index.js

function Emitter(object) {
	if (object) {
		return mixin(object);
	}

	this._callbacks = new Map();
}

function mixin(object) {
	Object.assign(object, Emitter.prototype);
	object._callbacks = new Map();
	return object;
}

Emitter.prototype.on = function (event, listener) {
	const callbacks = this._callbacks.get(event) ?? [];
	callbacks.push(listener);
	this._callbacks.set(event, callbacks);
	return this;
};

Emitter.prototype.once = function (event, listener) {
	const on = (...arguments_) => {
		this.off(event, on);
		listener.apply(this, arguments_);
	};

	on.fn = listener;
	this.on(event, on);
	return this;
};

Emitter.prototype.off = function (event, listener) {
	if (event === undefined && listener === undefined) {
		this._callbacks.clear();
		return this;
	}

	if (listener === undefined) {
		this._callbacks.delete(event);
		return this;
	}

	const callbacks = this._callbacks.get(event);
	if (callbacks) {
		for (const [index, callback] of callbacks.entries()) {
			if (callback === listener || callback.fn === listener) {
				callbacks.splice(index, 1);
				break;
			}
		}

		if (callbacks.length === 0) {
			this._callbacks.delete(event);
		} else {
			this._callbacks.set(event, callbacks);
		}
	}

	return this;
};

Emitter.prototype.emit = function (event, ...arguments_) {
	const callbacks = this._callbacks.get(event);
	if (callbacks) {
		// Create a copy of the callbacks array to avoid issues if it's modified during iteration
		const callbacksCopy = [...callbacks];

		for (const callback of callbacksCopy) {
			callback.apply(this, arguments_);
		}
	}

	return this;
};

Emitter.prototype.listeners = function (event) {
	return this._callbacks.get(event) ?? [];
};

Emitter.prototype.listenerCount = function (event) {
	if (event) {
		return this.listeners(event).length;
	}

	let totalCount = 0;
	for (const callbacks of this._callbacks.values()) {
		totalCount += callbacks.length;
	}

	return totalCount;
};

Emitter.prototype.hasListeners = function (event) {
	return this.listenerCount(event) > 0;
};

// Aliases
Emitter.prototype.addEventListener = Emitter.prototype.on;
Emitter.prototype.removeListener = Emitter.prototype.off;
Emitter.prototype.removeEventListener = Emitter.prototype.off;
Emitter.prototype.removeAllListeners = Emitter.prototype.off;

if (typeof module !== 'undefined') {
	module.exports = Emitter;
}