Simple Observable

Implement a simple Observable class with subscribe, next, complete, and unsubscribe functionality.

Requirements

• subscribe(observer) — register an observer with next and optional complete callbacks. Return an unsubscribe function.
• next(value) — emit a value to all active subscribers.
• complete() — notify all subscribers of completion and prevent further emissions.

Example

const obs = new Observable();
const unsub = obs.subscribe({ next: v => console.log(v) });
obs.next('hello'); // logs 'hello'
unsub();
obs.next('world'); // nothing (unsubscribed)