Client-only modules in NextJS - Performance & Optimization

Performance: Client-only modules

MEDIUM IMPACT

This affects page load speed by deferring non-essential JavaScript to client side, improving initial server render and reducing blocking time.

Importing a module that only works in the browser in a Next.js page

NextJS

import dynamic from 'next/dynamic';

const BrowserOnlyComponent = dynamic(() => import('browser-only-module'), { ssr: false });

export default function Page() {
  return <BrowserOnlyComponent />;
}

Defers loading to client only, skipping server render and reducing initial bundle size.

📈 Performance GainReduces server render blocking, saves 50-100kb on initial bundle, improves LCP by 200-400ms

Importing a module that only works in the browser in a Next.js page

NextJS

import BrowserOnlyComponent from 'browser-only-module';

export default function Page() {
  return <BrowserOnlyComponent />;
}

This imports the module during server-side rendering, causing errors or large bundles and blocking initial render.

📉 Performance CostBlocks server render, increases bundle size by 50-100kb, delays LCP by 200-400ms

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Import client-only module normally	N/A on server, large bundle on client	Multiple reflows if module triggers layout	High paint cost due to blocking JS	[X] Bad
Dynamic import with ssr: false	Minimal server DOM, client loads later	Single reflow after load	Lower paint cost, non-blocking	[OK] Good

Rendering Pipeline

Client-only modules are skipped during server rendering, so the server sends minimal HTML. The browser loads these modules asynchronously after initial paint, avoiding blocking style calculation and layout.

→Server Rendering

→JavaScript Parsing

→Layout

→Paint

⚠️ BottleneckServer Rendering and JavaScript Parsing

Core Web Vital Affected

LCP

This affects page load speed by deferring non-essential JavaScript to client side, improving initial server render and reducing blocking time.

Optimization Tips

1Always use dynamic imports with ssr: false for modules that only run in the browser.

2Avoid importing client-only code directly in server-rendered components to prevent blocking.

3Check bundle size and server render time to ensure client-only modules are deferred.

Performance Quiz - 3 Questions

What is the main benefit of using dynamic imports with ssr: false for client-only modules in Next.js?

A It preloads the module before the page loads to avoid delays.

B It bundles the module into the server output for faster server rendering.

C It defers loading to client, reducing server render blocking and improving LCP.

D It disables JavaScript entirely for faster page load.

DevTools: Performance

How to check: Record page load, look for long tasks blocking main thread during initial load and check when client-only module scripts are loaded.

What to look for: Look for reduced blocking time and earlier LCP when using dynamic imports with SSR disabled.

Practice

1. What is the main reason to use client-only modules in Next.js?

easy

A. They run only in the browser, avoiding server errors with browser APIs.

B. They improve server-side rendering speed by running on the server.

C. They allow using Node.js modules on the server.

D. They enable static site generation without JavaScript.

Solution

1. Step 1: Understand client-only module purpose

   Client-only modules run only in the browser to avoid errors from browser-specific APIs on the server.

2. Step 2: Compare options

   They run only in the browser, avoiding server errors with browser APIs. correctly states this purpose. Options B, C, and D describe unrelated or incorrect behaviors.

3. Final Answer:

   They run only in the browser, avoiding server errors with browser APIs. -> Option A

4. Quick Check:

   Client-only modules = browser-only code [OK]

Hint: Client-only means browser-only to avoid server API errors [OK]

Common Mistakes:

• Thinking client-only modules run on the server
• Confusing client-only with static generation
• Assuming client-only modules improve server speed

2. Which syntax correctly imports a client-only module named MyComponent using Next.js dynamic import?

easy

A. const MyComponent = require('./MyComponent');

B. import dynamic from 'next/dynamic'; const MyComponent = dynamic(() => import('./MyComponent'), { ssr: false });

C. import MyComponent from './MyComponent';

D. import dynamic from 'next/dynamic'; const MyComponent = dynamic(() => import('./MyComponent'));

Solution

1. Step 1: Recall dynamic import with ssr: false

   To load client-only modules, Next.js requires dynamic import with { ssr: false } to disable server-side rendering.

2. Step 2: Analyze options

   import dynamic from 'next/dynamic'; const MyComponent = dynamic(() => import('./MyComponent'), { ssr: false }); uses dynamic import with ssr: false correctly. import MyComponent from './MyComponent'; is a normal import (runs on server too). const MyComponent = require('./MyComponent'); uses require, not recommended in Next.js. import dynamic from 'next/dynamic'; const MyComponent = dynamic(() => import('./MyComponent')); misses ssr: false, so it runs on server.

