What can run in server components in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What output does this Server Component produce?

Consider this Next.js Server Component code. What will it render on the page?

NextJS

export default async function Greeting() {
  const name = await Promise.resolve('Alice');
  return <h1>Hello, {name}!</h1>;
}

A <h1>Hello, {name}!</h1>

B <h1>Hello, Alice!</h1>

C Error: Cannot use await in Server Component

D <h1>Hello, undefined!</h1>

💡 Hint

Server Components can use async/await to fetch data before rendering.

Server Components can run async code and resolve promises before rendering. Here, the awaited name 'Alice' is inserted into the output.

📝 Syntax

intermediate

Which code snippet is valid inside a Next.js Server Component?

Select the code snippet that is valid and will not cause a syntax or runtime error in a Server Component.

A const data = useState('hello'); return <p>{data}</p>;

B const data = fetch('/api/data').then(res => res.json()); return <p>{data}</p>;

C const fs = require('fs'); const data = fs.readFileSync('/file.txt', 'utf8'); return <p>{data}</p>;

D import fs from 'fs'; const data = await fs.promises.readFile('/file.txt', 'utf8'); return <p>{data}</p>;

💡 Hint

Server Components support async/await and Node.js APIs but do not support hooks like useState.

Option D correctly uses import and await with fs.promises in a Server Component. Option D uses require which is not supported in ES modules. Option D does not await the promise, so data is a Promise object. Option D uses useState which is a Client Component hook and not allowed in Server Components.

🔧 Debug

advanced

Why does this Server Component cause a runtime error?

Analyze the code below. Why will this Server Component cause an error when rendering?

NextJS

import { useState } from 'react';

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

A useState is not allowed in Server Components and causes a runtime error.

B The count variable is not initialized causing a ReferenceError.

C The component is missing async keyword for Server Components.

D The onClick handler is missing parentheses causing a syntax error.

💡 Hint

Server Components cannot use React hooks that require client-side interactivity.

useState is a React hook that only works in Client Components. Using it in a Server Component causes a runtime error because Server Components do not support state or event handlers.

🧠 Conceptual

advanced

Which of these can run inside a Next.js Server Component?

Select the option that correctly describes what can run inside a Server Component.

A Event handlers like onClick and onChange for user interactions.

B Direct DOM manipulation and browser-only APIs like window or document.

C Async data fetching, Node.js APIs, and rendering JSX without client-side hooks.

D React hooks like useState and useEffect for managing state and lifecycle.

💡 Hint

Think about what runs on the server vs what runs in the browser.

Server Components run on the server and can use async/await, Node.js APIs, and render JSX. They cannot use client-side hooks or browser APIs or event handlers.

❓ state_output

expert

What is the output of this mixed Server and Client Component setup?

Given the code below, what will be rendered on the page?

NextJS

import ClientCounter from './ClientCounter';

export default async function Page() {
  const message = await Promise.resolve('Welcome');
  return (
    <>
      <h1>{message}</h1>
      <ClientCounter />
    </>
  );
}

// ClientCounter.jsx
'use client';
import { useState } from 'react';
export default function ClientCounter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}

A <h1>Welcome</h1> and a button showing 'Count: 0' that increments on click

B Error: Cannot use useState in Server Component

C <h1>Welcome</h1> and a static button showing 'Count: 0' with no interaction

D <h1>{message}</h1> and a button showing 'Count: 0'

💡 Hint

Server Components can render Client Components which handle state and events.

The Server Component renders the awaited message and includes the Client Component. The Client Component uses useState and event handlers, so the button is interactive and shows count starting at 0.

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