Event Emitter II

Requirements

• Export EventEmitter as named and default. subscribe(event, callback) returns an object with unsubscribe(). Event names are strings and callbacks run synchronously.
• emit(event, ...args) calls subscriptions in registration order, with the exact arguments, and returns true if at least one subscription existed at the start; otherwise false. Callback return values are ignored.
• Each subscription has its own identity. Registering the same callback twice invokes it twice; unsubscribing one registration leaves the other active. unsubscribe() is idempotent.
• Emit from a snapshot: subscriptions added during emission wait for the next emit; subscriptions removed during emission still run in the current snapshot. Nested emits take their own fresh snapshot.
• Propagate callback errors immediately and stop the current emission. Do not remove subscriptions automatically on error. Keep events and emitter instances independent. Callbacks receive no emitter-bound this context.

Examples

Input: const emitter = new EventEmitter();
const subscription = emitter.subscribe("ready", (...args) => console.log(args));
emitter.emit("ready", 1, "ok");
subscription.unsubscribe();
emitter.emit("ready", 2);
Output: First emit logs [1, "ok"] and returns true; second returns false.

Input: const a = emitter.subscribe("change", callback);
const b = emitter.subscribe("change", callback);
a.unsubscribe();
Output: The registration returned as b remains active.