oauth - How do I get around Access-Control-Allow-Origin error with ReactJS? -

I have a flak backend that runs through Google's OAuth2 flow when a user visits / signups There is a button, so that they can press to start it. I know that the flow works because if I set it to start as a person's visit / signup (by redirecting to the 302 / start-google-oauth endpoint), everything goes smoothly is.

But if I delete the redirect and the user starts the free flow by pressing the button, then it does not work. Buttons start requesting for just one / gmail- google-oauth but then redirects to open their popup window returns an entry-control-permission-generation error:

  XMLHttpRequest https load Can not: //accounts.google.com/o/oauth2/auth?direct_uri = ... No 'Access-Control-Permission-Origin' header exists on the requested resource. Origin '& lt; Original url & gt; Therefore entry is not allowed  

Origin specified as the original URL is given as valid redirect in the app's credential setup.

What can be done with it?