Redux reducer registry to dynamically add reducers

reduxReducerRegistry.js

const placeholderReducer = (state = null) => state;
let reducers = {};

export default {
  getReducers() {
    return { ...reducers };
  },
  // Preserve initial state for not-yet-loaded reducers
  getInitialReducer(initialReducers, initialState) {
    const reducerNames = Object.keys(initialReducers);

    reducers = { ...initialReducers };

    Object.keys(initialState).forEach(key => {
      if (reducerNames.indexOf(key) === -1) {
        reducers[key] = placeholderReducer;
      }
    });

    return this.getReducers();
  },
  register(reducer, reducerName = reducer.name) {
    const existingReducer = reducers[reducerName];

    if (existingReducer && existingReducer !== placeholderReducer) {
      return;
    }

    reducers = {
      ...reducers,
      [reducerName]: reducer,
    };

    if (this.onRegisterHandler) {
      this.onRegisterHandler(this.getReducers());
    }
  },
  onRegister(cb) {
    this.onRegisterHandler = cb;
  }
};

Based off of the Twitter implementation referenced @ http://nicolasgallagher.com/redux-modules-and-code-splitting/