Response modification in NextJS

Introduction

Response modification lets you change what the server sends back to the browser. This helps you add headers, change status codes, or modify the content before the user sees it.

You want to add security headers like Content-Security-Policy to protect your site.

You need to set cookies or clear cookies in the response.

You want to change the status code to show errors or redirects.

You want to modify the response body before sending it to the client.

You want to add caching headers to improve performance.

Syntax

NextJS

export async function GET(request) {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();

  return new Response(JSON.stringify(data), {
    status: 200,
    headers: {
      'Content-Type': 'application/json',
      'Cache-Control': 'max-age=3600'
    }
  });
}

You create a new Response object to modify what is sent back.

Headers, status, and body can all be changed in the Response constructor.

Examples

Simple text response with status 200 and plain text content type.

NextJS

export async function GET() {
  return new Response('Hello World', {
    status: 200,
    headers: { 'Content-Type': 'text/plain' }
  });
}

Response with 404 status and custom status text, no body.

NextJS

export async function GET() {
  return new Response(null, {
    status: 404,
    statusText: 'Not Found'
  });
}

Sets a cookie in the response headers.

NextJS

export async function GET() {
  const headers = new Headers();
  headers.set('Set-Cookie', 'user=123; Path=/; HttpOnly');

  return new Response('Cookie set', {
    status: 200,
    headers
  });
}

Sample Program

This Next.js API route fetches data from an external API, then modifies the response by adding a custom header before sending it back to the client.

NextJS

export async function GET() {
  // Fetch data from external API
  const response = await fetch('https://jsonplaceholder.typicode.com/todos/1');
  const data = await response.json();

  // Modify response: add custom header and change status
  return new Response(JSON.stringify(data), {
    status: 200,
    headers: {
      'Content-Type': 'application/json',
      'X-Custom-Header': 'MyHeaderValue'
    }
  });
}

Important Notes

Always set the correct Content-Type header to match your response body.

You can create and modify Headers objects for more control.

Remember to handle errors when fetching or modifying responses.

Summary

Response modification lets you control what the server sends back.

You can change headers, status codes, and the response body.

This helps improve security, caching, and user experience.

Practice

1. In Next.js, what does modifying the response headers allow you to do?

easy

A. Change the React component state on the client

B. Control caching and security policies sent to the browser

C. Modify the URL path of the current page

D. Update the database directly from the response

Solution

1. Step 1: Understand response headers role

   Response headers tell the browser how to handle the data, like caching or security rules.

2. Step 2: Identify what modifying headers affects

   Changing headers controls browser behavior, not client state or URLs.

3. Final Answer:

   Control caching and security policies sent to the browser -> Option B

4. Quick Check:

   Headers control browser policies = A [OK]

Hint: Headers control browser rules like caching and security [OK]

Common Mistakes:

• Thinking headers change client-side state
• Confusing headers with URL routing
• Assuming headers update databases

2. Which of the following is the correct way to set a custom header in a Next.js API route response?

easy

A. res.setHeader('X-Custom-Header', 'value')

B. res.headers['X-Custom-Header'] = 'value'

C. res.header('X-Custom-Header', 'value')

D. res.addHeader('X-Custom-Header', 'value')

Solution

1. Step 1: Recall Next.js API response methods

   Next.js uses Node.js response objects where setHeader is the standard method.

2. Step 2: Compare options to Node.js response API

   Only setHeader is a valid method; others are incorrect or undefined.

3. Final Answer:

   res.setHeader('X-Custom-Header', 'value') -> Option A

4. Quick Check:

   Use setHeader to set headers = D [OK]

Hint: Use res.setHeader() to set headers in Next.js API [OK]

Common Mistakes:

• Using res.headers as an object directly
• Calling non-existent methods like header() or addHeader()
• Confusing client-side and server-side APIs

3. What will be the HTTP status code of this Next.js API response?

export default function handler(req, res) {
  res.status(404).json({ error: 'Not found' });
}

medium

A. 500

B. 200

C. 404

D. 302

Solution

1. Step 1: Identify the status method usage

   The code calls res.status(404) to set the HTTP status code to 404.

2. Step 2: Understand the effect of res.status()

   This sets the response status code before sending JSON data.

3. Final Answer:

   404 -> Option C

4. Quick Check:

   res.status(404) sets status code 404 [OK]

Hint: res.status(code) sets HTTP status code [OK]

Common Mistakes:

• Assuming default 200 status without checking code
• Confusing 404 with 500 or redirect codes
• Ignoring the status() method effect

4. Identify the error in this Next.js API route code that tries to modify the response:

export default function handler(req, res) {
  res.status(200);
  res.json({ message: 'Hello' });
  res.setHeader('X-Test', 'value');
}

medium

A. res.json() cannot be used with status()

B. res.status(200) is missing a return statement

C. res.setHeader() should be called after res.json()

D. Headers must be set before sending the response body

Solution

1. Step 1: Review response order rules

   Headers must be set before sending the response body with res.json().

2. Step 2: Identify incorrect header setting

   res.setHeader() is called after res.json(), which is too late to modify headers.

3. Final Answer:

   Headers must be set before sending the response body -> Option D

4. Quick Check:

   Set headers before body = C [OK]

Hint: Set headers before sending response body [OK]

Common Mistakes:

• Setting headers after res.json()
• Thinking res.status() sends response immediately
• Ignoring response flow order

5. You want to add caching headers to a Next.js API response only if the user is authenticated. Which code snippet correctly modifies the response based on this condition?

export default function handler(req, res) {
  const isAuth = req.headers.authorization === 'secret-token';
  // Add caching headers only if authenticated
  ???
  res.status(200).json({ data: 'Secure data' });
}

hard

A. if (isAuth) { res.setHeader('Cache-Control', 'private, max-age=3600'); }

B. if (!isAuth) { res.setHeader('Cache-Control', 'private, max-age=3600'); }

C. res.setHeader('Cache-Control', 'public, max-age=3600');

D. res.status(401).json({ error: 'Unauthorized' });

Solution

1. Step 1: Understand the condition for caching

   Caching headers should be added only if the user is authenticated (isAuth is true).

2. Step 2: Choose the correct conditional header setting

   if (isAuth) { res.setHeader('Cache-Control', 'private, max-age=3600'); } sets caching headers only when isAuth is true, matching the requirement.

3. Final Answer:

   if (isAuth) { res.setHeader('Cache-Control', 'private, max-age=3600'); } -> Option A

4. Quick Check:

   Set cache header only if authenticated = B [OK]

Hint: Use if (isAuth) to conditionally set headers [OK]

Common Mistakes:

• Setting cache headers when not authenticated
• Using public cache instead of private
• Returning 401 without setting headers when required