Response formatting in NextJS - Mini Project: Build & Apply

Next.js Response Formatting

📖 Scenario: You are building a simple Next.js API route that returns user data in a formatted JSON response. This is common when creating backend endpoints for web apps.

🎯 Goal: Create a Next.js API route that returns a JSON response with a status field and a data field containing user information.

📋 What You'll Learn

Create a Next.js API route handler function named handler

Define a user object with exact fields: id, name, and email

Create a response object with status set to success and data containing the user object

Return the response using Next.js res.status(200).json() method

💡 Why This Matters

🌍 Real World

API routes in Next.js are used to build backend endpoints that send JSON data to frontend apps or other services.

💼 Career

Understanding how to format and send JSON responses in Next.js API routes is essential for full-stack web development roles.

1

Create user data object

Create a constant called user with these exact fields and values: id set to 1, name set to "Alice", and email set to "[e-mail]".

NextJS

# Create the user object with id, name, and email
# Your code here

Hint

Use const user = { id: 1, name: "Alice", email: "[e-mail]" } to create the object.

2

Create response object

Create a constant called response that is an object with status set to "success" and data set to the user object.

NextJS

const user = { id: 1, name: "Alice", email: "[e-mail]" };
# Create the response object with status and data
# Your code here

Hint

Use const response = { status: "success", data: user } to create the response.

3

Create API route handler function

Create an exported async function called handler that takes req and res as parameters.

NextJS

const user = { id: 1, name: "Alice", email: "[e-mail]" };
const response = { status: "success", data: user };
# Create the exported async handler function with req and res
# Your code here

Hint

Use export default async function handler(req, res) { } to define the API route.

4

Send JSON response

Inside the handler function, send the response object as JSON with HTTP status 200 using res.status(200).json(response).

NextJS

const user = { id: 1, name: "Alice", email: "[e-mail]" };
const response = { status: "success", data: user };

export default async function handler(req, res) {
  # Send the JSON response with status 200
  # Your code here
}

Hint

Use res.status(200).json(response) to send the JSON response.

Practice

1. What is the main purpose of setting the Content-Type header in a Next.js API response?

easy

A. To set the status code of the response

B. To specify the HTTP method used in the request

C. To define the URL path of the API endpoint

D. To tell the client what type of data is being sent

Solution

1. Step 1: Understand the role of headers in HTTP responses

   Headers provide metadata about the response, including data format.

2. Step 2: Identify the purpose of Content-Type

   This header tells the client how to interpret the response body, e.g., JSON or HTML.

3. Final Answer:

   To tell the client what type of data is being sent -> Option D

4. Quick Check:

   Content-Type = Data format info [OK]

Hint: Content-Type always describes the data format sent [OK]

Common Mistakes:

• Confusing Content-Type with HTTP method
• Thinking Content-Type sets status code
• Mixing URL path with headers

2. Which of the following is the correct way to create a JSON response with status 200 in a Next.js API route?

easy

A. return new Response(data, { statusCode: 200, contentType: 'application/json' })

B. return Response(data, 200, { 'Content-Type': 'application/json' })

C. return new Response(JSON.stringify(data), { status: 200, headers: { 'Content-Type': 'application/json' } })

D. return new Response(JSON.stringify(data), 200, { 'Content-Type': 'text/plain' })

Solution

1. Step 1: Check the Response constructor syntax

   It takes the body as first argument, and an options object with status and headers.

2. Step 2: Verify correct headers and status usage

   Headers must include 'Content-Type' with 'application/json' for JSON data, and status should be 200.

3. Final Answer:

   return new Response(JSON.stringify(data), { status: 200, headers: { 'Content-Type': 'application/json' } }) -> Option C

4. Quick Check:

   Response(body, {status, headers}) = return new Response(JSON.stringify(data), { status: 200, headers: { 'Content-Type': 'application/json' } }) [OK]

Hint: Use new Response with JSON.stringify and proper headers [OK]

Common Mistakes:

• Passing data directly without JSON.stringify
• Using wrong header keys like contentType
• Incorrect argument order in Response

3. Consider this Next.js API handler code snippet:

export async function GET() {
  const data = { message: 'Hello' };
  return new Response(JSON.stringify(data), {
    status: 201,
    headers: { 'Content-Type': 'application/json' }
  });
}

What will be the HTTP status code and response body sent to the client?

medium

A. Status 201 with body '{"message":"Hello"}'

B. Status 200 with body '{"message":"Hello"}'

C. Status 201 with body 'Hello'

D. Status 200 with body 'Hello'

Solution

1. Step 1: Identify the status code set in the Response

   The code sets status to 201 explicitly in the Response options.

2. Step 2: Check the response body content

   The body is JSON.stringify(data), which converts { message: 'Hello' } to '{"message":"Hello"}'.

3. Final Answer:

   Status 201 with body '{"message":"Hello"}' -> Option A

4. Quick Check:

   Status and JSON body match Status 201 with body '{"message":"Hello"}' [OK]

Hint: Look for status in Response options and JSON.stringify body [OK]

Common Mistakes:

• Assuming default status 200 instead of 201
• Confusing raw string with JSON string
• Ignoring JSON.stringify usage

4. You wrote this Next.js API handler:

export async function POST() {
  const data = { success: true };
  return new Response(data, {
    status: 200,
    headers: { 'Content-Type': 'application/json' }
  });
}

What is the problem with this code?

medium

A. Headers object is missing required 'Accept' header

B. Response body must be a string or Blob, not an object

C. Status code 200 is invalid for POST requests

D. The function should not be async

Solution

1. Step 1: Check the Response body type

   The Response constructor expects a string, Blob, or similar, not a plain object.

2. Step 2: Identify the fix

   The object must be converted to a string using JSON.stringify before passing to Response.

3. Final Answer:

   Response body must be a string or Blob, not an object -> Option B

4. Quick Check:

   Response body type error = Response body must be a string or Blob, not an object [OK]

Hint: Always JSON.stringify objects before sending in Response body [OK]

Common Mistakes:

• Passing raw objects directly to Response
• Thinking status 200 is invalid for POST
• Adding unnecessary headers like Accept

5. You want to send a plain text response with status 404 from a Next.js API route. Which code snippet correctly formats this response?

hard

A. return new Response('Not Found', { status: 404, headers: { 'Content-Type': 'text/plain' } })

B. return new Response({ message: 'Not Found' }, { status: 404, headers: { 'Content-Type': 'application/json' } })

C. return new Response('Not Found', 404, { 'Content-Type': 'text/plain' })

D. return Response('Not Found', { status: 404, contentType: 'text/plain' })

Solution

1. Step 1: Confirm Response constructor usage

   It takes the body string first, then an options object with status and headers.

2. Step 2: Verify correct headers and status for plain text

   Status 404 is correct for 'Not Found', and Content-Type must be 'text/plain' for plain text.

3. Final Answer:

   return new Response('Not Found', { status: 404, headers: { 'Content-Type': 'text/plain' } }) -> Option A

4. Quick Check:

   Plain text + 404 status + correct headers = return new Response('Not Found', { status: 404, headers: { 'Content-Type': 'text/plain' } }) [OK]

Hint: Use string body with status and 'text/plain' header [OK]

Common Mistakes:

• Passing object instead of string for plain text
• Wrong argument order in Response
• Using contentType instead of headers key