Setting up authentication in Next.js 14 (App Router) is a nightmare if you don't understand the boundary between Server Components and Client Components.

I spent hours debugging useSession returning null or throwing hydration errors. Here is the exact, fool-proof way to set up NextAuth (Auth.js) with a MongoDB database.

1. Installation

bash npm install next-auth @auth/mongodb-adapter mongodb 2. Environment Variables Create a .env.local file and add your GitHub OAuth credentials and MongoDB URI:

env GITHUB_ID=your_github_client_id GITHUB_SECRET=your_github_client_secret MONGODB_URI=your_mongodb_connection_string NEXTAUTH_SECRET=any_long_random_string NEXTAUTH_URL=http://localhost:3000 3.…