Authentication Patterns

Q: What happens if you forget to await the cookies() call in Next.js 15+?

cookies() is async in Next.js 15+ and returns a Promise. Forgetting await means you call .get() on a Promise, which returns undefined. This silently breaks auth checks, making protected routes appear as if no token exists.

Q: Can you call redirect() inside a try/catch block?

redirect() throws a special NEXT_REDIRECT error internally. If wrapped in a try/catch, the redirect is caught and swallowed, so the user is never redirected. Either rethrow NEXT_REDIRECT errors or call redirect() outside the try/catch.

Q: Why does the Login Server Action set httpOnly: true and secure: true on the cookie?

httpOnly prevents JavaScript from reading the cookie, mitigating XSS attacks. secure ensures the cookie is only sent over HTTPS in production. sameSite: "lax" provides basic CSRF protection.

Q: Gotcha: What happens if you use runtime variables in the middleware config.matcher array?

The config.matcher must be statically analyzable at build time. Using variables or function calls causes the matcher to silently fail or throw a build error. Put conditional logic inside the middleware function body instead.

Q: How would you type the return value of verifySession in TypeScript?

JWTPayload from jose provides the base type with iss, sub, exp, etc. Return null on failure so callers can do a simple truthy check.

Q: How do you type the children prop in a layout component with TypeScript?

Use React.ReactNode for the children prop, which covers elements, strings, numbers, fragments, and null.

// middleware.ts
import { NextRequest, NextResponse } from "next/server";
 
const protectedPaths = ["/dashboard", "/settings", "/account"];
 
export function middleware(request: NextRequest) {
  const sessionToken = request.cookies.get("session-token")?.value;
  const { pathname } = request.nextUrl;
 
  const isProtected = protectedPaths.some((path) =>
    pathname.startsWith(path)
  );
 
  if (isProtected && !sessionToken) {
    const loginUrl = new URL("/login", request.url);
    loginUrl.searchParams.set("callbackUrl", pathname);
    return NextResponse.redirect(loginUrl);
  }
 
  return NextResponse.next();
}
 
export const config = {
  matcher: ["/dashboard/:path*", "/settings/:path*", "/account/:path*"],
};

Server-Side Auth in a Layout

// app/dashboard/layout.tsx
import { cookies } from "next/headers";
import { redirect } from "next/navigation";
import { verifySession } from "@/lib/auth";
 
export default async function DashboardLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const cookieStore = await cookies();
  const token = cookieStore.get("session-token")?.value;
 
  if (!token) {
    redirect("/login");
  }
 
  const session = await verifySession(token);
 
  if (!session) {
    redirect("/login");
  }
 
  return <>{children}</>;
}

Auth Utility (Server-Only)

// lib/auth.ts
import "server-only";
import { SignJWT, jwtVerify } from "jose";
 
const secret = new TextEncoder().encode(process.env.AUTH_SECRET);
 
export async function createSession(userId: string) {
  return new SignJWT({ userId })
    .setProtectedHeader({ alg: "HS256" })
    .setExpirationTime("7d")
    .sign(secret);
}
 
export async function verifySession(token: string) {
  try {
    const { payload } = await jwtVerify(token, secret);
    return payload;
  } catch {
    return null;
  }
}

// app/login/actions.ts
"use server";
 
import { cookies } from "next/headers";
import { redirect } from "next/navigation";
import { createSession } from "@/lib/auth";
 
export async function login(formData: FormData) {
  const email = formData.get("email") as string;
  const password = formData.get("password") as string;
 
