Use server directive in NextJS

Introduction

The use server directive tells Next.js that a function runs only on the server. This helps keep your app secure and fast by separating server code from client code.

When you want to write a function that fetches data securely on the server.

When you need to handle sensitive operations like database updates or authentication.

When you want to keep your client bundle small by not sending server-only code to the browser.

When you want to use server-only APIs or environment variables safely.

When you want to create server actions that can be called from client components.

Syntax

NextJS

"use server";

export async function myServerFunction() {
  // server-only code here
}

The directive "use server"; must be the very first line in the file or function.

Functions marked with use server can be imported and called from client components as Server Actions.

Examples

This function fetches user data only on the server.

NextJS

"use server";

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

This function runs only on the server to update the database safely.

NextJS

"use server";

export function updateDatabase() {
  // code to update database
}

Sample Program

This example shows a server function getServerTime that returns the current time as a string. The client component calls this server function when the button is clicked and shows the time.

NextJS

"use server";

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

// In a client component
'use client';
import { useState } from 'react';
import { getServerTime } from './serverFunctions';

export default function ShowServerTime() {
  const [time, setTime] = useState('');

  async function fetchTime() {
    const serverTime = await getServerTime();
    setTime(serverTime);
  }

  return (
    <div>
      <button onClick={fetchTime} aria-label="Get server time">Get Server Time</button>
      {time && <p>Server time is: {time}</p>}
    </div>
  );
}

Important Notes

Remember to keep use server functions free of client-only code like DOM access.

Server functions can be called from client components using Next.js server actions or API routes.

Summary

use server marks functions to run only on the server.

It helps keep sensitive code secure and improves app performance.

Use it when you want to separate server logic from client logic clearly.

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