Setting response headers in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of setting response headers in Node.js?

Response headers provide metadata about the response, such as content type, caching rules, and security policies. They help the browser understand how to handle the response.

beginner

How do you set a single response header in Node.js using the http module?

Use the response object's setHeader method, like: res.setHeader('Content-Type', 'application/json').

intermediate

What happens if you try to set a header after the response has been sent?

Node.js will throw an error because headers must be set before sending the response body. Once headers are sent, they cannot be changed.

intermediate

How can you set multiple headers at once in Node.js?

You can use res.writeHead(statusCode, headersObject) to set status and multiple headers together, for example: res.writeHead(200, {'Content-Type': 'text/html', 'Cache-Control': 'no-cache'}).

beginner

Why is it important to set the 'Content-Type' header correctly?

It tells the browser what kind of data is being sent (like HTML, JSON, or plain text), so the browser can display or process it properly.

Which method sets a single response header in Node.js?

A res.headerSet()

B res.getHeader()

C res.sendHeader()

D res.setHeader()

The correct method to set a single header is res.setHeader().

When must response headers be set in Node.js?

A After sending the response body

B Before sending the response body

C Any time during the request

D Only after the response ends

Headers must be set before sending the response body because they are sent first.

What does the 'Content-Type' header specify?

A The type of data in the response

B The size of the response

C The server's IP address

D The client's browser version

Content-Type tells the client what kind of data is in the response.

Which method sets multiple headers and status code at once?

A res.writeHead()

B res.setHeaders()

C res.sendHeaders()

D res.setStatus()

res.writeHead() sets the status code and multiple headers together.

What happens if you set a header after calling res.end()?

A The response restarts

B The header is set successfully

C An error occurs because headers are already sent

D The header is ignored silently

Headers must be set before res.end(); otherwise, Node.js throws an error.

Explain how to set response headers in Node.js and why it is important.

Think about how the browser knows what kind of data it receives.

Describe the difference between res.setHeader() and res.writeHead() in Node.js.

One is for single headers, the other for multiple headers and status.

Practice

1. What is the purpose of setting response headers in a Node.js server?

easy

A. To execute JavaScript code on the server

B. To provide the browser with important information about the response

C. To store data permanently on the server

D. To create new files on the server

Solution

1. Step 1: Understand what response headers do

   Response headers tell the browser details about the response, like content type or caching rules.

2. Step 2: Identify the correct purpose

   Only To provide the browser with important information about the response describes this role correctly; other options describe unrelated server tasks.

3. Final Answer:

   To provide the browser with important information about the response -> Option B

4. Quick Check:

   Response headers = browser info [OK]

Hint: Headers tell browser about response before body [OK]

Common Mistakes:

• Thinking headers run code on server
• Confusing headers with server storage
• Believing headers create files

2. Which of the following is the correct syntax to set a response header named Content-Type to application/json in Node.js?

easy

A. response.setHeader('Content-Type', 'application/json');

B. response.header('Content-Type', 'application/json');

C. response.set('Content-Type', 'application/json');

D. response.addHeader('Content-Type', 'application/json');

Solution

1. Step 1: Recall Node.js method for setting headers

   Node.js uses response.setHeader(name, value) to set headers.

2. Step 2: Match syntax with options

   Only response.setHeader('Content-Type', 'application/json'); uses setHeader correctly; others use incorrect method names.

3. Final Answer:

   response.setHeader('Content-Type', 'application/json'); -> Option A

4. Quick Check:

   Use setHeader(name, value) [OK]

Hint: Use setHeader method exactly as shown [OK]

Common Mistakes:

• Using header() instead of setHeader()
• Using set() or addHeader() which don't exist
• Wrong method name casing

3. What will be the value of the Content-Type header sent to the client after running this code snippet?

const http = require('http');
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('Content-Type', 'application/json');
  res.end('{}');
});
server.listen(3000);

medium

A. 'text/html'

B. Both 'text/html' and 'application/json'

C. No Content-Type header is sent

D. 'application/json'

Solution

1. Step 1: Understand header overwriting behavior

   Setting the same header twice overwrites the previous value in Node.js.

2. Step 2: Identify final header value

   The second setHeader call sets 'Content-Type' to 'application/json', replacing 'text/html'.

3. Final Answer:

   'application/json' -> Option D

4. Quick Check:

   Last setHeader call wins [OK]

Hint: Last setHeader call overwrites previous [OK]

Common Mistakes:

• Thinking both headers are sent
• Assuming first header stays
• Believing no header is sent

4. Identify the error in this Node.js code that tries to set a response header:

const http = require('http');
const server = http.createServer((req, res) => {
  res.write('Hello');
  res.setHeader('Content-Type', 'text/plain');
  res.end();
});
server.listen(3000);

medium

A. res.end() is missing a parameter

B. The method setHeader does not exist on res

C. Headers must be set before writing the response body

D. The server.listen port number is invalid

Solution

1. Step 1: Check order of header setting and response writing

   Headers must be set before sending any part of the response body.

2. Step 2: Identify the error in code order

   res.write('Hello') sends body first, so setHeader after it causes error.

3. Final Answer:

   Headers must be set before writing the response body -> Option C

4. Quick Check:

   Set headers before body [OK]

Hint: Set headers before any res.write or res.end [OK]

Common Mistakes:

• Setting headers after writing body
• Thinking setHeader is invalid method
• Assuming res.end needs data always

5. You want to set multiple headers including Content-Type as application/json and Cache-Control as no-cache in a Node.js server. Which code snippet correctly sets both headers before sending the response?

hard

A. res.writeHead(200, {
'Content-Type': 'application/json',
'Cache-Control': 'no-cache'
});
res.end(JSON.stringify(data));

B. res.end(JSON.stringify(data));
res.setHeader('Content-Type', 'application/json');
res.setHeader('Cache-Control', 'no-cache');

C. res.setHeader('Content-Type', 'application/json');
res.end(JSON.stringify(data));
res.setHeader('Cache-Control', 'no-cache');

D. res.setHeader({'Content-Type': 'application/json', 'Cache-Control': 'no-cache'});
res.end(JSON.stringify(data));

Solution

1. Step 1: Understand setting multiple headers at once

   writeHead allows setting status and multiple headers before sending response.

2. Step 2: Check order and correctness

   res.writeHead(200, { 'Content-Type': 'application/json', 'Cache-Control': 'no-cache' }); res.end(JSON.stringify(data)); sets both headers before res.end, correctly using writeHead.

3. Step 3: Identify errors in other options

   res.setHeader({'Content-Type': 'application/json', 'Cache-Control': 'no-cache'}); res.end(JSON.stringify(data)); incorrectly passes an object to setHeader (expects name, value); B and C set headers after res.end (which sends headers).

4. Final Answer:

   res.writeHead(200, { 'Content-Type': 'application/json', 'Cache-Control': 'no-cache' }); res.end(JSON.stringify(data)); -> Option A

5. Quick Check:

   Use writeHead for multiple headers before response [OK]

Hint: Use writeHead to set many headers at once before response [OK]

Common Mistakes:

• Using setHeader with object (like Express)
• Setting headers after res.end
• Confusing setHeader and writeHead usage