A simple pattern for having a reducer as a map of functions instead of a switch statement. One benefit is being able to grab any of the reducer's functions independently for testing.

make-reducer.js

export default function (reducerMap = {}, initialState) =>
  (state = initialState, { type = '', ...payload }) =>
    (reducerMap[type] || (() => state))(state, payload) // if type isn't found in the reducer, just use a noop

users.js

import Immutable from 'immutable'
import makeReducer from './make-reducer'

const INITIAL_STATE = Immutable.Map()

export default makeReducer({
  'users/update-users': (state, { users }) => {
    return Immutable.fromJS(users)
  },
  'users/update-user': (state, { user, id }) => {
    return state.set(id, user)
  })
}, INITIAL_STATE)