This is a very basic example of a custom authentication implementation using an external backend API (using express) and nextjs.

Demo

Start the backend API:

cd back/
npm install
npm run dev

Start the nextjs app:

cd front/
cp .env.sample .env
npm install
npm run dev

Open http://localhost:3000 with your browser and register an account.

Implementation details

This is a very basic implementation of register/signin flow. You should add proper validation in the different endpoints.
Using an in memory "DB" to store the user. You might want to change that.
accessToken are never invalidated on the backend API. You might want to support that.
Session only contains the email and accessToken. You might want to include other things like roles, ...
Sessions are encrypted and stored in a jwt token in a cookie. This is the reason why we proxy the register/signin calls from client -> next server api -> external backend api
When calling a backend protected endpoint, use the Authorization: 'Bearer session.accessToken' either from useSession() for client components or from getServerSession() for server components.