Server-only modules in NextJS

Introduction

Server-only modules let you run code only on the server, keeping it safe and hidden from users. This helps protect secrets and improve performance.

When you need to keep API keys or database passwords secret.

When you want to run code that should never be sent to the browser.

When fetching data from a database or external service securely.

When performing server-side calculations or processing.

When you want to reduce the amount of code sent to the client.

Syntax

NextJS

import 'server-only';

// Your server-only code here
export async function getServerData() {
  // fetch or process data
  const data = {}; // example placeholder
  return data;
}

Use import 'server-only'; at the top to mark the module as server-only.

Server-only modules cannot be imported into client components.

Examples

This module safely returns a secret key only on the server.

NextJS

import 'server-only';

export async function getSecret() {
  return process.env.SECRET_KEY;
}

This module fetches data from an API only on the server side.

NextJS

import 'server-only';

export async function fetchData() {
  const res = await fetch('https://api.example.com/data');
  return res.json();
}

Sample Program

This server-only module exports a function that returns the current server time as a string. It runs only on the server, so the time is always accurate and hidden from the client.

NextJS

import 'server-only';

export async function getServerTime() {
  return new Date().toISOString();
}

Important Notes

Server-only modules help keep sensitive data safe by never sending it to the browser.

Trying to import server-only modules into client components will cause errors.

Use server-only modules for backend logic, database calls, and secret management.

Summary

Server-only modules run only on the server and are hidden from the browser.

They protect secrets and improve app security.

Mark them with import 'server-only'; at the top.

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