Authentication and authorization are fundamental to building secure web applications. NextAuth.js is a powerful and flexible library that simplifies the process of adding authentication to your Next.js applications, supporting various providers like Google, GitHub, email, and more.

NextAuth.js is a client-side and server-side authentication library for Next.js applications. It handles common authentication patterns, such as OAuth, email/password, magic links, and JWTs, abstracting away much of the complexity involved in setting up secure sign-in and session management.

Understanding these core concepts is crucial for effectively using NextAuth.js:

Providers: These are the services your users can use to sign in (e.g., Google, GitHub, Email). NextAuth.js offers adapters for many popular providers.

Callbacks: Functions that allow you to customize how the user object is modified before being saved to the session or returned to the client. This is where you can add custom data like user roles or permissions.

Session Management: NextAuth.js handles the creation, storage, and validation of user sessions, typically using JWTs. This ensures that authenticated users are recognized across different requests.

API Routes: NextAuth.js uses Next.js API routes to create endpoints for authentication (e.g.,

/api/auth/[...nextauth]

The setup involves configuring the NextAuth.js handler and providing your authentication credentials.

1. Install NextAuth.js:
   bash

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   npm install next-auth
   # or
   yarn add next-auth

2. Create the API Route: Create a file at
   code

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

   (or
   code

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   app/api/auth/[...nextauth]/route.ts

   for App Router) and configure your providers and callbacks.

Example configuration for

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

import NextAuth from 'next-auth';
import GoogleProvider from 'next-auth/providers/google';
export const authOptions = {
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_CLIENT_ID,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET,
    }),
    // ... other providers
  ],
  callbacks: {
    async jwt({ token, account, profile }) {
      // Persist the OAuth access token to the token right after signin
      if (account) {
        token.accessToken = account.access_token;
      }
      return token;
    },
    async session({ session, token, user }) {
      // Send properties to the client, like an access_token from a provider.
      session.accessToken = token.accessToken;
      return session;
    }
  }
};
export default NextAuth(authOptions);

You can protect routes by checking the user's session status. NextAuth.js provides hooks and utilities for this.

Using the

useSession

import { useSession, signIn, signOut } from 'next-auth/react';
function MyComponent() {
  const { data: session, status } = useSession();
  if (status === 'loading') {
    return 
Loading...
;
  }
  if (session) {
    return (
      <>
        Signed in as {session.user.email} 
         signOut()}>Sign out
      
    );
  }
  return (
    <>
      Not signed in 
       signIn()}>Sign in
    
  );
}

For server-side protection (e.g., in

getServerSideProps

getSession

While NextAuth.js primarily handles authentication (verifying who a user is), authorization (determining what a user can do) is typically implemented by checking user roles or permissions, often added within the

callbacks

NextAuth.js supports features like custom email providers, JWT encryption, database adapters for persistent sessions, and more, allowing for highly customized authentication flows.