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Introduction to Express.js · learncode.live

What is Express.js?

Express.js is a fast, unopinionated, minimalist web framework for Node.js. It provides a robust set of features for building web applications and APIs, including routing, middleware support, template engines, and static file serving.

Express acts as a thin layer on top of Node.js’s built-in http module, abstracting away boilerplate code while keeping the low-level control that Node.js developers appreciate.

┌──────────────────────────────────────┐
│           Your Application           │
├──────────────────────────────────────┤
│            Express.js                │
├──────────────────────────────────────┤
│         Node.js (HTTP core)          │
├──────────────────────────────────────┤
│            Operating System          │
└──────────────────────────────────────┘

Why Express?

FeatureBenefit
MinimalistSmall footprint, no unnecessary abstraction
UnopinionatedYou decide the structure - no forced patterns
Middleware-basedRequest pipeline is composable and flexible
Huge ecosystemThousands of middleware packages on npm
Battle-testedPowers companies like Uber, IBM, and Fox Sports
Great docsWell-documented with a large community

Key Concepts

Routing

Routing determines how an application responds to a client request at a specific endpoint (URI + HTTP method):

GET  /users       → list users
POST /users       → create a user
GET  /users/:id   → get a specific user

Middleware

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.

Request → Middleware 1 → Middleware 2 → Route Handler → Response
                ↓             ↓
          (can end)     (can end)

Request & Response

Express extends Node.js’s native req and res objects with convenient methods:

req.params    // Route parameters
req.query     // URL query string
req.body      // Request body (with body-parser)
req.headers   // HTTP headers

res.send()    // Send any response
res.json()    // Send JSON
res.render()  // Render a template
res.redirect()// Redirect to another URL

Express vs plain Node.js

Here’s the same server built with plain Node.js and with Express:

Plain Node.js:

const http = require('http');

const server = http.createServer((req, res) => {
  if (req.url === '/' && req.method === 'GET') {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.end('Hello World');
  } else if (req.url === '/api/users' && req.method === 'GET') {
    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify([{ id: 1, name: 'John' }]));
  } else {
    res.writeHead(404);
    res.end('Not Found');
  }
});

server.listen(3000);

With Express:

const express = require('express');
const app = express();

app.get('/', (req, res) => res.send('Hello World'));
app.get('/api/users', (req, res) => res.json([{ id: 1, name: 'John' }]));

app.listen(3000);

Express eliminates manual URL parsing, status code management, and content-type headers.

Who uses Express?

  • APIs - RESTful and GraphQL APIs
  • Single Page Applications - backend serving SPAs
  • Microservices - lightweight service endpoints
  • Real-time apps - combined with Socket.io
  • Proxies - middleware-based request forwarding

When NOT to use Express

  • High-performance real-time - consider Fastify or plain Node.js
  • Strict structure needed - consider NestJS (built on Express but opinionated)
  • Full-stack meta-framework - consider Next.js or Remix

Key Takeaways

  • Express is a minimal, unopinionated Node.js web framework
  • It provides routing, middleware, and convenience methods on top of Node’s http module
  • Express code is significantly cleaner and shorter than plain Node.js
  • It has a massive ecosystem of third-party middleware
  • Use it for APIs, SPAs, microservices, and traditional web apps
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