  // Validate credentials against your database
  const user = await authenticateUser(email, password);
 
  if (!user) {
    return { error: "Invalid credentials" };
  }
 
  const token = await createSession(user.id);
  const cookieStore = await cookies();
 
  cookieStore.set("session-token", token, {
    httpOnly: true,
    secure: process.env.NODE_ENV === "production",
    sameSite: "lax",
    maxAge: 60 * 60 * 24 * 7, // 7 days
    path: "/",
  });
 
  redirect("/dashboard");
}

Deep Dive

How It Works

Middleware runs at the edge before any page or API route renders. It can read cookies and headers but cannot access a database directly. Use it for fast, coarse-grained checks (is a token present?).
Server Components and layouts run on the Node.js server. They can perform full session verification against a database or JWT secret.
The server-only import guarantees that lib/auth.ts can never be bundled into client code, preventing secret leakage.
Cookies are the primary session transport in App Router. The cookies() function is async in Next.js 15+ and must be awaited.
redirect() throws internally, so code after it is unreachable. No explicit return is needed after redirect().

Variations

Using NextAuth.js (Auth.js v5):

// auth.ts
import NextAuth from "next-auth";
import GitHub from "next-auth/providers/github";
 
export const { handlers, signIn, signOut, auth } = NextAuth({
  providers: [GitHub],
});
 
// middleware.ts
export { auth as middleware } from "./auth";

Role-Based Access:

// app/admin/layout.tsx
import { verifySession } from "@/lib/auth";
import { cookies } from "next/headers";
import { redirect } from "next/navigation";
 
export default async function AdminLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const cookieStore = await cookies();
  const token = cookieStore.get("session-token")?.value;
  const session = await verifySession(token!);
 
  if (session?.role !== "admin") {
    redirect("/unauthorized");
  }
 
  return <>{children}</>;
}

TypeScript Notes

cookies().get() returns { name: string; value: string } | undefined. Always use optional chaining.
Server Actions that return data (like { error: string }) cannot also call redirect() in the same code path, because redirect() throws.
Use import "server-only" at the top of any module containing secrets to get a compile-time error if it is imported from a Client Component.

Gotchas

Middleware cannot access databases or Node.js APIs. It runs in the Edge Runtime by default. Only check for token presence; verify tokens in Server Components or Route Handlers.
cookies() is async in Next.js 15+. Forgetting await produces a runtime error.
Middleware matcher must be static. You cannot use runtime variables in the config.matcher array. Use conditional logic inside the middleware function instead.
redirect() inside a try/catch will be swallowed because it throws a special NEXT_REDIRECT error. Either rethrow it or call redirect() outside the try/catch.
Static pages cannot check auth at request time. Use export const dynamic = "force-dynamic" or perform auth in middleware for statically generated pages.

Alternatives

Approach	Pros	Cons
Middleware-only	Fast, runs before rendering	Cannot verify tokens against DB
Layout-based auth	Full server access, can query DB	Runs after middleware, slight latency
NextAuth.js (Auth.js)	Built-in providers, session management	Extra dependency, migration churn
Clerk or Supabase Auth	Managed service, less code	Vendor lock-in, cost at scale
Iron Session	Encrypted cookies, no JWT	Smaller community, manual setup

FAQs

Why does the middleware only check for the presence of a token instead of fully verifying it?

Middleware runs in the Edge Runtime, which cannot access databases or most Node.js APIs.
A full JWT verify with a database lookup is too heavy for the edge.
Middleware performs a coarse check (is the cookie present?), then Server Components do the full verification.

What happens if you forget to `await` the `cookies()` call in Next.js 15+?

cookies() is async in Next.js 15+ and returns a Promise.
Forgetting await means you call .get() on a Promise, which returns undefined.
This silently breaks auth checks, making protected routes appear as if no token exists.

Why is `import "server-only"` used at the top of `lib/auth.ts`?

It guarantees a compile-time error if any Client Component imports this module.
This prevents your AUTH_SECRET and JWT signing logic from leaking into the browser bundle.

Can you call `redirect()` inside a try/catch block?

redirect() throws a special NEXT_REDIRECT error internally.
If wrapped in a try/catch, the redirect is caught and swallowed, so the user is never redirected.
Either rethrow NEXT_REDIRECT errors or call redirect() outside the try/catch.

