Topic 19 of 40

Request & Response

Q: What is the mechanical difference between `res.setHeader()` and `res.writeHead()`?

`setHeader()` allows you to queue up multiple headers individually over multiple lines of code. `writeHead()` mathematically finalizing the headers, writing the Status Code and all queued headers into the TCP socket instantly, preventing any further header modifications.

Overview

The Request (req) and Response (res) objects represent the entire HTTP transaction. The Request contains everything the user sent you: the URL, the HTTP Method (GET, POST), Headers (Auth Tokens), and the Body (Form data). The Response is the pipeline going back to the user: you set the Status Code (200, 404, 500), configure security Headers (CORS), and finally stream the physical payload (JSON, HTML) back to their browser.

Syntax

javascript

const http = require('http');

const server = http.createServer((req, res) => {
    
    // --- 1. INSPECTING THE REQUEST (req) ---
    console.log(req.method); // 'GET', 'POST', 'DELETE'
    console.log(req.url);    // '/api/users?sort=asc'
    console.log(req.headers['user-agent']); // Identifies the browser!
    
    // --- 2. CRAFTING THE RESPONSE (res) ---
    // A standard JSON API response
    const payload = JSON.stringify({ error: false, message: "Success" });
    
    // Set headers manually
    res.setHeader('Content-Type', 'application/json');
    res.setHeader('X-Powered-By', 'Node.js');
    
    // Set the status code and end the stream!
    res.statusCode = 200;
    res.end(payload);
});

Common Pitfalls

Trying to send headers AFTER you have already sent the body. HTTP protocol strictly dictates that Headers must be sent first, followed by a blank line, followed by the Body. If you call res.write('Hello') and then try to call res.setHeader(...), Node will throw a fatal ERR_HTTP_HEADERS_SENT crash.
Assuming req.body exists natively. In raw Node.js, the Request body does NOT exist as a parsed object. Because the request is a Stream, the body comes in as raw binary chunks. You have to manually listen to req.on('data') and concatenate the chunks to read a JSON payload (Express automates this).

Interview Questions

What is the mechanical difference between res.setHeader() and res.writeHead()?

setHeader() allows you to queue up multiple headers individually over multiple lines of code. writeHead() mathematically finalizing the headers, writing the Status Code and all queued headers into the TCP socket instantly, preventing any further header modifications.

Real-World Example

Extracting a Bearer Token (JWT) from the raw Request Headers.

example

javascript

const authHeader = req.headers['authorization'];

if (!authHeader || !authHeader.startsWith('Bearer ')) {
    res.statusCode = 401; // Unauthorized
    return res.end('Missing or invalid token.');
}

const token = authHeader.split(' ')[1]; // Extracts the actual JWT string
console.log("Token received:", token);

Check Your Knowledge

Test your understanding of Request & Response with these quick questions.

Because the `res` (Response) object is fundamentally a Writable Stream, which of the following stream methods must you execute to formalize the end of the HTTP transaction?

Take Node.js Quiz

Topic 19 of 40

Request & Response

Overview

Syntax

javascript

const http = require('http');

const server = http.createServer((req, res) => {
    
    // --- 1. INSPECTING THE REQUEST (req) ---
    console.log(req.method); // 'GET', 'POST', 'DELETE'
    console.log(req.url);    // '/api/users?sort=asc'
    console.log(req.headers['user-agent']); // Identifies the browser!
    
    // --- 2. CRAFTING THE RESPONSE (res) ---
    // A standard JSON API response
    const payload = JSON.stringify({ error: false, message: "Success" });
    
    // Set headers manually
    res.setHeader('Content-Type', 'application/json');
    res.setHeader('X-Powered-By', 'Node.js');
    
    // Set the status code and end the stream!
    res.statusCode = 200;
    res.end(payload);
});

Common Pitfalls

Trying to send headers AFTER you have already sent the body. HTTP protocol strictly dictates that Headers must be sent first, followed by a blank line, followed by the Body. If you call res.write('Hello') and then try to call res.setHeader(...), Node will throw a fatal ERR_HTTP_HEADERS_SENT crash.
Assuming req.body exists natively. In raw Node.js, the Request body does NOT exist as a parsed object. Because the request is a Stream, the body comes in as raw binary chunks. You have to manually listen to req.on('data') and concatenate the chunks to read a JSON payload (Express automates this).

Interview Questions

What is the mechanical difference between res.setHeader() and res.writeHead()?

Real-World Example

Extracting a Bearer Token (JWT) from the raw Request Headers.

example

javascript

const authHeader = req.headers['authorization'];

if (!authHeader || !authHeader.startsWith('Bearer ')) {
    res.statusCode = 401; // Unauthorized
    return res.end('Missing or invalid token.');
}

const token = authHeader.split(' ')[1]; // Extracts the actual JWT string
console.log("Token received:", token);

Check Your Knowledge

Test your understanding of Request & Response with these quick questions.

Because the `res` (Response) object is fundamentally a Writable Stream, which of the following stream methods must you execute to formalize the end of the HTTP transaction?

Take Node.js Quiz