function replaceReducer(nextReducer) {
    if (typeof nextReducer !== 'function') {
      throw new Error('Expected the nextReducer to be a function.');
    }

    currentReducer = nextReducer;
    dispatch({ type: ActionTypes.REPLACE });
  }
Replaces the reducer currently used by the store to calculate the state.

You might need this if your app implements code splitting and you want to
load some of the reducers dynamically. You might also need this if you
implement a hot reloading mechanism for Redux.

@param {Function} nextReducer The reducer for the store to use instead.
@returns {void}