How does the `callbackUrl` pattern work in the middleware redirect?

When an unauthenticated user hits a protected path, the middleware redirects to /login.
The original pathname is appended as ?callbackUrl=/dashboard (or whatever path was requested).
After successful login, your login handler can read this param and redirect back to the original page.

What is the difference between middleware-based auth and layout-based auth?

Middleware runs before any rendering, at the edge, and can only do lightweight checks.
Layout-based auth runs on the Node.js server, can query a database, and fully verify a session.
Best practice is to combine both: middleware for a fast gate, layout for full verification.

Why does the Login Server Action set `httpOnly: true` and `secure: true` on the cookie?

httpOnly prevents JavaScript from reading the cookie, mitigating XSS attacks.
secure ensures the cookie is only sent over HTTPS in production.
sameSite: "lax" provides basic CSRF protection.

Gotcha: What happens if you use runtime variables in the middleware `config.matcher` array?

The config.matcher must be statically analyzable at build time.
Using variables or function calls causes the matcher to silently fail or throw a build error.
Put conditional logic inside the middleware function body instead.

How would you type the return value of `verifySession` in TypeScript?

import { JWTPayload } from "jose";
 
export async function verifySession(
  token: string
): Promise<JWTPayload | null> {
  try {
    const { payload } = await jwtVerify(token, secret);
    return payload;
  } catch {
    return null;
  }
}

JWTPayload from jose provides the base type with iss, sub, exp, etc.
Return null on failure so callers can do a simple truthy check.

How do you type the `children` prop in a layout component with TypeScript?

export default async function DashboardLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  // ...
}

Use React.ReactNode for the children prop, which covers elements, strings, numbers, fragments, and null.

Why can't statically generated pages check auth at request time?

Static pages are pre-rendered at build time and served as plain HTML files.
There is no server-side execution at request time to read cookies or verify sessions.
Use export const dynamic = "force-dynamic" or protect the route via middleware instead.

How does NextAuth.js (Auth.js v5) simplify the middleware setup?

You export auth as middleware directly, which handles session checks automatically.
Auth.js manages providers, session storage, and token rotation out of the box.
The trade-off is an extra dependency and keeping up with Auth.js migration changes.

Route Handlers - protect API endpoints with the same auth utilities
Environment Variables - storing AUTH_SECRET safely
Error Handling - handling auth failures gracefully
Next.js Middleware Docs

// middleware.ts
import { NextRequest, NextResponse } from "next/server";
 
const protectedPaths = ["/dashboard", "/settings", "/account"];
 
export function middleware(request: NextRequest) {
  const sessionToken = request.cookies.get("session-token")?.value;
  const { pathname } = request.nextUrl;
 
  const isProtected = protectedPaths.some((path) =>
    pathname.startsWith(path)
  );
 
  if (isProtected && !sessionToken) {
    const loginUrl = new URL("/login", request.url);
    loginUrl.searchParams.set("callbackUrl", pathname);
    return NextResponse.redirect(loginUrl);
  }
 
  return NextResponse.next();
}
 
export const config = {
  matcher: ["/dashboard/:path*", "/settings/:path*", "/account/:path*"],
};

Server-Side Auth in a Layout

// app/dashboard/layout.tsx
import { cookies } from "next/headers";
import { redirect } from "next/navigation";
import { verifySession } from "@/lib/auth";
 
export default async function DashboardLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const cookieStore = await cookies();
  const token = cookieStore.get("session-token")?.value;
 
  if (!token) {
    redirect("/login");
  }
 
  const session = await verifySession(token);
 
  if (!session) {
    redirect("/login");
  }
 
  return <>{children}</>;
}

Auth Utility (Server-Only)

