Response modification in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Next.js API route return?

Consider this Next.js API route handler that modifies the response before sending:

export async function GET() {
  const res = new Response(JSON.stringify({ message: 'Hello' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  });
  res.headers.set('X-Custom-Header', 'TestValue');
  return res;
}

What will the client receive in the response headers?

NextJS

export async function GET() {
  const res = new Response(JSON.stringify({ message: 'Hello' }), {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  });
  res.headers.set('X-Custom-Header', 'TestValue');
  return res;
}

A Response with status 200, Content-Type 'application/json', and header 'X-Custom-Header' set to 'TestValue'

B Response with status 200 and Content-Type 'application/json' only, no 'X-Custom-Header'

C Response with status 200 and header 'X-Custom-Header' only, no Content-Type

D Response with status 500 due to header modification error

💡 Hint

Think about how the Response object headers can be modified before returning.

The Response object allows header modification before returning. Setting X-Custom-Header adds it to the response headers along with the original Content-Type.

📝 Syntax

intermediate

Which option correctly modifies the response status in Next.js API route?

Given this Next.js API route snippet, which option correctly changes the response status to 404?

export async function GET() {
  const res = new Response('Not Found');
  // Modify status here
  return res;
}

NextJS

export async function GET() {
  const res = new Response('Not Found');
  // Modify status here
  return res;
}

A res.setStatus(404);

B res.statusCode = 404;

C res = new Response('Not Found', { status: 404 });

D res.status = 404;

💡 Hint

Remember that Response objects are immutable for status after creation.

The Response object status cannot be changed after creation. You must create a new Response with the desired status.

🔧 Debug

advanced

Why does this Next.js API route fail to set a cookie?

Examine this Next.js API route code:

export async function GET() {
  const res = new Response('OK');
  res.headers.append('Set-Cookie', 'token=abc123; HttpOnly');
  return res;
}

Why might the cookie not be set in the browser?

NextJS

export async function GET() {
  const res = new Response('OK');
  res.headers.append('Set-Cookie', 'token=abc123; HttpOnly');
  return res;
}

A Because the Response object headers are immutable and cannot be appended after creation

B Because the Response body must be JSON to set cookies

C Because 'HttpOnly' cookies cannot be set from Next.js API routes

D Because the 'Set-Cookie' header must be set during Response creation, not appended later

💡 Hint

Consider when headers can be set on the Response object.

The Response headers are immutable after creation. To set cookies, you must include the 'Set-Cookie' header in the Response constructor options.

❓ state_output

advanced

What is the output of this Next.js middleware modifying response headers?

Given this Next.js middleware code:

import { NextResponse } from 'next/server';

export function middleware(request) {
  const response = NextResponse.next();
  response.headers.set('X-Middleware', 'Active');
  return response;
}

What will the client receive in the response headers?

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const response = NextResponse.next();
  response.headers.set('X-Middleware', 'Active');
  return response;
}

A Response without 'X-Middleware' header because headers cannot be set in middleware

B Response with header 'X-Middleware' set to 'Active'

C Response with status 500 due to header modification error

D Response with header 'X-Middleware' set to 'next'

💡 Hint

Think about how NextResponse.next() allows header modification.

NextResponse.next() returns a mutable response object. You can set headers on it before returning, so the client receives the custom header.

🧠 Conceptual

expert

Which statement about response modification in Next.js App Router is true?

Choose the correct statement about modifying responses in Next.js App Router server actions or route handlers.

A Response objects are immutable; to change status or headers, create a new Response with desired options

B You can modify the response headers after returning the Response object by mutating it asynchronously

C Next.js automatically merges headers set in multiple middleware and route handlers without conflicts

D You must always use the <code>res</code> object from Express to modify responses in Next.js App Router

💡 Hint

Think about immutability of Response objects in web standards.

Response objects follow web standards and are immutable after creation. To modify status or headers, you must create a new Response object with the desired properties.