SignupForm.propTypes = {
  username: PropTypes.string.isRequired,
  email: PropTypes.string.isRequired,
  password: PropTypes.string.isRequired,
  passwordConfirmation: PropTypes.string.isRequired,
  onUsernameChange: PropTypes.func.isRequired,
  onEmailChange: PropTypes.func.isRequired,
  onPasswordChange: PropTypes.func.isRequired,
  onPasswordConfirmationChange: PropTypes.func.isRequired,
  submit: PropTypes.func.isRequired,
  usernameError: PropTypes.string,
  emailError: PropTypes.string,
  passwordError: PropTypes.string,
  passwordConfirmationError: PropTypes.string,
};