Setting response headers in Node.js

Introduction

Response headers tell the browser important information about the data it receives. Setting them helps control how the browser handles the response.

When you want to tell the browser the type of content you are sending, like JSON or HTML.

When you want to allow or restrict access from other websites (CORS).

When you want to control caching behavior to make your site faster or always fresh.

When you want to set cookies or security policies for your site.

Syntax

Node.js

response.setHeader(name, value);

name is the header name as a string, like 'Content-Type'.

value is the header value as a string, like 'application/json'.

Examples

This tells the browser the response is HTML.

Node.js

response.setHeader('Content-Type', 'text/html');

This tells the browser not to cache the response.

Node.js

response.setHeader('Cache-Control', 'no-cache');

This allows any website to access the response (CORS).

Node.js

response.setHeader('Access-Control-Allow-Origin', '*');

Sample Program

This Node.js server sets two headers: it tells the browser the response is JSON and disables caching. Then it sends a JSON message.

Node.js

import http from 'node:http';

const server = http.createServer((request, response) => {
  response.setHeader('Content-Type', 'application/json');
  response.setHeader('Cache-Control', 'no-store');
  const data = { message: 'Hello, world!' };
  response.end(JSON.stringify(data));
});

server.listen(3000, () => {
  console.log('Server running at http://localhost:3000/');
});

Important Notes

Headers must be set before sending the response body.

Header names are case-insensitive but usually written in standard capitalization.

Setting headers incorrectly can cause browsers to misinterpret your response.

Summary

Response headers give the browser important info about the response.

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

Always set headers before sending the response body.

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