Combine root reducer with nested reducers

combineRootWithReducers.js

const combineRootWithReducers = ({
  root = () => ({}), // reducer which should be on the root level of global state
  reducers = {}, // other reducers
  initial = {}, // initial state of global state
  rootLast = false // if true, root reducer will be called after other reducers
}) => {
  /* 
	this method allows you to combine basic reducers with a reducer which 
	manages state on the root level of global state
	{
	  banana: ..., // first reducer
	  apple: ..., // second reducer
	  authUser: ..., // some value managed by root reducer
	}
	** notes **
	- keys of root values and keys of reducers should be distinct
  */
  const combined = (state = initial, action) => {
    let nextState = { ...state };
    const rootWrapper = x => root(x, action);
    const reducersWrapper = x =>
      Object.entries(reducers).reduce(
        (ns, [name, reducer]) => ({
          ...ns,
          [name]: reducer(ns[name], action)
        }),
        x
      );
    if (!rootLast) {
      nextState = rootWrapper(nextState);
      nextState = reducersWrapper(nextState);
    } else {
      nextState = reducersWrapper(nextState);
      nextState = rootWrapper(nextState);
    }
    return nextState;
  };
  return combined;
};

export default combineRootWithReducers;