Exit App in React Navigation

index.js

const AppNavigator = createStackNavigator(
  {
    Screen: Screen,
    Login: Login,
    Intro: Intro,
  },
  {
    initialRouteName: 'Screen',
    transitionConfig,
    navigationOptions: {
      gesturesEnabled: false
    }
  }
)

const defaultGetStateForAction = AppNavigator.router.getStateForAction

AppNavigator.router.getStateForAction = (action, state) => {
  if (state &&
    action.type === NavigationActions.BACK &&
    (
      state.routes[state.index].routeName === 'Intro' ||
      state.routes[state.index].routeName === 'Home' ||
      state.routes[state.index].routeName === 'Login'
    )
  ) {
    BackHandler.exitApp()
    return null
  }

  return defaultGetStateForAction(action, state)
}