Solution.ts

type Callback = (...args: any[]) => any;
type Subscription = {
    unsubscribe: () => void;
};

class EventEmitter {
    private d: Map<string, Set<Callback>> = new Map();

    subscribe(eventName: string, callback: Callback): Subscription {
        this.d.set(eventName, (this.d.get(eventName) || new Set()).add(callback));
        return {
            unsubscribe: () => {
                this.d.get(eventName)?.delete(callback);
            },
        };
    }

    emit(eventName: string, args: any[] = []): any {
        const callbacks = this.d.get(eventName);
        if (!callbacks) {
            return [];
        }
        return [...callbacks].map(callback => callback(...args));
    }
}

/**
 * const emitter = new EventEmitter();
 *
 * // Subscribe to the onClick event with onClickCallback
 * function onClickCallback() { return 99 }
 * const sub = emitter.subscribe('onClick', onClickCallback);
 *
 * emitter.emit('onClick'); // [99]
 * sub.unsubscribe(); // undefined
 * emitter.emit('onClick'); // []
 */