if let errCode = FIRAuthErrorCode(rawValue: err!._code) {

                switch errCode {
                case .errorCodeInvalidEmail:
                    print("invalid email")
                case .errorCodeEmailAlreadyInUse:
                    print("in use")
                default:
                    print("Other error!")
                }

            }

Auth.auth().createUser(withEmail: email, password: password) { (user: User?, error) in

        if error != nil {


            if let errCode = AuthErrorCode(rawValue: error!._code) {

                switch errCode {
                case .invalidEmail:

                    print("invalid email")
                    // Create an alert message
                    let alertMessage = UIAlertController(title: "Invalid Email", message: "Please check the entered email address", preferredStyle: .alert)
                    // Attach an action on alert message
                    alertMessage.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in
                        alertMessage.dismiss(animated: true, completion: nil)
                    }))
                    // Display the alert message
                    self.present(alertMessage, animated: true, completion: nil)

                case .emailAlreadyInUse:

                    print("in use")
                    // Create an alert message
                    let alertMessage = UIAlertController(title: "Existed Email", message: "The email existed in our database, login instead of registering", preferredStyle: .alert)
                    // Attach an action on alert message
                    alertMessage.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in
                        alertMessage.dismiss(animated: true, completion: nil)
                    }))
                    // Display the alert message
                    self.present(alertMessage, animated: true, completion: nil)

                    case .weakPassword:

                        print("password is weak")
                        // Create an alert message
                        let alertMessage = UIAlertController(title: "Password is weak", message: "Use upper and lower characters along with numbers", preferredStyle: .alert)
                        // Attach an action on alert message
                        alertMessage.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in
                            alertMessage.dismiss(animated: true, completion: nil)
                        }))
                        // Display the alert message
                        self.present(alertMessage, animated: true, completion: nil)

                default:
                    print("Other error!")
                }

            }
        }