onLoginOrRegister = () => {
  const { phoneNumber } = this.state;
  firebase.auth().signInWithPhoneNumber(phoneNumber)
    .then((confirmResult) => {
      // This means that the SMS has been sent to the user
      // You need to:
      //   1) Save the `confirmResult` object to use later
      this.setState({ confirmResult });
      //   2) Hide the phone number form
      //   3) Show the verification code form
    })
    .catch((error) => {
      const { code, message } = error;
      // For details of error codes, see the docs
      // The message contains the default Firebase string
      // representation of the error
    });
}
onVerificationCode = () => {
  const { confirmResult, verificationCode } = this.state;
  confirmResult.confirm(verificationCode)
    .then((user) => {
      // If you need to do anything with the user, do it here
      // The user will be logged in automatically by the
      // `onAuthStateChanged` listener we set up in App.js earlier
    })
    .catch((error) => {
      const { code, message } = error;
      // For details of error codes, see the docs
      // The message contains the default Firebase string
      // representation of the error
    });
}