Use server directive in NextJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the use server directive do in Next.js?

It marks a function or component to run only on the server side, ensuring it does not run in the browser. This helps with security and performance.

beginner

Where should you place the use server directive in your Next.js code?

At the very top of the file or function before any other code, as a special directive line: 'use server'.

intermediate

Why is it important to use the use server directive for server-only functions?

It prevents server-only code from being bundled and sent to the browser, reducing bundle size and avoiding exposing sensitive logic.

beginner

Can a component marked with use server access browser APIs like window?

No. Components or functions with use server run only on the server and do not have access to browser-specific APIs.

intermediate

How does use server relate to React Server Components in Next.js?

It explicitly marks functions or components as server-only, which is a key part of React Server Components to separate server and client logic.

What is the correct way to mark a function as server-only in Next.js?

A import server from 'next/server'

B 'use server' at the top of the file

C Add a comment // server-only

D Use <ServerOnly> component wrapper

The 'use server' directive at the top of the file or function marks it as server-only.

What happens if you do NOT use 'use server' for a server-only function?

A The function might be bundled and sent to the browser

B The function runs only on the server anyway

C The function will run twice on server and client

D Next.js throws a compile error

Without 'use server', Next.js may include the function in client bundles, exposing server logic.

Can a 'use server' function use browser APIs like localStorage?

A Yes, always

B Only if wrapped in useEffect

C No, it runs only on the server

D Only in development mode

'use server' functions run only on the server and cannot access browser APIs.

Where should the 'use server' directive be placed in a file?

A At the very top before any code

B After imports

C Anywhere inside the function

D Inside the component return

'use server' must be at the top of the file or function before any other code.

What is a benefit of using 'use server' in Next.js?

A Improves client-side rendering speed

B Allows server-only code to run in the browser

C Automatically converts code to TypeScript

D Reduces client bundle size and hides server logic

'use server' helps keep server code off the client, reducing bundle size and improving security.

Explain what the 'use server' directive does in Next.js and why it is important.

Think about where your code runs and what you want to hide from the browser.

Describe how to correctly use the 'use server' directive in a Next.js component or function.

It's a special comment line that must come first.

Practice

1. What is the main purpose of the use server directive in Next.js?

easy

A. To enable client-side rendering of components.

B. To import CSS stylesheets into components.

C. To mark functions that should only run on the server for security and performance.

D. To declare global variables accessible on both client and server.

Solution

1. Step 1: Understand the role of use server

   The directive marks functions to run only on the server side, not on the client.

2. Step 2: Recognize benefits

   This separation helps keep sensitive code secure and improves app performance by avoiding client execution.

3. Final Answer:

   To mark functions that should only run on the server for security and performance. -> Option C

4. Quick Check:

   use server = server-only functions [OK]

Hint: Remember: use server means run code only on server [OK]

Common Mistakes:

• Thinking it enables client-side rendering
• Confusing it with CSS imports
• Assuming it shares variables globally

2. Which of the following is the correct way to declare a server-only function using the use server directive in Next.js?

easy

A. 'use server'; function fetchData() { return 'data'; }

B. "use server" function fetchData() { return 'data'; }

C. use server; function fetchData() { return 'data'; }

D. use server() function fetchData() { return 'data'; }

Solution

1. Step 1: Identify correct directive syntax

   The use server directive must be a string literal: 'use server';

2. Step 2: Check function declaration

   The function follows normal JavaScript syntax after the directive.

3. Final Answer:

   'use server'; function fetchData() { return 'data'; } -> Option A

4. Quick Check:

   Directive as string literal = correct syntax [OK]

Hint: Use quotes around use server directive [OK]

Common Mistakes:

• Omitting quotes around use server
• Using semicolon incorrectly
• Trying to call use server as a function

3. Given the code below, what will be the output when the component renders?

"use client";
import { useState } from 'react';

export default function Page() {
  const [count, setCount] = useState(0);

  async function increment() {
    'use server';
    // server-only logic
    return count + 1;
  }

  return  setCount(count + 1)}>Count: {count};
}

medium

A. Button shows count starting at 0 and increments on click.

B. Error because server function is called inside client component.

C. Button shows count but does not update on click.

D. Button shows count starting at 1 immediately.

Solution

1. Step 1: Identify client and server code

   The component uses "use client" so it runs on client. The increment function is marked 'use server' but is not called in this code.

2. Step 2: Understand state update

   The button's onClick updates state locally with setCount(count + 1), so count increments on click.

3. Final Answer:

   Button shows count starting at 0 and increments on click. -> Option A

4. Quick Check:

   Client state updates on click = Button shows count starting at 0 and increments on click. [OK]

Hint: Server functions don't run unless explicitly called [OK]

Common Mistakes:

• Assuming server function runs automatically
• Expecting error from server function inside client
• Thinking count starts at 1

4. What is wrong with the following Next.js server function?

'use server'
export async function getData() {
  const res = await fetch('/api/data');
  return res.json();
}

medium

A. Missing semicolon after 'use server' directive.

B. The fetch URL should be absolute or use Next.js fetch options.

C. Cannot use fetch inside server functions.

D. Async functions cannot be marked with 'use server'.

Solution

1. Step 1: Check directive syntax

   Missing semicolon is not a syntax error in JavaScript, so not critical here.

2. Step 2: Analyze fetch usage

   In server functions, fetch should use absolute URLs or Next.js fetch options to avoid errors.

3. Final Answer:

   The fetch URL should be absolute or use Next.js fetch options. -> Option B

4. Quick Check:

   Server fetch needs absolute URL or options [OK]

Hint: Use absolute URLs in server fetch calls [OK]

Common Mistakes:

• Thinking semicolon is mandatory after directive
• Believing fetch is disallowed on server
• Assuming async can't be used with use server

5. You want to create a Next.js API route that uses a server-only function to fetch user data securely. Which approach correctly uses the use server directive to keep the fetch logic server-side while exposing only safe data to the client?

hard

A. Fetch data in client components and mark the fetch function with 'use server' to secure it.

B. Mark the API route file with 'use server' and call fetch directly in client components.

C. Use 'use server' in client components to fetch data and pass it to API routes.

D. Declare 'use server' inside the API route handler and call fetch there, then return safe data.

Solution

1. Step 1: Understand server-only logic placement

   The server-only fetch logic should be inside the API route handler marked with 'use server' to keep it secure.

2. Step 2: Avoid client-side fetch for sensitive data

   Client components should not fetch sensitive data directly or use 'use server' since it only works server-side.

3. Final Answer:

   Declare 'use server' inside the API route handler and call fetch there, then return safe data. -> Option D

4. Quick Check:

   Server fetch in API route = secure data [OK]

Hint: Keep fetch in API route with use server [OK]

Common Mistakes:

• Trying to use use server in client components
• Fetching sensitive data directly on client
• Marking entire API route file incorrectly