index.js

import { combineReducers } from 'redux'

const subscribed = (state = false, action) => {
  switch (action.type) {
    case 'SET_SUBSCRIBED':
      return true
    default:
      return state
  }
}

const rootReducer = combineReducers({
  subscribed
})

export default rootReducer

index.spec.js

it('should handle the default action for passed in state', () => {
  const action = { type: 'RANDOM-123' }

  deepFreeze(action)

  expect(reducer(true, action)).toEqual({ subscribed: true })
})

Expected { subscribed: false } to equal { subscribed: true }