Server-only modules in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Next.js server component importing a server-only module?

Consider a Next.js server component that imports a server-only module to fetch data. What will be rendered on the client?

NextJS

import getData from './server-only-module';

export default async function Page() {
  const data = await getData();
  return <div>{data.message}</div>;
}

// server-only-module.js
import 'server-only';
export default async function getData() {
  return { message: 'Hello from server!' };
}

A <div>Hello from server!</div>

B <div>[object Promise]</div>

C Error: Module not found on client

D <div>undefined</div>

💡 Hint

Think about where server-only modules run and how data is fetched before rendering.

The server-only module runs on the server, so the data is fetched before rendering. The component renders the message correctly as "Hello from server!" on the client.

📝 Syntax

intermediate

Which import statement correctly imports a server-only module in Next.js?

Given a server-only module at lib/server-utils.js, which import statement is valid in a server component?

A import utils from 'lib/server-utils';

B import utils from './server-utils';

C import utils from '../lib/server-utils';

D import utils from '/lib/server-utils';

💡 Hint

Consider relative paths and module resolution in Next.js.

Using a relative path like ../lib/server-utils correctly imports the module from the parent directory's lib folder. Absolute or root-based imports without configuration will cause errors.

🔧 Debug

advanced

Why does this Next.js server component fail to compile when importing a server-only module in a client component?

Examine the code below. Why does it cause a build error?

NextJS

// client-component.jsx
'use client';
import secret from '../lib/server-secret';

export default function ClientComponent() {
  return <div>{secret}</div>;
}

// lib/server-secret.js
import 'server-only';
const secret = 'top-secret';
export default secret;

A Client components cannot import server-only modules, causing a build error.

B The 'use client' directive is missing in the server-secret module.

C The import path '../lib/server-secret' is invalid.

D The secret variable is not exported correctly.

💡 Hint

Think about the difference between client and server components and module usage.

Client components cannot import server-only modules because server-only modules may use Node.js APIs unavailable in the browser. This causes a build error.

❓ state_output

advanced

What is the value of 'result' after running this Next.js server component code?

Given the server-only module and server component below, what is the value of result?

NextJS

// server-only-module.js
import 'server-only';
export function compute() {
  return 42;
}

// page.jsx
import { compute } from './server-only-module';

export default function Page() {
  const result = compute();
  return <div>{result}</div>;
}

A undefined

B 42

C Error: compute is not a function

D null

💡 Hint

Consider how server-only modules export functions and how they are used in server components.

The compute function returns 42. The server component calls it and assigns the result to result. So result is 42.

🧠 Conceptual

expert

Which statement best describes the behavior of server-only modules in Next.js?

Choose the most accurate description of server-only modules in Next.js.

A Server-only modules are deprecated and replaced by client components in Next.js.

B Server-only modules can be imported by both client and server components without restrictions.

C Server-only modules run on the client but fetch data from the server asynchronously.

D Server-only modules run exclusively on the server and cannot be imported by client components.

💡 Hint

Think about where server-only modules execute and their import limitations.

Server-only modules run only on the server and cannot be imported by client components because they may use Node.js APIs unavailable in browsers.

Practice

1. What is the main purpose of server-only modules in Next.js?

easy

A. To enable server code to be bundled with client JavaScript

B. To run code on both server and client for faster rendering

C. To allow client-side code to access server secrets directly

D. To run code only on the server and keep it hidden from the browser

Solution

1. Step 1: Understand server-only module purpose

   Server-only modules run exclusively on the server and are not sent to the browser.

2. Step 2: Identify security benefit

   This keeps secrets safe and improves app security by hiding sensitive code from clients.