3. Final Answer:

   import dynamic from 'next/dynamic'; const MyComponent = dynamic(() => import('./MyComponent'), { ssr: false }); -> Option B

4. Quick Check:

   Use dynamic + ssr: false for client-only imports [OK]

Hint: Use dynamic import with ssr: false for client-only modules [OK]

Common Mistakes:

• Forgetting { ssr: false } disables server rendering
• Using normal import for client-only modules
• Using require instead of import

3. Given this code snippet in Next.js, what will be the rendered output?

import dynamic from 'next/dynamic';
const ClientOnly = dynamic(() => import('./ClientOnlyComponent'), { ssr: false });

export default function Page() {
  return (
    

      
Welcome
      
    
  );
}

Assuming ClientOnlyComponent renders <p>Loaded on client</p>.

medium

A. WelcomeLoaded on client

B. Welcome

C. WelcomeLoading...

D. Error: ClientOnlyComponent cannot be rendered on server

Solution

1. Step 1: Understand dynamic import with ssr: false

   The component ClientOnly loads only on the client, so server renders without it initially, showing a loading placeholder.

2. Step 2: Predict final render output

   Before hydration, the placeholder Loading... is shown. After hydration, ClientOnlyComponent renders <p>Loaded on client</p>. The initial render output includes the heading and loading placeholder.

3. Final Answer:

   <div><h1>Welcome</h1><p>Loading...</p></div> -> Option C

4. Quick Check:

   Client-only modules show loading placeholder on server render [OK]

Hint: Client-only components show loading placeholder before hydration [OK]

Common Mistakes:

• Expecting client-only content on server render
• Assuming error occurs without server rendering
• Thinking loading placeholder always shows

4. You wrote this code to import a client-only module:

import dynamic from 'next/dynamic';
const Widget = dynamic(() => import('./Widget'));

export default function Home() {
  return ;
}

What is the main problem here?

medium

A. Missing { ssr: false } causes server to try rendering Widget, which may fail.

B. Dynamic import syntax is incorrect; should use require instead.

C. Widget component must be imported normally, not dynamically.

D. You must wrap Widget in a Suspense component for dynamic imports.

Solution

1. Step 1: Check dynamic import options

   The code uses dynamic import but omits { ssr: false }, so Next.js tries to render Widget on the server.

2. Step 2: Understand consequences

   If Widget uses browser-only APIs, server rendering will fail or cause errors. Adding { ssr: false } prevents server rendering.

3. Final Answer:

   Missing { ssr: false } causes server to try rendering Widget, which may fail. -> Option A

4. Quick Check:

   Always add ssr: false for client-only modules [OK]

Hint: Add { ssr: false } to disable server rendering for client-only modules [OK]

Common Mistakes:

• Forgetting ssr: false option
• Thinking dynamic import syntax is wrong without require
• Assuming Suspense is mandatory for dynamic imports

5. You want to use a browser-only API inside a Next.js component, but only on the client side. Which approach correctly ensures this without server errors?

import dynamic from 'next/dynamic';

const BrowserOnly = dynamic(() => import('./BrowserOnlyComponent'), { ssr: false });

export default function Page() {
  return (
    

      
Client-only Example
      
    
  );
}

What else should you do inside BrowserOnlyComponent to safely use browser APIs?

hard

A. Wrap browser API calls inside a try-catch in the component body.

B. Call browser APIs directly in the component body without hooks.

C. Use getServerSideProps to fetch browser API data on server.

D. Use React's useEffect hook to run browser API code after component mounts.

Solution

1. Step 1: Understand client-only import

   Dynamic import with { ssr: false } ensures the component loads only on the client, avoiding server errors.

2. Step 2: Use useEffect for browser APIs

   Inside the component, browser APIs should be called inside useEffect to run after mounting, ensuring the DOM and window objects exist.

3. Final Answer:

   Use React's useEffect hook to run browser API code after component mounts. -> Option D

4. Quick Check:

   Browser APIs inside useEffect = safe client use [OK]

Hint: Put browser API calls inside useEffect to run after mount [OK]

Common Mistakes:

• Calling browser APIs directly in render causing errors
• Trying to use browser APIs on server via getServerSideProps
• Relying on try-catch instead of proper lifecycle hooks