text 15 min

Introduction to Express

Why It Matters

Express is the smallest useful layer most Node.js teams put between raw HTTP and an application. It gives you routing, middleware, request parsing hooks, response helpers, and a predictable way to compose web/API behavior without hiding Node's request and response model.

Core Concepts

  • Express applications are functions that receive requests and produce responses.
  • Routes match an HTTP method plus a path, then run one or more handlers.
  • Middleware is code that runs before, after, or around route handlers.
  • Express does not enforce architecture; your project structure must keep handlers, services, and data access readable.
  • The underlying server is still Node's node:http server, so timeouts, sockets, streaming, and process lifecycle still matter.

Flow to Remember

A client opens a TCP connection, sends an HTTP request, Node parses it, Express matches middleware and routes in registration order, your handler writes a response, then Node flushes bytes back to the client.

Syntax and Examples

js
import express from 'express';
import { createServer } from 'node:http';

const app = express();
app.use(express.json());

app.get('/health', (req, res) => {
  res.json({ status: 'ok', uptimeSeconds: process.uptime() });
});

app.post('/echo', (req, res) => {
  res.status(201).json({ received: req.body });
});

const server = createServer(app);
server.listen(3000, () => {
  console.log('API listening on http://localhost:3000');
});

Use Cases and Tradeoffs

  • Build JSON APIs, server-rendered sites, webhooks, internal tools, and backend-for-frontend services.
  • Choose Express when you want control and a large ecosystem. Choose a more opinionated framework when you need conventions for dependency injection, validation, or modules.
  • Because Express is minimal, production readiness comes from deliberate choices around validation, logging, security headers, and shutdown.

Common Mistakes

  • Putting all logic inside route handlers until the app becomes impossible to test.
  • Assuming Express catches every async error without checking the Express version and handler style.
  • Registering middleware after routes that need it.
  • Returning stack traces or raw database errors to clients.

Practical Challenge

Create an Express app with /health, /api/time, and /api/echo routes. Add JSON parsing only for routes under /api, then test invalid JSON and observe the response.

Recap

  • Express wraps Node HTTP with routing and middleware, not with magic.
  • Registration order is behavior.
  • A maintainable Express app separates transport concerns from business logic.
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