What can run in server components in NextJS

Introduction

Server components run code on the server to prepare content before sending it to the browser. This helps make pages faster and lighter.

When you want to fetch data from a database securely without exposing credentials to the browser.

When you need to run code that uses Node.js APIs not available in the browser.

When you want to generate HTML on the server to improve page load speed.

When you want to keep sensitive logic hidden from users.

When you want to reduce JavaScript sent to the browser for better performance.

Syntax

NextJS

export default async function MyServerComponent() {
  // Server-only code here
  const data = await fetchDataFromDatabase();
  return <div>{data}</div>;
}

Server components run only on the server and never in the browser.

You can use async/await to fetch data directly inside server components.

Examples

Fetch data from an API on the server and display it.

NextJS

export default async function ServerComponent() {
  const res = await fetch('https://api.example.com/data');
  const data = await res.json();
  return <div>{data.message}</div>;
}

Use Node.js file system API to read a file on the server.

NextJS

import fs from 'fs';

export default function ReadFile() {
  const content = fs.readFileSync('file.txt', 'utf8');
  return <pre>{content}</pre>;
}

Access environment variables safely on the server.

NextJS

export default function ServerOnlyLogic() {
  const secret = process.env.SECRET_KEY;
  return <div>Secret is hidden from browser</div>;
}

Sample Program

This server component reads a text file on the server and shows its content inside a <pre> block. The file reading happens only on the server, so the browser never sees the file system code.

NextJS

import fs from 'fs';

export default function ServerComponentExample() {
  const data = fs.readFileSync('data.txt', 'utf8');
  return <main>
    <h1>Server Component Example</h1>
    <p>File content:</p>
    <pre>{data}</pre>
  </main>;
}

Important Notes

Server components cannot use browser-only APIs like window or document.

They can import and use Node.js modules freely.

Server components can call other server components or client components.

Summary

Server components run only on the server and prepare HTML before sending it to the browser.

They can safely use server-only code like database access, file system, and environment variables.

This helps improve performance and security by reducing client-side JavaScript.

Practice

1. Which of the following can you safely run inside a Next.js server component?

easy

A. Manipulating the DOM directly

B. Accessing a database directly

C. Adding event listeners for user clicks

D. Using browser APIs like localStorage

Solution

1. Step 1: Understand server component environment

   Server components run only on the server, so they can use server-only features like database access.

2. Step 2: Identify client-only features

   Browser APIs, event listeners, and DOM manipulation require client-side JavaScript and cannot run in server components.

3. Final Answer:

   Accessing a database directly -> Option B

4. Quick Check:

   Server-only code = Access database [OK]

Hint: Server components run server code only, no browser APIs [OK]

Common Mistakes:

• Thinking client APIs like localStorage work in server components
• Trying to add event listeners in server components
• Assuming DOM manipulation is possible on server

2. Which syntax correctly imports a server-only module inside a Next.js server component?

easy

A. import localStorage from 'localStorage';

B. import { useState } from 'react';

C. import window from 'window';

D. import fs from 'fs';

Solution

1. Step 1: Recognize server-only modules

   Modules like 'fs' (file system) are server-only and can be imported in server components.

2. Step 2: Identify invalid imports

   useState is client-side React hook, window and localStorage are browser globals, not importable modules.

3. Final Answer:

   import fs from 'fs'; -> Option D

4. Quick Check:

   Server module import = fs [OK]

Hint: Only import Node.js modules like fs in server components [OK]

Common Mistakes:

• Importing client hooks like useState in server components
• Trying to import browser globals as modules
• Confusing client and server imports

3. Given this server component code, what will be rendered?

import fs from 'fs';
export default function Page() {
  const data = fs.readFileSync('data.txt', 'utf8');
  return <div>{data}</div>;
}

medium

A. Contents of data.txt file inside a div

B. Error: fs module not found

C. Empty div with no content

D. Client-side error about fs usage

Solution

1. Step 1: Understand server component execution

   The code runs on the server, so fs.readFileSync reads the file content successfully.

2. Step 2: Render the file content

   The content read from 'data.txt' is returned inside a div and sent as HTML to the client.

3. Final Answer:

   Contents of data.txt file inside a div -> Option A

4. Quick Check:

   Server read file = content rendered [OK]

Hint: Server components can read files and render content directly [OK]

Common Mistakes:

• Thinking fs is unavailable in server components
• Expecting client-side errors for server code
• Assuming empty output without file content

4. What is wrong with this server component code?

export default function Page() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}

medium

A. Both A and B

B. onClick handlers are not allowed in server components

C. useState cannot be used in server components

D. No error, code works fine

Solution

1. Step 1: Check React hooks usage in server components

   useState is a client-side hook and cannot be used in server components.

2. Step 2: Check event handlers in server components

   onClick handlers require client-side JavaScript and are not supported in server components.

3. Final Answer:

   Both A and B -> Option A

4. Quick Check:

   useState + onClick invalid in server components [OK]

Hint: No React hooks or event handlers in server components [OK]

Common Mistakes:

• Using useState in server components
• Adding client event handlers in server components
• Assuming server components can handle user events

5. You want to fetch data from a database and display it in a Next.js app. Which approach best uses server components to improve performance and security?

hard

A. Fetch data in the browser using fetch API directly

B. Fetch data inside a client component using useEffect

C. Fetch data inside a server component and pass it as props to a client component

D. Fetch data in a client component and store it in localStorage

Solution

1. Step 1: Understand server component advantages

   Server components can securely fetch data on the server, avoiding exposing secrets or extra client JS.

2. Step 2: Use server component to fetch and pass data

   Fetching data in server component and passing it as props to client components optimizes performance and security.

3. Final Answer:

   Fetch data inside a server component and pass it as props to a client component -> Option C

4. Quick Check:

   Server fetch + props passing = best practice [OK]

Hint: Fetch data server-side, then pass to client components [OK]

Common Mistakes:

• Fetching data only on client causing slower load
• Exposing secrets by fetching in client
• Storing sensitive data in localStorage