I am using react newforms for react form handling. I find it really good but couldn't figure out how to trigger an error on a given input field if the server responses with an error.
For example you got a login form and you decide not to show an alert box under the form but throw a custom error that newforms catches and displays an error message Username does not exist right below the input field. (like it does on clientside validation)
You can use form.addError() for this:
form.addError('username', 'Username does not exist.')
If there isn't another state change happening associated with the server response, you may need to force your component to re-render.
It also supports async validation, so it's possible to do the server call inside a custom validation method and get the form to add the error for you (suitability depends on when the server call needs to happen).
http://newforms.readthedocs.org/en/latest/validation.html#asynchronous-validation