I am building a custom signup form where I would like to show the errors (if there any) and a spinner, when auth0 is authenticating the user.
How this could be done? What is the parameter, which tells that authentication has ended and where is errors are stored? I am building the page with ReactJS
That would depend largely of the authentication API endpoints and/or libraries being used.
For example, if you used the method webAuth.client.login() available in Auth0.js (v8) you would be able to pass a callback function that is invoked when the authentication request completes and that could be used to signal the spinner process to end.
You’ll need to check the documentation of the specific endpoints you’re using to learn how you could achieve something similar when using them.