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## 1. What a route is

> A **route** in Express.js is a **mapping** between:
>
> - an **HTTP method** (`GET`, `POST`, `PUT`, `DELETE`, …);
> - a **URL path** (`/`, `/users`, `/api/products/:id`);
> - and a **handler function**,
> that runs when a request matches it.

In other words, a route tells Express:
*"If a request for* `GET /users` *comes in, run this code."*

### The simplest route example

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

app.get('/', (req, res) => {
  res.send('Home page');
});

app.get('/about', (req, res) => {
  res.send('About us');
});

app.listen(3000, () => console.log('Server running'));
```

If you open these in a browser:

- `http://localhost:3000/` → `Home page`
- `http://localhost:3000/about` → `About us`

## 2. A route's format

```javascript
app.METHOD(PATH, HANDLER)
```

| Element | What it is | Example |
|---|---|---|
| `METHOD` | The HTTP method (lowercase) | `get`, `post`, `put`, `delete` |
| `PATH` | The path the route responds to | `'/'`, `'/users'`, `'/users/:id'` |
| `HANDLER` | The `(req, res)` handler function | `(req, res) => res.send('OK')` |

## 3. Examples of different routes

### GET, retrieve data

```javascript
app.get('/users', (req, res) => {
  res.send('List of users');
});
```

### POST, create

```javascript
app.post('/users', (req, res) => {
  res.send('User created');
});
```

### PUT, update

```javascript
app.put('/users/:id', (req, res) => {
  res.send(`User ${req.params.id} updated`);
});
```

### DELETE, remove

```javascript
app.delete('/users/:id', (req, res) => {
  res.send(`User ${req.params.id} deleted`);
});
```

## 4. Dynamic routes

Routes can contain **parameters**, marked with `:name`.

```javascript
app.get('/products/:id', (req, res) => {
  res.send(`Product with ID: ${req.params.id}`);
});
```

`GET /products/123` → response: `Product with ID: 123`

Parameters can be read via `req.params`.

## 5. Grouping routes (Router)

To keep code organized, it's convenient to move routes into separate files using `express.Router()`:

```javascript
const express = require('express');
const router = express.Router();

router.get('/', (req, res) => res.send('List of products'));
router.get('/:id', (req, res) => res.send(`Product ${req.params.id}`));

module.exports = router;
```

In the main file:

```javascript
const productsRouter = require('./routes/products');
app.use('/products', productsRouter);
```

Now `GET /products` and `GET /products/123` are handled by this module.

## 6. Routes can have several handlers

You can pass **several functions** (middleware) that run one after another:

```javascript
app.get('/admin',
  (req, res, next) => {
    console.log('Checking authorization...');
    next();
  },
  (req, res) => {
    res.send('Admin panel');
  }
);
```

## 7. Support for patterns and RegExp in routes

Express supports **pattern routes** and **regular expressions**:

```javascript
app.get('/ab*cd', (req, res) => {
  res.send('Matched the pattern /ab*cd');
});
```

This matches `/abcd`, `/ab123cd`, `/abXYZcd`, and so on.

## 8. Summary

> A **route** in Express.js is a rule that tells the server
> how to **respond to a specific HTTP request**.
>
> It consists of:
>
> - an HTTP method (`GET`, `POST`, `PUT`, `DELETE`, etc.),
> - a path (`/`, `/users/:id`),
> - a handler function (`(req, res)`).
>
> Routes:
>
> - drive the application's logic;
> - support dynamic parameters;
> - can be grouped into modules via `Router`;
> - support middleware and pattern paths.

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