Let’s say that you use Immutable.js Map as a data-structure of your state and you have following reducers:

import { Map } from 'immutable'

const initialState = Map({})

const reducers = Map({
  'current': currentReducer,
	'items': itemsReducer,
	'offices': officesReducer,
})

Default combineReducers function that comes with Redux will not work as it doesn’t know how to read and create new versions of state.

But it is easy to write you own combineReducers for Immutable.js:

const combineReducers = function(state = initialState, action) {
  reducers.forEach((reducer, key) => {
    state = state.set(key, reducer(state.get(key), action))
  })

  return state
}

on in a more readable way:

const combineReducers = function(state = initialState, action) {
  reducers.forEach((reducer, key) => {
    var oldState = state.get(key)
    var newState = reducer(oldState, action)
    state = state.set(key, newState)
  })

  return state
}

Reducers that get entire state instead of a single attribute

If for some reason you need a reducer to get entire state as an attribute then it’s easy to do with a simple convention: is a reducer name starts with an asterisk * then that reducer will receive entire state as first argument:

const reducers = Map({
  '*all': everythingReducer, // <== this reducer will receive entire state as first argument
	'items': itemsReducer,
	'offices': officesReducer,
})

and the combineReducers for that is:

const combineReducers = function(state = initialState, action) {
  reducers.forEach((reducer, key) => {
    // If reducer key starts with * then pass entire state to reducer
    var oldState = key[0] === '*' ? state : state.get(key)
    var newState = reducer(oldState, action)
    if (key[0] === '*') {
      state = newState
    } else {
      state = state.set(key, newState)
    }
  })

  return state
}

But this is an antipattern. I was using this function initially as I thought that I need it so that an action could change multiple attributes of state. But there is a better way.

Same action can change multiple state attributes

If you have an action (say CHANGE_CURRENT_PAGE) that should modify multiple state top-level attributes (say sidebarShown and currentPage) then you’re much better off by checking for the same action in 2 separate reducers. This way each reducer will be responsible for it’s own part of the state.