Node.js Request and Response Objects

The req and res Objects

req is an instance of http.IncomingMessage, a readable stream with extra properties describing the request: method, url, headers, and the HTTP version. res is an instance of http.ServerResponse, a writable stream with methods and properties for building the reply: statusCode, setHeader(), write(), and end().

Because both are streams, req emits 'data'/'end' events for reading a body (covered in the http module lesson), and res supports .write() for streaming a response in chunks before finishing with .end().

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

  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Request logged.');
});

Header names in req.headers are always lowercase, req.headers['user-agent'], not req.headers['User-Agent'].

Common req and res Properties

req.method, req.url, req.headers
res.statusCode, res.statusMessage
res.setHeader(name, value)
res.getHeader(name)
res.write(chunk)
res.end(finalChunk)

• req.headers is a plain object with lowercase keys.
• res.statusCode defaults to 200 if never explicitly set.
• res.setHeader() must run before the first res.write()/res.end() call.
• res.writeHead(statusCode, headers) can set status and multiple headers at once.

Request & Response Cheatsheet

The properties and methods you'll use on nearly every request.

Reading Query Strings

req.url includes the query string as raw text; parsing it into a usable object is done with the built-in URL class, treating req.url as a relative reference against a base URL (typically built from req.headers.host).

import { URL } from 'node:url';

const url = new URL(req.url, `http://${req.headers.host}`);
const page = url.searchParams.get('page') || '1';
console.log('Requested page:', page);

Setting Content-Type and Status Codes Correctly

Clients (and API consumers) rely on accurate Content-Type headers and status codes to know how to interpret a response. Getting these consistent across every route is one of the main jobs a framework like Express automates for you.

• 200 OK for a successful GET/PUT/PATCH.
• 201 Created for a successful POST that created a new resource.
• 204 No Content for a successful request with no response body (common for DELETE).
• 404 Not Found when no matching resource or route exists.

Common Mistakes

• Assuming header names preserve their original casing, req.headers keys are always lowercased.
• Forgetting res.statusCode defaults to 200, and never explicitly setting it for error cases.
• Calling res.setHeader() after res.write()/res.end() has already run, which throws an error.
• Manually parsing query strings with string splitting instead of the built-in URL class.

Key Takeaways

• req (IncomingMessage) exposes the request's method, URL, and headers as a readable stream.
• res (ServerResponse) is a writable stream with status code, headers, and body-writing methods.
• Header keys on req.headers are always lowercase.
• Use the built-in URL class to parse query strings reliably instead of manual string manipulation.