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## 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');

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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