3. Final Answer:

   To run code only on the server and keep it hidden from the browser -> Option D

4. Quick Check:

   Server-only modules hide code from browser = A [OK]

Hint: Server-only means code runs only on server, hidden from browser [OK]

Common Mistakes:

• Thinking server-only modules run on client too
• Assuming server-only modules expose secrets to browser
• Confusing server-only with shared server-client code

2. Which line correctly marks a Next.js module as server-only?

easy

A. export const client = true;

B. export const runtime = 'edge';

C. import 'server-only';

D. export default function ServerOnly() {}

Solution

1. Step 1: Recall server-only marking syntax

   Next.js uses import 'server-only'; to mark server-only modules.

2. Step 2: Compare options

   import 'server-only'; matches the correct syntax; others either mark edge runtime or client code.

3. Final Answer:

   import 'server-only'; -> Option C

4. Quick Check:

   Server-only marker = import 'server-only' [OK]

Hint: Look for import 'server-only' to mark server-only [OK]

Common Mistakes:

• Using 'edge' runtime which is for edge functions, not server-only
• Confusing client export with server-only marking
• Missing the import keyword

3. Given this server-only module code in Next.js, what will happen if you try to import it in a client component?

import 'server-only';

export function secretData() {
  return 'top-secret';
}

medium

A. The client component will fail to compile with an error

B. The client component will receive the secret data as a string

C. The secretData function will run on the client without errors

D. The secretData function will be bundled into client JavaScript

Solution

1. Step 1: Understand server-only import restrictions

   Server-only modules cannot be imported into client components; Next.js blocks this to protect secrets.

2. Step 2: Predict behavior on import

   Trying to import causes a compile-time error because the module is not allowed in client code.

3. Final Answer:

   The client component will fail to compile with an error -> Option A

4. Quick Check:

   Importing server-only in client causes error = D [OK]

Hint: Server-only modules cannot be imported by client components [OK]

Common Mistakes:

• Assuming server-only code runs on client
• Thinking secret data is exposed to client
• Believing server-only modules are bundled with client code

4. You wrote this server-only module but get an error when running your Next.js app:

export function getSecret() {
  return 'hidden';
}

What is missing to make this module server-only?

medium

A. You need to add import 'server-only'; at the top

B. You must rename the function to start with 'server'

C. You should import React in this module

D. You need to export the function as default

Solution

1. Step 1: Identify server-only module requirement

   Next.js requires import 'server-only'; to mark a module as server-only.

2. Step 2: Check given code

   The code lacks this import, so Next.js treats it as shared or client code, causing errors if used improperly.

3. Final Answer:

   You need to add import 'server-only'; at the top -> Option A

4. Quick Check:

   Missing 'server-only' import causes error = A [OK]

Hint: Always add import 'server-only'; to mark server-only [OK]

Common Mistakes:

• Thinking function name controls server-only status
• Assuming React import is needed for server-only modules
• Believing default export affects server-only behavior

5. You want to fetch a secret API key only on the server in Next.js and prevent it from being sent to the client. Which approach correctly uses server-only modules to achieve this?

// secret.js
import 'server-only';
export function getApiKey() {
  return process.env.API_KEY;
}

// page.jsx
'use client';
import { getApiKey } from './secret';

export default function Page() {
  const key = getApiKey();
  return <p>API Key: {key}</p>;
}

hard

A. This code will cause an error because getApiKey is called in a client component

B. You should move getApiKey call inside a server component or server action

C. This code works and safely shows the API key on the client

D. You must export getApiKey as default to work

Solution

1. Step 1: Understand server-only module usage

   Server-only modules cannot be imported or called directly in client components without error.

2. Step 2: Analyze the code

   The page.jsx is a client component calling getApiKey(), which is server-only, causing an error.

3. Step 3: Correct approach

   Call getApiKey() inside a server component or server action to keep the secret on the server.

4. Final Answer:

   You should move getApiKey call inside a server component or server action -> Option B

5. Quick Check:

   Server-only calls must be inside server components = B [OK]

Hint: Call server-only functions only inside server components or actions [OK]

Common Mistakes:

• Calling server-only functions directly in client components
• Exposing secrets by rendering them in client JSX
• Assuming default export fixes server-only usage