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## 1. What Request and Response are

> In Express.js:
>
> - `req` **(request)** is the object representing the **incoming HTTP request** from the client;
> - `res` **(response)** is the object the server uses to **send a response** back to the client.

When a client makes a request (say, `GET /users?id=10`), Express automatically creates these two objects and passes them into your route handler.

### Example:

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

app.get('/hello', (req, res) => {
  console.log(req.method); // "GET"
  console.log(req.url);    // "/hello"
  res.send('Hello, Express!');
});

app.listen(3000);
```

Client → server:

```javascript
GET /hello
```

The server responds:

```javascript
Hello, Express!
```

## 2. The `req` (Request) object

> `req` is the object representing the **HTTP request** that came in from the client.

It holds everything the client sent: the URL, parameters, body, headers, and so on.

### `req`'s main properties

| Property | Description | Example |
|---|---|---|
| `req.method` | The request's HTTP method | `'GET'`, `'POST'`, `'PUT'`, `'DELETE'` |
| `req.url` | The full request URL | `'/users?name=tim'` |
| `req.path` | Just the path, no query | `'/users'` |
| `req.query` | The query-parameters object | `{ name: 'tim' }` |
| `req.params` | The route parameters | `/users/:id` → `{ id: '123' }` |
| `req.body` | The request body (for POST/PUT) | `{ name: 'John' }` *(with* `express.json()`*)* |
| `req.headers` | The HTTP headers | `{ 'content-type': 'application/json' }` |
| `req.cookies` | Cookies (with cookie-parser attached) | `{ token: 'abc123' }` |
| `req.ip` | The client's IP address | `'127.0.0.1'` |
| `req.protocol` | `'http'` or `'https'` | `'http'` |
| `req.get(headerName)` | Get a specific header | `req.get('User-Agent')` |

### An example request with parameters:

```javascript
app.get('/user/:id', (req, res) => {
  console.log(req.params.id); // path: /user/42 → "42"
  console.log(req.query);     // query: ?name=Tim → { name: 'Tim' }
  res.send('User found');
});
```

## 3. The `res` (Response) object

> `res` is the object Express uses to **send a response back to the client**.

You can:

- return text, JSON, HTML;
- set the status, headers, cookies;
- end the response.

### `res`'s main methods

| Method | Purpose | Example |
|---|---|---|
| `res.send(data)` | Send text, HTML, or JSON | `res.send('Hello!')` |
| `res.json(obj)` | Send a JSON object | `res.json({ ok: true })` |
| `res.status(code)` | Set the response status | `res.status(404).send('Not Found')` |
| `res.set(header, value)` | Set an HTTP header | `res.set('X-Powered-By', 'Express')` |
| `res.redirect(url)` | Redirect the client | `res.redirect('/login')` |
| `res.download(file)` | Send a file for download | `res.download('report.pdf')` |
| `res.sendFile(path)` | Send a file as the response | `res.sendFile(__dirname + '/index.html')` |
| `res.end()` | End the response with no body | `res.end()` |

### An example with JSON and a status

```javascript
app.post('/api/users', (req, res) => {
  const user = req.body;
  console.log(user);
  res.status(201).json({
    message: 'User created',
    data: user
  });
});
```

Response:

```javascript
{
  "message": "User created",
  "data": { "name": "Tim", "age": 25 }
}
```

## 4. How Express ties req and res together

Every time a client makes an HTTP request:

```javascript
Client → [Express App] → Middleware → Route Handler → res.send()
```

Express creates a pair of objects:

- `req`, to carry the request's data;
- `res`, to send back the response.

They only live **for the duration of handling one request**.

## 5. An example using both objects

```javascript
app.post('/feedback', (req, res) => {
  const { name, message } = req.body;
  console.log(`From ${name}: ${message}`);

res.status(200).send(`Thank you, ${name}, your message has been received!`);
});
```

## 6. Summary

> `req` **(Request)** is the incoming-request object:
> it holds the method, path, parameters, body, headers, and other client data.
>
> `res` **(Response)** is the response object:
> used to send data to the client and set the status and headers.
>
> Together they form the **foundation of the client-server interaction** in Express.js.

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