Creating the server

Create a file called index.js and paste this in:

import express from 'express';

const app = express();

app.get('/', (req, res) => {
  res.send('Hello World!');
});

const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server running at http://localhost:${PORT}`);
});

Run it with:

node index.js

Open your browser to http://localhost:3000 and you will see "Hello World!". That is your server. Four meaningful lines of code, and you have a working HTTPWhat is http?The protocol browsers and servers use to exchange web pages, API data, and other resources, defining how requests and responses are formatted. Ask AI for more server running in Node.js.

The req object

req is short for request. Every time a browser or APIWhat is api?A set of rules that lets one program talk to another, usually over the internet, by sending requests and getting responses. Ask AI for more client hits your server, Express builds an req object from the raw HTTPWhat is http?The protocol browsers and servers use to exchange web pages, API data, and other resources, defining how requests and responses are formatted. Ask AI for more request and hands it to your route handlerWhat is route handler?A Next.js file named route.js inside the app/ directory that handles HTTP requests directly - the App Router equivalent of API routes. Ask AI for more. Here is a tour of the most useful properties:

app.get('/inspect', (req, res) => {
  console.log(req.method);           // 'GET'
  console.log(req.path);             // '/inspect'
  console.log(req.url);              // '/inspect?foo=bar' (includes query string)
  console.log(req.query);            // { foo: 'bar' } from ?foo=bar
  console.log(req.headers);          // All HTTP headers as an object
  console.log(req.get('User-Agent')); // One specific header
  console.log(req.ip);               // Client IP address

  res.send('Logged to console');
});

You will use req.query constantly for things like paginationWhat is pagination?Splitting a large set of results into smaller pages so the server and client only handle a manageable chunk at a time. Ask AI for more (?page=2&limit=10) and filters. req.headers matters when you start building authenticationWhat is authentication?Verifying who a user is, typically through credentials like a password or token. Ask AI for more with tokens.

The res object

res is short for response. It is your toolkit for building the reply you send back to the client. The most important methods:

app.get('/demo', (req, res) => {
  // Send a plain text or HTML string
  res.send('Simple text response');

  // Send JSON - automatically sets Content-Type: application/json
  res.json({ message: 'Hello', ok: true });

  // Set a status code (chain with send or json)
  res.status(404).send('Not found');
  res.status(201).json({ id: 42, created: true });

  // Redirect the client to another URL
  res.redirect('/new-url');
  res.redirect(301, '/permanent-redirect');

  // Set a custom response header
  res.set('X-Powered-By', 'My Express App');
});

Common response methods at a glance

Combining routes on one path

Sometimes you want a single URL to handle multiple HTTPWhat is http?The protocol browsers and servers use to exchange web pages, API data, and other resources, defining how requests and responses are formatted. Ask AI for more methods. Instead of writing app.get and app.post separately, you can chain them with app.route():

app.route('/users')
  .get((req, res) => {
    res.json([{ id: 1, name: 'Alice' }]);
  })
  .post((req, res) => {
    res.status(201).json({ id: 2, name: 'Bob' });
  });

This keeps related handlers together and makes your route file easier to scan.