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node-js / Request & Response Objects
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Request & Response Objects

Every time a client makes a request to your server, Node.js creates two objects: the **request object** (`req`) and the **response object** (`res`). These objects are your main tools for understanding what the client wants and for sending back a response. Let's explore them in detail.

The Request Object (req)

The request object contains all information about the incoming request: the URL, HTTP method, headers, query parameters, and more.

Property/MethodDescription
`req.url`The URL path (e.g., `/about?name=john` includes query string).
`req.method`HTTP method (GET, POST, PUT, DELETE, etc.).
`req.headers`An object containing all HTTP headers.
`req.httpVersion`HTTP version used by the client.
`req.socket`Information about the connection (remote address, port).

Think of the request object as the "order form" a customer fills out. It tells you what they want (URL), how they want it (method), and any special instructions (headers).

Request Object Examples
const http = require('http');

const server = http.createServer((req, res) => {
  console.log('URL:', req.url);
  console.log('Method:', req.method);
  console.log('Headers:', req.headers);
  console.log('User-Agent:', req.headers['user-agent']);
  res.end('Check your console!');
});

server.listen(3000);

Parsing the URL

Often you need to separate the path from query parameters. Node.js provides the `url` module for this.
const url = require('url');

const server = http.createServer((req, res) => {
  const parsedUrl = url.parse(req.url, true); <!-- true parses query string -->
  console.log('Path:', parsedUrl.pathname);
  console.log('Query:', parsedUrl.query);
  res.end('OK');
});

The Response Object (res)

The response object is used to send data back to the client. You set status codes, headers, and the response body using its methods.

MethodDescription
`res.writeHead(statusCode, headers)`Sets the HTTP status code and headers.
`res.setHeader(name, value)`Sets a single header.
`res.write(chunk)`Sends a chunk of the response body (can be called multiple times).
`res.end([data])`Signals that the response is complete. Optionally sends final data.
`res.statusCode`Property to set status code (alternative to writeHead).

Response Examples
<!-- Method 1: Using writeHead -->
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({ message: 'Hello' }));

<!-- Method 2: Setting properties individually -->
res.statusCode = 200;
res.setHeader('Content-Type', 'text/html');
res.write('
');
res.write('Hello World');
res.write('');
res.end();

Common HTTP Status Codes

CodeMeaning
200OK – Success
201Created – Resource created successfully
301Moved Permanently – Redirect
400Bad Request – Client error
404Not Found
500Internal Server Error

Two Minute Drill

  • `req` gives you request details: url, method, headers.
  • `res` lets you build and send the response.
  • Use `url.parse()` to split path and query string.
  • Set status codes appropriately (200, 404, 500, etc.).
  • Always call `res.end()` to finish the response.
  • Headers are case-insensitive but commonly written in lowercase.

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