// lib/auth.ts
import "server-only";
import { SignJWT, jwtVerify } from "jose";
 
const secret = new TextEncoder().encode(process.env.AUTH_SECRET);
 
export async function createSession(userId: string) {
  return new SignJWT({ userId })
    .setProtectedHeader({ alg: "HS256" })
    .setExpirationTime("7d")
    .sign(secret);
}
 
export async function verifySession(token: string) {
  try {
    const { payload } = await jwtVerify(token, secret);
    return payload;
  } catch {
    return null;
  }
}

// app/login/actions.ts
"use server";
 
import { cookies } from "next/headers";
import { redirect } from "next/navigation";
import { createSession } from "@/lib/auth";
 
export async function login(formData: FormData) {
  const email = formData.get("email") as string;
  const password = formData.get("password") as string;
 
  // Validate credentials against your database
  const user = await authenticateUser(email, password);
 
  if (!user) {
    return { error: "Invalid credentials" };
  }
 
  const token = await createSession(user.id);
  const cookieStore = await cookies();
 
  cookieStore.set("session-token", token, {
    httpOnly: true,
    secure: process.env.NODE_ENV === "production",
    sameSite: "lax",
    maxAge: 60 * 60 * 24 * 7, // 7 days
    path: "/",
  });
 
  redirect("/dashboard");
}

Deep Dive

How It Works

Middleware runs at the edge before any page or API route renders. It can read cookies and headers but cannot access a database directly. Use it for fast, coarse-grained checks (is a token present?).
Server Components and layouts run on the Node.js server. They can perform full session verification against a database or JWT secret.
The server-only import guarantees that lib/auth.ts can never be bundled into client code, preventing secret leakage.
Cookies are the primary session transport in App Router. The cookies() function is async in Next.js 15+ and must be awaited.
redirect() throws internally, so code after it is unreachable. No explicit return is needed after redirect().

Variations

Using NextAuth.js (Auth.js v5):

// auth.ts
import NextAuth from "next-auth";
import GitHub from "next-auth/providers/github";
 
export const { handlers, signIn, signOut, auth } = NextAuth({
  providers: [GitHub],
});
 
// middleware.ts
export { auth as middleware } from "./auth";

Role-Based Access:

// app/admin/layout.tsx
import { verifySession } from "@/lib/auth";
import { cookies } from "next/headers";
import { redirect } from "next/navigation";
 
export default async function AdminLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const cookieStore = await cookies();
  const token = cookieStore.get("session-token")?.value;
  const session = await verifySession(token!);
 
  if (session?.role !== "admin") {
    redirect("/unauthorized");
  }
 
  return <>{children}</>;
}

TypeScript Notes

cookies().get() returns { name: string; value: string } | undefined. Always use optional chaining.
Server Actions that return data (like { error: string }) cannot also call redirect() in the same code path, because redirect() throws.
Use import "server-only" at the top of any module containing secrets to get a compile-time error if it is imported from a Client Component.

Gotchas

Middleware cannot access databases or Node.js APIs. It runs in the Edge Runtime by default. Only check for token presence; verify tokens in Server Components or Route Handlers.
cookies() is async in Next.js 15+. Forgetting await produces a runtime error.
Middleware matcher must be static. You cannot use runtime variables in the config.matcher array. Use conditional logic inside the middleware function instead.
redirect() inside a try/catch will be swallowed because it throws a special NEXT_REDIRECT error. Either rethrow it or call redirect() outside the try/catch.
Static pages cannot check auth at request time. Use export const dynamic = "force-dynamic" or perform auth in middleware for statically generated pages.

Alternatives

Approach	Pros	Cons
Middleware-only	Fast, runs before rendering	Cannot verify tokens against DB
Layout-based auth	Full server access, can query DB	Runs after middleware, slight latency
NextAuth.js (Auth.js)	Built-in providers, session management	Extra dependency, migration churn
Clerk or Supabase Auth	Managed service, less code	Vendor lock-in, cost at scale
Iron Session	Encrypted cookies, no JWT	Smaller community, manual setup

FAQs

Why does the middleware only check for the presence of a token instead of fully verifying it?

