Client-only modules in NextJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a client-only module in Next.js?

A client-only module is a part of your Next.js app that runs only in the browser, not on the server. It is used for code that depends on browser APIs or user interactions.

beginner

How do you mark a module as client-only in Next.js 14+?

You add the directive 'use client' at the top of the module file. This tells Next.js to load and run this module only on the client side.

intermediate

Why should you avoid using server-only modules inside client-only modules?

Server-only modules use Node.js or server APIs that don't exist in the browser. Using them in client-only modules causes errors because the browser can't run server code.

intermediate

What happens if you forget to add 'use client' in a module that uses browser-only APIs?

Next.js will try to run the module on the server, causing errors like 'window is not defined' because browser APIs don't exist on the server.

beginner

Give an example of when to use a client-only module in Next.js.

Use client-only modules for things like handling user input, animations, or accessing localStorage, which only work in the browser.

What directive do you add to a Next.js module to make it client-only?

A 'use client'

B 'use server'

C 'client-only'

D 'browser-only'

The correct directive is 'use client' placed at the top of the module.

Which of these is a reason to use a client-only module?

A To run server-side authentication

B To run database queries

C To render static HTML

D To access browser APIs like localStorage

Browser APIs like localStorage only work in client-side code.

What error might you see if you use browser APIs in a server module?

A 'database connection failed'

B 'module not found'

C 'window is not defined'

D 'syntax error'

'window is not defined' happens because the server has no window object.

Where does a client-only module run in a Next.js app?

A In the browser only

B On the server only

C Both server and browser

D In the database

Client-only modules run exclusively in the browser.

What happens if you forget 'use client' in a module that uses React hooks?

A It runs fine without any issues

B Next.js treats it as a server component and errors may occur

C The module is ignored

D The app crashes immediately

React hooks require client components; without 'use client', Next.js treats the module as server-side.

Explain what a client-only module is in Next.js and why you would use it.

Think about where the code runs and what it needs to access.

Describe the consequences of not adding 'use client' to a module that uses browser features.

What happens when server tries to run browser code?

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