A simple JS Event Bus

eventbus.js

class EventBus {
	subscriptions = {};

	on(eventType, callback) {
		const id = Symbol('id');

		if (!this.subscriptions[eventType]) this.subscriptions[eventType] = {};
		this.subscriptions[eventType][id] = callback;

		const thiz = this;

		return {
			id,
			eventType,
			off() {
				thiz.off(eventType, id);
			},
		};
	};

	off(eventType, id) {
		if (!id) {
			delete this.subscriptions[eventType];
			return;
		}

		delete this.subscriptions[eventType][id];
		if (Object.getOwnPropertySymbols(this.subscriptions[eventType]).length === 0) {
			delete this.subscriptions[eventType];
		}
	}

	once(eventType, callback) {
		let s = undefined;
		const wrap = (arg) => {
			try {
				callback(arg);
			} finally {
				s.off();
			}
		}

		s = this.on(eventType, wrap);

		return s;
	}

	emit(eventType, arg) {
		if (!this.subscriptions[eventType]) return;

		Object.getOwnPropertySymbols(this.subscriptions[eventType])
			.forEach(key => this.subscriptions[eventType][key](arg));
	};
}

export default EventBus;