What is Express.js and why is it used?

What is Express.js?

Express.js is a minimal, fast, and unopinionated web framework for Node.js. It provides a thin layer of fundamental web application features on top of Node.js's built-in http module, without obscuring any of Node.js's core capabilities.

Why Use Express.js?

Without Express, a basic Node.js HTTP server looks like:

// Pure Node.js — verbose and manual
const http = require('http');

const server = http.createServer((req, res) => {
  if (req.method === 'GET' && req.url === '/users') {
    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify({ users: [] }));
  } else {
    res.writeHead(404);
    res.end('Not Found');
  }
});
server.listen(3000);

With Express:

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

app.get('/users', (req, res) => {
  res.json({ users: [] });
});

app.listen(3000);

Core Features

Feature	Description
Routing	Define URL handlers with HTTP methods
Middleware	Composable request/response pipeline
Static files	Serve HTML, CSS, images with one line
Template engines	Render HTML with EJS, Pug, Handlebars
Error handling	Centralized error management
REST API support	JSON parsing, status codes, headers

Hello World

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

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

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Express.js vs Alternatives

Framework	Style	Best For
Express.js	Minimal, flexible	REST APIs, full-stack apps
Fastify	Fast, schema-based	High-performance APIs
Koa	Modern, async-first	Lightweight middleware chains
NestJS	Opinionated, TypeScript	Enterprise-scale apps
Hapi	Configuration-heavy	Secure enterprise APIs

Express Application Object

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

// app is an instance of Express Application
// Key properties and methods:
app.use()      // mount middleware
app.get()      // handle GET route
app.post()     // handle POST route
app.listen()   // start HTTP server
app.set()      // configure settings
app.locals     // app-level variables

Summary

Express.js simplifies Node.js web development by providing routing, middleware, and utilities for building HTTP servers. It's the most popular Node.js framework with a massive ecosystem and is the foundation for frameworks like NestJS and LoopBack.

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What is Express.js and why is it used?