Authentication with NextAuth.jsLesson 5.1

How to set up NextAuth.js v5 in a Next.js App Router project

Auth.js v5, auth() function, providers, AUTH_SECRET, session strategy, route handler setup, auth.config.ts pattern

Installing Auth.js v5

Auth.js v5 (formerly NextAuth.js) is the standard auth library for Next.js. Install it:

npm install next-auth@beta
npx auth secret  # Generates AUTH_SECRET in .env

Configuration Files

// auth.config.ts (edge-compatible, no Node.js APIs)
import type { NextAuthConfig } from 'next-auth'

export const authConfig: NextAuthConfig = {
  pages: {
    signIn: '/login',
  },
  callbacks: {
    authorized({ auth, request: { nextUrl } }) {
      const isLoggedIn = !!auth?.user
      const isOnDashboard = nextUrl.pathname.startsWith('/dashboard')
      if (isOnDashboard) return isLoggedIn
      return true
    },
  },
  providers: [],
}

// auth.ts (full Node.js — has Prisma/DB access)
import NextAuth from 'next-auth'
import { authConfig } from './auth.config'
import GitHub from 'next-auth/providers/github'

export const { auth, signIn, signOut, handlers } = NextAuth({
  ...authConfig,
  providers: [GitHub],
})

// app/api/auth/[...nextauth]/route.ts
import { handlers } from '@/auth'
export const { GET, POST } = handlers

Environment Variables

AUTH_SECRET=your-generated-secret
AUTH_GITHUB_ID=your-github-oauth-app-id
AUTH_GITHUB_SECRET=your-github-oauth-app-secret

The split between auth.config.ts and auth.ts is required because middleware runs in the Edge Runtime, which cannot use Node.js APIs like Prisma. Put edge-safe logic in auth.config.ts and import it in both auth.ts and middleware.ts.