A Developer's Guide to Auth.js

Auth.js (which you know as next-auth) is a complete, open-source authentication solution for modern web applications, built primarily for Next.js. It's designed to be flexible, secure, and easy to use, handling everything from OAuth sign-ins (Google, GitHub, etc.) to email/passwordless links and credentials-based logic.

It provides a set of simple, powerful APIs to manage user sessions, protect routes, and interact with various authentication providers, all while remaining unopinionated about your database or backend stack.

Core Concepts & Features

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  Providers: These are the "strategies" you use to sign users in. Auth.js supports dozens out-of-the-box (e.g., GoogleProvider, AppleProvider) and a generic CredentialsProvider for your own custom login forms.
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  Adapters: These are what connect Auth.js to your database. By using an adapter (like @auth/prisma-adapter), Auth.js can automatically create users, link accounts, manage sessions, and more directly in your database.
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  Session Management: It handles session state securely, using either database sessions (default with an Adapter) or JSON Web Tokens (JWTs). It provides both server-side helpers and a React hook (useSession) to access the session.
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  Callbacks: This is the heart of customization. Callbacks are functions you can define to intercept the authentication flow at critical points, allowing you to modify session data, control access, or add custom logic.
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  Security by Default: Auth.js automatically handles CSRF protection, secure cookies (HttpOnly, SameSite), and helps mitigate common auth vulnerabilities.