Response modification in NextJS - Mini Project: Build & Apply

Modify HTTP Response Headers in Next.js API Route

📖 Scenario: You are building a Next.js API route that returns a JSON response. You want to add a custom HTTP header to the response to provide extra information to the client.

🎯 Goal: Create a Next.js API route that sends a JSON response with a custom HTTP header X-Custom-Header set to MyValue.

📋 What You'll Learn

Create a Next.js API route handler function named GET.

Set a custom header X-Custom-Header with the value MyValue on the response.

Return a JSON response with a message { message: 'Hello, Next.js!' }.

💡 Why This Matters

🌍 Real World

Adding custom headers is useful for sending extra information like security policies, caching instructions, or metadata to clients in web applications.

💼 Career

Understanding how to modify HTTP responses is important for backend and full-stack developers working with Next.js or other server frameworks to control client-server communication.

1

Create the API route handler function

Create an async function called GET that takes a single parameter request.

NextJS

export async function GET(request) {
  // Your code here
}

Hint

This function will handle GET requests to your API route.

2

Create the JSON response body

Inside the GET function, create a constant called jsonData and set it to the object { message: 'Hello, Next.js!' }.

NextJS

export async function GET(request) {
  // Create jsonData object
  // Your code here
}

Hint

This object will be sent as the JSON response body.

3

Create the Response object with custom header

Still inside the GET function, create a constant called response and set it to a new Response object. Pass JSON.stringify(jsonData) as the body, and an object with headers containing 'Content-Type': 'application/json' and 'X-Custom-Header': 'MyValue'.

NextJS

export async function GET(request) {
  const jsonData = { message: 'Hello, Next.js!' };
  // Create response with headers
  // Your code here
}

Hint

The Response constructor lets you set headers along with the body.

4

Return the Response object

At the end of the GET function, return the response object.

NextJS

export async function GET(request) {
  const jsonData = { message: 'Hello, Next.js!' };
  const response = new Response(JSON.stringify(jsonData), {
    headers: {
      'Content-Type': 'application/json',
      'X-Custom-Header': 'MyValue'
    }
  });
  // Return the response
  // Your code here
}

Hint

Returning the Response sends it back to the client.

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