Why authentication matters in NextJS - Performance Evidence

Performance: Why authentication matters

MEDIUM IMPACT

Authentication affects initial page load speed and interaction responsiveness by controlling access to content and resources.

Protecting user data and controlling access in a Next.js app

NextJS

import { cookies } from 'next/headers';
export default async function Page() {
  const cookieStore = cookies();
  const token = cookieStore.get('authToken')?.value;
  if (!token) {
    return <div>Please login</div>;
  }
  const response = await fetch('http://localhost:3000/api/user', {
    headers: {
      Authorization: `Bearer ${token}`,
    },
  });
  const user = await response.json();
  return <div>Welcome {user.name}</div>;
}

Server-side authentication check prevents unnecessary client fetches and renders content only for authenticated users.

📈 Performance GainImproves LCP by rendering content immediately; reduces INP by avoiding extra client fetches

Protecting user data and controlling access in a Next.js app

NextJS

'use client';
import { useState, useEffect } from 'react';

export default function Page() {
  const [user, setUser] = useState(null);

  useEffect(() => {
    fetch('/api/user').then(res => res.json()).then(setUser);
  }, []);

  if (!user) {
    return <div>Please login</div>;
  }
  return <div>Welcome {user.name}</div>;
}

Fetching user data on the client side delays content rendering and exposes protected data before authentication is confirmed.

📉 Performance CostBlocks LCP until user data is fetched; increases INP due to delayed interaction readiness

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Client-side auth fetch	More DOM nodes due to delayed rendering	Multiple reflows as content updates	Higher paint cost due to delayed content	[X] Bad
Server-side auth check	Minimal DOM nodes initially	Single reflow on initial render	Lower paint cost with immediate content	[OK] Good

Rendering Pipeline

Authentication affects the rendering pipeline by determining what content is rendered and when, impacting server-side rendering and client hydration.

→Server-side Rendering

→Data Fetching

→Hydration

→Interaction Readiness

⚠️ BottleneckData Fetching and Authentication Validation

Core Web Vital Affected

LCP, INP

Authentication affects initial page load speed and interaction responsiveness by controlling access to content and resources.

Optimization Tips

1Perform authentication checks on the server to speed up initial content rendering.

2Avoid client-side user data fetching before rendering protected content.

3Use authentication to control resource loading and reduce unnecessary data transfer.

Performance Quiz - 3 Questions

How does client-side authentication fetching affect Largest Contentful Paint (LCP)?

A It improves LCP by loading data asynchronously.

B It delays LCP because content waits for user data fetch.

C It has no effect on LCP.

D It speeds up LCP by caching user data.

DevTools: Performance

How to check: Record a page load with authentication; look for delays in LCP and long tasks during data fetching.

What to look for: Check if main content appears quickly and if there are long blocking tasks caused by client-side auth fetches.

Practice

1. Why is authentication important in a Next.js application?

easy

A. It automatically fixes bugs in the code.

B. It speeds up the loading time of pages.

C. It confirms the identity of users and protects private data.

D. It changes the app's color scheme based on user preference.

Solution

1. Step 1: Understand the purpose of authentication

   Authentication is used to confirm who a user is when they access an app.

2. Step 2: Recognize the importance of protecting data

   It helps protect private or sensitive data by allowing only authorized users to see it.

3. Final Answer:

   It confirms the identity of users and protects private data. -> Option C

4. Quick Check:

   Authentication = Confirm identity and protect data [OK]

Hint: Authentication means confirming who the user is [OK]

Common Mistakes:

• Confusing authentication with app speed
• Thinking authentication changes UI colors
• Believing authentication fixes code bugs

2. Which of the following is the correct way to import the NextAuth library in a Next.js app?

easy

A. require('next-auth');

B. include 'next-auth';

C. import nextAuth from 'next-auth';

D. import NextAuth from 'next-auth';

Solution

1. Step 1: Identify the ES module import syntax

   Next.js uses ES module syntax with import to load libraries.

2. Step 2: Match the correct import statement

   The correct import is import NextAuth from 'next-auth'; with exact casing and syntax.

3. Final Answer:

   import NextAuth from 'next-auth'; -> Option D

4. Quick Check:

   Use ES module import syntax for NextAuth [OK]

Hint: Use ES module import, not require or include [OK]

Common Mistakes:

• Using CommonJS require instead of import
• Wrong casing in import statement
• Using include which is not valid in JS

3. Given this Next.js code snippet using next-auth, what will be rendered if the user is not signed in?

import { useSession } from 'next-auth/react';

export default function Profile() {
  const { data: session } = useSession();
  if (!session) {
    return <p>Please sign in to view your profile.</p>;
  }
  return <p>Welcome, {session.user.name}!</p>;
}

medium

A. Please sign in to view your profile.

B. Loading user data...

C. Welcome, [user's name]!

D. Error: session not found

Solution

1. Step 1: Check the session state when user is not signed in

   If the user is not signed in, session will be null or undefined.

2. Step 2: Follow the conditional rendering logic

   The code returns the message <p>Please sign in to view your profile.</p> when !session is true.

3. Final Answer:

   Please sign in to view your profile. -> Option A

4. Quick Check:

   Not signed in = show sign-in prompt [OK]

Hint: If no session, show sign-in message [OK]

Common Mistakes:

• Assuming user name shows without sign-in
• Expecting loading text instead of sign-in prompt
• Thinking an error will be thrown

4. What is wrong with this Next.js authentication check?

import { useSession } from 'next-auth/react';

export default function Dashboard() {
  const session = useSession();
  if (!session) {
    return <p>Access denied.</p>;
  }
  return <p>Dashboard content</p>;
}

medium

A. The session variable should be declared with var.

B. useSession must be destructured to get data property.

C. The component should be a class component.

D. The return statements should be inside useEffect.

Solution

1. Step 1: Check how useSession is used

   useSession returns an object with a data property containing the session info.

2. Step 2: Identify the correct destructuring

   The code should use const { data: session } = useSession(); to get the session data.

3. Final Answer:

   useSession must be destructured to get data property. -> Option B

4. Quick Check:

   Destructure useSession to access session data [OK]

Hint: Destructure useSession to get session data [OK]

Common Mistakes:

• Using useSession without destructuring
• Trying to put return inside useEffect
• Thinking class components are required
• Using var instead of const or let

5. You want to protect a Next.js page so only signed-in users can access it. Which approach correctly enforces this using next-auth?

hard

A. Use getServerSideProps to check session and redirect if not signed in.

B. Render the page normally and hide content with CSS if user is not signed in.

C. Use a client-side setTimeout to check session after page loads.

D. Allow all users to access and show an alert if not signed in.

Solution

1. Step 1: Understand server-side protection

   Using getServerSideProps allows checking the session before rendering the page.

2. Step 2: Redirect unauthorized users

   If no session is found, redirecting to sign-in page prevents unauthorized access securely.

3. Final Answer:

   Use getServerSideProps to check session and redirect if not signed in. -> Option A

4. Quick Check:

   Server-side session check = secure page protection [OK]

Hint: Check session server-side to protect pages [OK]

Common Mistakes:

• Hiding content with CSS does not secure data
• Using client-side delay risks exposing content
• Allowing access and just showing alerts is insecure