JWT vs session strategy in NextJS - When to Use Which

The Big Idea

Discover how to keep users logged in effortlessly and securely without reinventing the wheel!

The Scenario

Imagine building a website where users log in, and you have to remember who they are on every page they visit.

You try to do this by manually checking and storing user info on every request without any system.

The Problem

Manually tracking users is tricky and risky. You might forget to check login status, lose user info, or expose sensitive data.

It's slow and hard to keep secure, especially when many users visit at once.

The Solution

JWT and session strategies handle user identity smoothly and safely.

Sessions store user info on the server, while JWTs keep it in a secure token on the user's side.

Both make remembering users automatic and protect your app from mistakes.

Before vs After

✗ Before

if (request.cookies['user']) { user = request.cookies['user']; } else { redirectToLogin(); }

✓ After

const token = getToken(); if (verifyToken(token)) { user = decodeToken(token); } else { redirectToLogin(); }

What It Enables

It lets your app safely recognize users anytime, making login smooth and secure without extra hassle.

Real Life Example

Think of an online store where you add items to your cart, leave, and come back later without losing your cart or needing to log in again.

Key Takeaways

Manual user tracking is error-prone and insecure.

Sessions and JWTs automate and secure user identity management.

Choosing the right method improves user experience and app safety.

Practice

1. What is a key difference between JWT and session strategies in Next.js authentication?

easy

A. JWT stores user info on the client, sessions store it on the server

B. JWT requires server storage, sessions do not

C. Sessions are better for scaling across devices than JWT

D. JWT tokens expire immediately after login

Solution

1. Step 1: Understand JWT storage

   JWT stores user information inside a token on the client side, allowing stateless authentication.

2. Step 2: Understand session storage

   Sessions keep user information on the server, maintaining state and control centrally.

3. Final Answer:

   JWT stores user info on the client, sessions store it on the server -> Option A

4. Quick Check:

   Storage location difference = B [OK]

Hint: Remember: JWT = client, session = server [OK]

Common Mistakes:

• Thinking sessions store data on client
• Believing JWT requires server storage
• Confusing scaling benefits

2. Which code snippet correctly initializes a session in Next.js using a session strategy?

easy

A. const session = localStorage.getItem('session');

B. const token = jwt.sign(payload, secret);

C. import { getSession } from 'next-auth/react'; const session = await getSession();

D. import jwt from 'jsonwebtoken'; const token = jwt.verify(tokenString, secret);

Solution

1. Step 1: Identify session initialization

   Using 'getSession' from 'next-auth/react' is the correct way to get session data in Next.js.

2. Step 2: Check other options

   Options B and D relate to JWT token creation and verification, not sessions. const session = localStorage.getItem('session'); uses localStorage, which is client-side and not a session strategy.

3. Final Answer:

   import { getSession } from 'next-auth/react'; const session = await getSession(); -> Option C

4. Quick Check:

   Session retrieval uses getSession() [OK]

Hint: Sessions use getSession(), JWT uses jwt.sign() [OK]

Common Mistakes:

• Confusing JWT token code with session code
• Using localStorage as session storage
• Missing async/await with getSession

3. Given this Next.js API route using JWT, what will be the response if the token is expired?

import jwt from 'jsonwebtoken';
export default function handler(req, res) {
  try {
    const token = req.headers.authorization?.split(' ')[1];
    jwt.verify(token, process.env.JWT_SECRET);
    res.status(200).json({ message: 'Access granted' });
  } catch (err) {
    res.status(401).json({ error: 'Invalid or expired token' });
  }
}

medium

A. 200 with message 'Access granted'

B. 401 with error 'Invalid or expired token'

C. 500 server error

D. 403 forbidden without message

Solution

1. Step 1: Understand jwt.verify behavior

   If the token is expired, jwt.verify throws an error caught by the catch block.

2. Step 2: Check catch block response

   The catch block sends a 401 status with error message 'Invalid or expired token'.

3. Final Answer:

   401 with error 'Invalid or expired token' -> Option B

4. Quick Check:

   Expired token triggers 401 error [OK]

Hint: Expired JWT triggers catch block with 401 [OK]

Common Mistakes:

• Assuming expired token returns 200
• Confusing 403 and 401 status codes
• Missing try-catch around jwt.verify

4. Identify the bug in this Next.js session handling code snippet:

import { getSession } from 'next-auth/react';
export default async function handler(req, res) {
  const session = getSession();
  if (!session) {
    res.status(401).json({ error: 'Not authenticated' });
  } else {
    res.status(200).json({ message: 'Welcome!' });
  }
}

medium

A. Missing await before getSession()

B. Using getSession() instead of getServerSession()

C. No error handling for session retrieval

D. Incorrect status code for authenticated user

Solution

1. Step 1: Check the context

   This is a Next.js API route (server-side).

2. Step 2: Identify correct function for server-side

   While getServerSession() is recommended for server-side session retrieval, the immediate bug in the code is missing await before the async getSession() call, causing session to be a Promise instead of resolved value.

3. Final Answer:

   Missing await before getSession() -> Option A

4. Quick Check:

   Async function requires await to get session value [OK]

Hint: Async calls need await [OK]

Common Mistakes:

• Forgetting await on async functions
• Confusing getSession and getServerSession
• Ignoring promise returned by getSession

5. You want to build a Next.js app that supports multiple devices per user and scales easily without server state. Which strategy fits best and why?

hard

A. Use sessions because they store data on the server for better control

B. Use sessions with database storage for multi-device support

C. Use JWT but store tokens only on the server

D. Use JWT because tokens store user info on client, enabling stateless scaling

Solution

1. Step 1: Analyze multi-device and scaling needs

   Supporting multiple devices and easy scaling requires stateless authentication without server session storage.

2. Step 2: Match strategy to needs

   JWT stores user info in tokens on the client, allowing stateless, scalable authentication across devices.

3. Final Answer:

   Use JWT because tokens store user info on client, enabling stateless scaling -> Option D

4. Quick Check:

   Stateless multi-device = JWT [OK]

Hint: Stateless multi-device apps use JWT [OK]

Common Mistakes:

• Choosing sessions for stateless scaling
• Thinking JWT tokens must be stored on server
• Assuming sessions easily scale without extra setup