Observable

Requirements

• Export Observable<T> as named and default. Construct it with a producer function receiving a subscriber with next(value), error(reason), complete(), and a readonly closed flag. No producer runs until subscribe(observer). Each subscription invokes the producer independently.
• subscribe accepts an object with optional next, error, and complete callbacks and returns a subscription with readonly closed and unsubscribe(). Deliver next synchronously while open. error and complete close the subscription before notifying the matching callback, and ignore all later notifications.
• Calling unsubscribe closes silently without calling complete. Repeated unsubscribe or terminal calls are harmless. The producer may return a cleanup function; run it exactly once on unsubscribe, error, or completion. Handle synchronous termination before the producer returns its cleanup function.
• If the producer throws while open, deliver the exact thrown reason through error and close the subscription. An omitted error callback consumes the reason without throwing globally. Missing next/complete callbacks are allowed. Both producer and callbacks must be functions when supplied; invalid arguments throw TypeError before starting the producer.
• Callbacks and cleanup functions are assumed not to throw, and a producer returns only void or a cleanup function. Async producers may capture the subscriber and notify later; closed prevents delivery after cancellation. No operators, promises, scheduler, multicasting, or RxJS dependency is required.

Examples

Input: const values = []; const source = new Observable(s => { s.next(1); s.next(2); s.complete(); return () => values.push("cleanup"); }); const sub = source.subscribe({ next: x => values.push(x) }); [values, sub.closed];
Output: [[1, 2, "cleanup"], true]

Input: let emit; const source = new Observable(s => { emit = s; }); const sub = source.subscribe({ next: console.log }); sub.unsubscribe(); emit.next(3);
Output: No value is delivered after unsubscribe.