(combineReducers({ reducerA1, reducerA2 }))
const reducerB = filterReducer(combineReducers({ reducerB1, reducerB2 }))

NOTE: This does break with the idea that the reducer will always provide the same output given the same inputs. It would probably be better to accept the list of keys when creating the reducer:

const filterReducer2 = (reducer, keys) => {
  let lastState = undefined;
  return (state, action) => {
    if (lastState === undefined || state == undefined) {
      lastState = reducer(state, action);
      return lastState;
    }
    var filteredState = {};
    keys.forEach( (key) => {
      filteredState[key] = state[key];
    });
    return lastState = reducer(filteredState, action);
  };
}

const reducerA = filterReducer2(
  combineReducers({ reducerA1, reducerA2 }),
  ['reducerA1', 'reducerA2'])
const reducerB = filterReducer2(
  combineReducers({ reducerB1, reducerB2 }),
  ['reducerB1', 'reducerB2'])