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## 1. Subscribing via `.on(eventName, listener)`

`.on()` registers a handler (listener) for a given event.
When the event fires via `.emit()`, every handler subscribed through `.on()` gets called.

### Example:

```javascript
const EventEmitter = require('events');
const emitter = new EventEmitter();

// Subscribe to the 'message' event
emitter.on('message', (data) => {
  console.log('Message received:', data);
});

// Emit the event
emitter.emit('message', 'Hello, Node.js!');
```

Output:

```javascript
Message received: Hello, Node.js!
```

## 2. Subscribing via `.once(eventName, listener)`

`.once()` is like `.on()`,
but the handler fires **only once**, then removes itself automatically.

### Example:

```javascript
emitter.once('connect', () => {
  console.log('Connection established (once only)');
});

emitter.emit('connect'); // fires
emitter.emit('connect'); // won't fire again
```

## 3. Several subscribers on the same event

Several handlers can subscribe to the same event,
they fire **in registration order**:

```javascript
emitter.on('update', () => console.log('First handler'));
emitter.on('update', () => console.log('Second handler'));

emitter.emit('update');
```

Output:

```javascript
First handler
Second handler
```

## 4. Removing a subscription

To **unsubscribe** from an event, use `.off()` or `.removeListener()`:

```javascript
function handler() {
  console.log('An event happened');
}

emitter.on('event', handler);
emitter.emit('event'); // fires

emitter.off('event', handler);
emitter.emit('event'); // doesn't fire
```

## 5. Where subscribing to events is used in Node.js

Many built-in objects are `EventEmitter`s, with the same `.on()` and `.once()` methods:

```javascript
const fs = require('fs');

const stream = fs.createReadStream('file.txt');

// Subscribing to system events
stream.on('data', (chunk) => console.log('Got a chunk of data'));
stream.on('end', () => console.log('Reading finished'));
stream.on('error', (err) => console.error('Error:', err));
```

The same works with:

- `http.Server` (`on('request', handler)`)
- `net.Socket` (`on('connect', handler)`)
- `process` (`on('exit', handler)`)

## Summary

> To **subscribe to an event** in Node.js,
> use the `.on(eventName, listener)` method of an `EventEmitter` object.
>
> - `.on()`, a permanent subscription
> - `.once()`, a one-time subscription
> - `.off()` / `.removeListener()`, unsubscribing
>
> This mechanism is used **everywhere**:
> in streams, HTTP, the filesystem, processes, and custom events.

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