Passport authentication with client side generation

[XT3RM1NATOR]

I am facing an issue now because i have client side generation with frontend in react and backend with node.js. In all the documentation the set up is configured so that when a user lets say presses on a button the request straight up comes to the server and the server redirects the user to a new oauth2 page lets say. However there is no answer on how to handle it with client side generation when the requests only come to the server for the data, and the server cannot redirect the user anywhere. How do i handle authentication in this case?