class EventEmitter {

    constructor() {
        this.eventListenersMap = new Map();
    }

    on(event, callback) {

        // create list of listeners for event if it doesn't exist
        if(!this.eventListenersMap.has(event)){
            this.eventListenersMap.set(event, []);
        }

        // add listener for event
        this.eventListenersMap.get(event).push(callback);

    }

    off(event, callback) {

        // remove callback from listeners for this event
        if(this.eventListenersMap.has(event)){
            const callbacks = this.eventListenersMap.get(event).filter(cb => cb !== callback);
            this.eventListenersMap.set(event, callbacks);
        }

    }

    once(event, callback) {

        // internal callback to handle the event
        const internalCallback = (...data) => {

            // we received an event, so lets remove the event listener
            this.off(event, internalCallback);

            // fire the original callback provided by the user
            setTimeout(() => callback(...data), 0);

        };

        // listen to this event
        this.on(event, internalCallback);

    }

    emit(event, ...data) {

        // invoke each listener for this event
        if(this.eventListenersMap.has(event)){
            for(const eventListener of this.eventListenersMap.get(event)){
                setTimeout(() => eventListener(...data), 0);
            }
        }

    }

}

export default EventEmitter;