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## 1. What `on()` does

> `.on(eventName, listener)` registers a handler (function) for a given event.
> Every time an event named `eventName` fires via `.emit()`,
> every handler added through `.on()` is called in registration order.

### Example:

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

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

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

Output:

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

## 2. Syntax

```javascript
emitter.on(eventName, listener)
```

- `eventName`, a string, the event's name (e.g. `'data'`, `'error'`, `'end'`).
- `listener`, the handler function that gets called when the event occurs.

## 3. Example with several listeners

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

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

Output:

```javascript
First listener
Second listener
```

## 4. How `on()` differs from `addListener()`

| Method | Difference |
|---|---|
| `.on()` | The modern, more readable method name |
| `.addListener()` | The old name, kept for compatibility (works exactly the same way) |

In fact, `on()` **is an alias (synonym)** for `addListener()`:

```javascript
EventEmitter.prototype.on === EventEmitter.prototype.addListener // true
```

That is:

```javascript
emitter.on('event', listener);
```

and

```javascript
emitter.addListener('event', listener);
```

do **exactly the same thing**.

## 5. Which one to use, and when

- Use `.on()`, it's the modern, familiar choice (used throughout the official docs and every example).
- `.addListener()` remains for backward compatibility with earlier Node.js versions.

## Summary

> `on()` subscribes a function to an event (called on every `emit()`).
> `addListener()` is a completely identical method, just an older name.
> There is **no difference in behavior at all**, they're aliases for the same method.
>
> `.on()` is recommended as the modern standard.

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