function userFollowersReducer(
  events: Event[],
  initialState = []
): string[] {
  return events.reduce((followers: string[], e: Event): string[] => {
    switch (e.type) {
      case 'USER_FOLLOWED':
        // add userId in array
        return [...followers, e.payload.followedUserId]
      case 'USER_UNFOLLOWED':
        // remove userId from array
        return followers.filter(userId => userId !== e.payload.unfollowedUserId)
      default:
        // keep array the same in case if any other event
        return followers
    }
  }, initialState)
}