Response formatting in NextJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is response formatting in Next.js?

Response formatting in Next.js means shaping the data or content sent back from the server or API route so it is clear and useful for the client. It often involves setting headers and structuring JSON or HTML properly.

beginner

How do you send a JSON response in a Next.js API route?

You use the NextResponse object with the json() method, like: return NextResponse.json({ key: 'value' }); This sets the right headers and formats the data as JSON.

intermediate

Why is setting the Content-Type header important in response formatting?

Setting Content-Type tells the browser or client what kind of data is being sent (like JSON or HTML). This helps the client understand how to process or display the response correctly.

intermediate

What is the role of NextResponse in Next.js response formatting?

NextResponse is a helper from Next.js that helps create HTTP responses easily. It supports methods like json() to format data and set headers automatically.

intermediate

How can you customize HTTP status codes in Next.js API responses?

You can pass a second argument to NextResponse.json() with a status property, like: NextResponse.json(data, { status: 404 }) to send a 404 Not Found status with your response.

Which method formats a JSON response in Next.js API routes?

A NextResponse.json()

B Response.send()

C fetch()

D JSON.stringify()

NextResponse.json() formats the response as JSON and sets the correct headers automatically.

What header is important to set when sending JSON data?

A Content-Type: application/json

B Accept: text/html

C Authorization: Bearer token

D Cache-Control: no-cache

Content-Type: application/json tells the client the response is JSON data.

How do you send a 404 status with a JSON response in Next.js?

A fetch('/404')

B NextResponse.status(404).json(data)

C Response.status(404).send(data)

D NextResponse.json(data, { status: 404 })

Passing { status: 404 } as the second argument to NextResponse.json() sets the HTTP status code.

What does NextResponse help with?

A Fetching data from APIs

B Creating and formatting HTTP responses

C Routing pages in Next.js

D Styling components

NextResponse helps create HTTP responses with proper formatting and headers.

Why is response formatting important?

A It changes the URL

B It speeds up the server

C It ensures clients understand the data sent

D It styles the page

Proper formatting helps clients correctly process and display the response data.

Explain how to format a JSON response with a custom status code in a Next.js API route.

Think about how NextResponse.json() accepts a second argument for status.

Describe why setting the Content-Type header matters when sending responses in Next.js.

Consider how browsers know what to do with the data they receive.

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