Customise session objects via callbacks in next-auth

In NextAuth.js, you can customize session objects using callbacks. NextAuth.js provides several callbacks that allow you to customize various aspects of the authentication flow, including the session object. The callbacks object in the NextAuth.js configuration allows you to define these callbacks.

To customize the session object, you can use the session callback. The session callback is called every time a session is created or updated, allowing you to modify the session object before it is stored.

Here's an example of how to use the session callback to customize the session object in NextAuth.js:

1. Create a NextAuth.js configuration file (e.g., next-auth.config.js) and define the session callback:

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// next-auth.config.js

export default {
  providers: [
    // Add your authentication providers here
  ],
  callbacks: {
    async session(session, user) {
      // Modify the session object before it is stored
      session.user.role = 'user'; // Set a custom role for the user
      session.customProperty = 'Custom Value'; // Add a custom property to the session object

      return session;
    },
  },
};

2. In your Next.js API route, use the useSession hook to access the customized session object:

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// pages/api/auth/[...nextauth].js

import NextAuth from 'next-auth';
import Providers from 'next-auth/providers';

export default NextAuth({
  providers: [
    // Add your authentication providers here
  ],
  callbacks: {
    async session(session, user) {
      // Modify the session object before it is stored
      session.user.role = 'user'; // Set a custom role for the user
      session.customProperty = 'Custom Value'; // Add a custom property to the session object

      return session;
    },
  },
});

Now, the session callback will be executed every time a user logs in or the session is updated, and the session object will be customized according to your logic.

Keep in mind that the session object should not exceed a certain size (e.g., 16KB) to ensure it can be efficiently stored and managed. Avoid adding large amounts of data to the session object. If you need to store additional data for the user, consider using a separate database or a caching mechanism.

Additionally, the session object will be serialized and deserialized for each request, so make sure that the data you add to the session can be efficiently serialized and deserialized.