Middleware runs in the Edge Runtime, which cannot access databases or most Node.js APIs.
A full JWT verify with a database lookup is too heavy for the edge.
Middleware performs a coarse check (is the cookie present?), then Server Components do the full verification.

What happens if you forget to `await` the `cookies()` call in Next.js 15+?

cookies() is async in Next.js 15+ and returns a Promise.
Forgetting await means you call .get() on a Promise, which returns undefined.
This silently breaks auth checks, making protected routes appear as if no token exists.

Why is `import "server-only"` used at the top of `lib/auth.ts`?

It guarantees a compile-time error if any Client Component imports this module.
This prevents your AUTH_SECRET and JWT signing logic from leaking into the browser bundle.

Can you call `redirect()` inside a try/catch block?

redirect() throws a special NEXT_REDIRECT error internally.
If wrapped in a try/catch, the redirect is caught and swallowed, so the user is never redirected.
Either rethrow NEXT_REDIRECT errors or call redirect() outside the try/catch.

How does the `callbackUrl` pattern work in the middleware redirect?

When an unauthenticated user hits a protected path, the middleware redirects to /login.
The original pathname is appended as ?callbackUrl=/dashboard (or whatever path was requested).
After successful login, your login handler can read this param and redirect back to the original page.

What is the difference between middleware-based auth and layout-based auth?

Middleware runs before any rendering, at the edge, and can only do lightweight checks.
Layout-based auth runs on the Node.js server, can query a database, and fully verify a session.
Best practice is to combine both: middleware for a fast gate, layout for full verification.

Why does the Login Server Action set `httpOnly: true` and `secure: true` on the cookie?

httpOnly prevents JavaScript from reading the cookie, mitigating XSS attacks.
secure ensures the cookie is only sent over HTTPS in production.
sameSite: "lax" provides basic CSRF protection.

Gotcha: What happens if you use runtime variables in the middleware `config.matcher` array?

The config.matcher must be statically analyzable at build time.
Using variables or function calls causes the matcher to silently fail or throw a build error.
Put conditional logic inside the middleware function body instead.

How would you type the return value of `verifySession` in TypeScript?

import { JWTPayload } from "jose";
 
export async function verifySession(
  token: string
): Promise<JWTPayload | null> {
  try {
    const { payload } = await jwtVerify(token, secret);
    return payload;
  } catch {
    return null;
  }
}

JWTPayload from jose provides the base type with iss, sub, exp, etc.
Return null on failure so callers can do a simple truthy check.

How do you type the `children` prop in a layout component with TypeScript?

export default async function DashboardLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  // ...
}

Use React.ReactNode for the children prop, which covers elements, strings, numbers, fragments, and null.

Why can't statically generated pages check auth at request time?

Static pages are pre-rendered at build time and served as plain HTML files.
There is no server-side execution at request time to read cookies or verify sessions.
Use export const dynamic = "force-dynamic" or protect the route via middleware instead.

How does NextAuth.js (Auth.js v5) simplify the middleware setup?

You export auth as middleware directly, which handles session checks automatically.
Auth.js manages providers, session storage, and token rotation out of the box.
The trade-off is an extra dependency and keeping up with Auth.js migration changes.

Route Handlers - protect API endpoints with the same auth utilities
Environment Variables - storing AUTH_SECRET safely
Error Handling - handling auth failures gracefully
Next.js Middleware Docs

Authentication Patterns

Recipe

Working Example

Middleware Session Check

Server-Side Auth in a Layout

Auth Utility (Server-Only)

Deep Dive

How It Works

Variations

TypeScript Notes

Gotchas

Alternatives

FAQs

Authentication Patterns

Recipe

Working Example

Middleware Session Check

Server-Side Auth in a Layout

Auth Utility (Server-Only)

Deep Dive

How It Works

Variations

TypeScript Notes

Gotchas

Alternatives

FAQs

Authentication Patterns

Search Documentation

Authentication Patterns