Express.js

Express.js is a minimal web framework for Node.js.

It gives you a clean way to:

• define routes (URLs)
• add middleware (code that runs before/after routes)
• send responses (HTML, JSON, files)

Install

npm install express

Your First Server

import express from 'express';

const app = express();
const port = 3000;

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

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

Routing (The Core Concept)

A route matches a request by:

• HTTP method (GET/POST/PUT/PATCH/DELETE)
• path (like /users)

Examples:

app.get('/health', (req, res) => res.json({ ok: true }));

app.post('/users', (req, res) => {
  res.status(201).json({ message: 'User created' });
});

Route Parameters (:id)

Use route params when you want to address one specific resource.

app.get('/users/:id', (req, res) => {
  const { id } = req.params;
  res.json({ id, name: 'Example' });
});

Request:

GET /users/42

req.params:

{ "id": "42" }

Query Parameters (?search=...)

Use query params for filtering, sorting, pagination.

app.get('/users', (req, res) => {
  const { role, page = '1' } = req.query;
  res.json({ role, page });
});

Request:

GET /users?role=admin&page=2

Request / Response Objects

Every route handler receives:

• req (request): incoming data
• res (response): what you send back

Common things you use:

app.get('/info', (req, res) => {
  res.json({
    method: req.method,
    path: req.path,
    headers: req.headers,
  });
});

JSON APIs (Reading the Request Body)

To read JSON bodies, you need the JSON middleware:

app.use(express.json());

app.post('/echo', (req, res) => {
  res.json({ youSent: req.body });
});

Middleware

Middleware is a function that runs during request processing.

It can:

• log requests
• check auth
• attach data to req
• stop the request with a response
• or pass control to the next handler

Logging Middleware Example

app.use((req, res, next) => {
  console.log(`${req.method} ${req.path}`);
  next();
});

Route-Specific Middleware Example

function requireApiKey(req, res, next) {
  if (req.headers['x-api-key'] !== 'secret') {
    return res.status(401).json({ error: 'Unauthorized' });
  }
  next();
}

app.get('/private', requireApiKey, (req, res) => {
  res.json({ secretData: 123 });
});

Routers (Organizing Routes)

As your API grows, you usually group routes into a router.

Example:

import express from 'express';

const usersRouter = express.Router();

usersRouter.get('/', (req, res) => res.json([{ id: 1, name: 'Alice' }]));
usersRouter.get('/:id', (req, res) => res.json({ id: req.params.id }));

export default usersRouter;

Then mount it:

import usersRouter from './routes/users.js';

app.use('/users', usersRouter);

This creates:

• GET /users
• GET /users/:id

Status Codes (Quick Examples)

res.status(200).json({ ok: true });
res.status(201).json({ created: true });
res.status(404).json({ error: 'Not found' });

Error Handling (Simple Pattern)

Express treats a function with four arguments as an error handler.

app.get('/boom', (req, res) => {
  throw new Error('Something broke');
});

app.use((err, req, res, next) => {
  console.error(err);
  res.status(500).json({ error: 'Internal Server Error' });
});

Serving Static Files

If you have a folder like public/, you can serve files from it:

app.use(express.static('public'));

Then public/logo.png becomes:

GET /logo.png

Where Express Is Used

• REST APIs for web and mobile apps
• Backend-for-frontend (BFF) servers
• Prototypes and small-to-medium backends (and sometimes parts of larger systems)

Industry examples (companies using Node/Express-style backends):

• Netflix: has used Node.js widely for web services
• PayPal: early well-known Node.js adoption for web apps
• Uber: uses Node.js for various services

Related Frameworks (Mentions)

• NestJS: A more structured Node.js framework (TypeScript-first) inspired by Angular.

References

• Express.js Official Site
• Express API Reference