bar.js

import React from 'react'

export function funcA(state, action) {
  // do stuff
  return state_new
}

bar_test.js

import expect from 'expect'
import deepFreeze from 'deep-freeze'

import { funcA } from './bar'

function test_funcA() {
  const state_0 = {}
  const state_1 = {}

  expect(
    funcA(state_0, action_A)
  ).toEqual(state_1)
}

test_funcA()
console.log('All tests passed')

// https://egghead.io/lessons/javascript-redux-avoiding-object-mutations-with-object-assign-and-spread