How to implement Custom Provider in NextAuth?

NextAuth.js is a complete solution for authentication, session management, and login for Next.js applications. If you want to add a custom authentication provider to NextAuth.js, follow these steps:

Step 1: Install NextAuth.js

If you haven't installed NextAuth.js yet, you can install it using npm or yarn:

bash
npm install next-auth

or

bash
yarn add next-auth

Step 2: Create NextAuth.js Configuration

In your Next.js application, create a file, typically [...nextauth].js (usually placed in the pages/api/auth directory), and configure NextAuth.js. In the configuration, you can add a providers array and configure your custom provider within it.

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

export default NextAuth({
  // Configure one or more authentication providers
  providers: [
    Providers.Credentials({
      // The name to display on the sign-in form (e.g., 'Sign in with...')
      name: 'Credentials',
      // The credentials are used to generate a suitable form on the sign-in page.
      // You can specify any fields you expect to be submitted.
      // e.g., domain, username, password, 2FA token, etc.
      // You can pass any HTML attribute to the input tag via the object.
      credentials: {
        username: { label: "Username", type: "text" },
        password: {  label: "Password", type: "password" }
      },
      authorize: async (credentials) => {
        // Add logic here to look up the user from the credentials supplied
        const user = { id: 1, name: 'J Smith', email: 'jsmith@example.com' }

        if (user) {
          // Any user object returned here will be saved in the JSON Web Token
          return Promise.resolve(user)
        } else {
          return Promise.resolve(null)
        }
      }
    }),
    // ...add more providers here
  ],

  // Add custom pages if needed
  pages: {
    signIn: '/auth/signin',
    signOut: '/auth/signout',
    error: '/auth/error', // Error code passed in query string as ?error=
    // ...
  },

  // ...add more options here
})

In the above example, we use Providers.Credentials to create a custom authentication provider based on credentials. This provider allows you to accept any form of credentials (e.g., username and password) and implement the verification logic in the authorize function.

Step 3: Implement the Authorization Logic

In the authorize function, you need to implement the logic to verify user credentials. This typically involves checking a database or calling an external service to confirm the user's identity.

Ensure that you return the user object when verification is successful and null when verification fails. The returned user object will be used to generate the JSON Web Token and for the user's session.

Step 4: Use the Custom Provider

Once you have configured the custom provider and implemented the authorization logic, you can use NextAuth.js's React hooks and components, such as useSession, signIn, and signOut, to manage the user's authentication state.

javascript
import { signIn, signOut, useSession } from 'next-auth/client';

// In a component
function MyComponent() {
  const [session, loading] = useSession();

  const handleSignIn = async () => {
    // Call the signIn function and pass 'credentials' with the form data
    const result = await signIn('credentials', {
      redirect: false, // Do not redirect
      username: 'myUsername',
      password: 'myPassword',
    });

    if (!result.error) {
      // Login successful!
    }
  };

  const handleSignOut = () => {
    signOut();
  };

  if (loading) return <div>Loading...</div>;
  if (!session) return <button onClick={handleSignIn}>Sign In</button>;

  return (
    <div>
      <p>Welcome, {session.user.name}!</p>
      <button onClick={handleSignOut}>Sign Out</button>
    </div>
  );
}

Note: The above example shows a basic implementation. You should replace the placeholder user data with your actual authentication logic.