What is the observer pattern and how is it different from EventEmitter?

Question

Accepted Answer

**Observer pattern** — one-to-many dependency where observers subscribe to a subject. **EventEmitter** — Node.js implementation of the observer pattern.

**Observer pattern (custom):**
```javascript
class Subject {
    constructor() {
        this.observers = [];
    }

subscribe(observer) {
        this.observers.push(observer);
        return () => {
            this.observers = this.observers.filter(o => o !== observer);
        };
    }

notify(data) {
        this.observers.forEach(o => o(data));
    }
}

const subject = new Subject();
const unsub = subject.subscribe(data => console.log(data));
subject.notify("hello");
unsub(); // unsubscribe
```

**EventEmitter:**
```javascript
const EventEmitter = require("events");
const emitter = new EventEmitter();
emitter.on("data", (msg) => console.log(msg));
emitter.emit("data", "hello");
```

**Key differences:**
- EventEmitter is event-name based.
- Observer is simpler, no event names.
- EventEmitter has once(), removeAllListeners().

Node.js: What is the observer pattern and how is it different from EventEmitter?