What is Express.js?

Node.jsJunior~2 min read

1. What Express.js is

Express.js is a minimalist, flexible web framework for Node.js that simplifies building server applications, REST APIs, and web services.

It provides a convenient interface for:

handling HTTP requests and responses;
routing;
working with middleware (in-between handlers);
and integrating with databases, templates, JSON, and more.

2. Why Express came about

Without Express, plain Node.js looks something like this:

javascript

const http = require('http');

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

server.listen(3000);

The problems:

routes have to be handled manually (req.url, req.method);
there's no proper middleware system;
there's no JSON parsing, body parser, cookies, etc.

Express solves all of that out of the box.

3. A simple Express server example

javascript

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

// Middleware for JSON parsing
app.use(express.json());

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

// A POST route
app.post('/users', (req, res) => {
  res.json({ message: 'User created', data: req.body });
});

// Start the server
app.listen(3000, () => console.log('Server running on port 3000'));

Advantages:

less code;
a readable route structure (app.get, app.post);
built-in JSON and middleware support;
easy to extend.

4. Express.js's main features

Feature	Description
Routing	Convenient request routing (`app.get('/path', ...)`)
Middleware	Functions that process a request before/after a route
Working with JSON	Automatic request-body parsing
Template engines	Support for ejs, pug, handlebars, and others
Static files	Simple file serving (`app.use(express.static('public'))`)
Error handling	Centralized error handling
Integration	Compatible with any DB (MongoDB, PostgreSQL, MySQL, etc.)

5. What Middleware is in Express

Middleware is functions that run between the request and the response.

For example:

javascript

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // pass control onward
});

Every middleware can:

change req or res;
stop execution;
call next(), to pass control to the next handler.

6. Express as the base for a REST API

Express is a great fit for building RESTful APIs:

javascript

app.get('/api/users', (req, res) => {...});
app.post('/api/users', (req, res) => {...});
app.put('/api/users/:id', (req, res) => {...});
app.delete('/api/users/:id', (req, res) => {...});

That's why Express is the de facto standard for Node.js API backends.

7. Where Express is used

Area	Application
REST API	Backends for SPAs and mobile apps
SSR	Server-side rendering of React/Vue/Nunjucks
Middleware chains	Authentication, logging, error handling
Microservices	Lightweight microservices with a JSON API
Backend-for-Frontend	A layer between the frontend and external APIs

8. Summary

Express.js is a web framework for Node.js that makes building servers and APIs simple, fast, and structured.

It's used for:

handling HTTP requests;

routing;

middleware;

building REST APIs;

serving static files and templates.

It's the foundation of most Node.js backends (Next.js, NestJS, Sails, Feathers, LoopBack, and more).

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