FetchUserId.swift

func fetchUserId(completion: @escaping (Int?, Error?) -> Void) {
    let request = URLRequest(url: URL(string: "http://facebook.com/me"))
    URLSession.shared.dataTask(with: request) { data, response, error in
        if let data = data {
            completion(Int(data), nil)
        } else if error = error {
            completion(nil, error)
        }
    }.resume()
}

fetchUserId { userId, error in
    print(userId, error)
}