Add .listenerCount() method

Fixes #58

Author: sindresorhus

index.d.ts

@@ -58,13 +58,35 @@ type Emitter = {
 	*/
 	listeners(event: string): Array<(...arguments_: any[]) => void>;
 
+	/**
+	Get the count of listeners for a specific event.
+
+	@param event - The name of the event.
+	@returns The number of listeners for the event.
+	*/
+	listenerCount(event: string): number; // eslint-disable-line @typescript-eslint/unified-signatures
+
+	/**
+	Get the count of all event handlers in total.
+
+	@returns The total number of event handlers.
+	*/
+	listenerCount(): number;
+
 	/**
 	Check if there are any handlers registered for a specific event.
 
 	@param event - The name of the event.
 	@returns `true` if there are one or more handlers, `false` otherwise.
 	*/
-	hasListeners(event: string): boolean;
+	hasListeners(event: string): boolean; // eslint-disable-line @typescript-eslint/unified-signatures
+
+	/**
+	Check if there are any handlers registered for any event.
+
+	@returns `true` if there are one or more handlers for any event, `false` otherwise.
+	*/
+	hasListeners(): boolean;
 };
 
 type EmitterConstructor = {

index.js

@@ -78,8 +78,21 @@ 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.listeners(event).length > 0;
+	return this.listenerCount(event) > 0;
 };
 
 // Aliases

readme.md

@@ -73,10 +73,22 @@ Emit an event, invoking all handlers registered for it.
 
 Retrieve the event handlers registered for a specific event.
 
+### Emitter#listenerCount(event)
+
+Get the count of listeners for a specific event.
+
+### Emitter#listenerCount()
+
+Get the count of all event handlers in total.
+
 ### Emitter#hasListeners(event)
 
 Check if there are any handlers registered for a specific event.
 
+### Emitter#hasListeners()
+
+Check if there are any handlers registered for any event.
+
 ## FAQ
 
 ### How does it differ from the Node.js EventEmitter?

test.js

@@ -366,3 +366,48 @@ test('Alias methods should work as expected', t => {
 	emitter.emit('foo');
 	t.false(isCalled);
 });
+
+test('Emitter.listenerCount returns the correct count for a specific event', t => {
+	const emitter = new Emitter();
+
+	emitter.on('foo', () => {});
+	emitter.on('foo', () => {});
+	emitter.once('bar', () => {});
+
+	t.is(emitter.listenerCount('foo'), 2);
+	t.is(emitter.listenerCount('bar'), 1);
+	t.is(emitter.listenerCount('baz'), 0);
+});
+
+test('Emitter.listenerCount returns the correct count for all events', t => {
+	const emitter = new Emitter();
+
+	emitter.on('foo', () => {});
+	emitter.on('bar', () => {});
+	emitter.once('baz', () => {});
+
+	t.is(emitter.listenerCount(), 3);
+});
+
+test('Emitter.hasListeners returns true when there are listeners for a specific event', t => {
+	const emitter = new Emitter();
+
+	emitter.on('foo', () => {});
+	emitter.on('bar', () => {});
+	emitter.once('baz', () => {});
+
+	t.true(emitter.hasListeners('foo'));
+	t.true(emitter.hasListeners('bar'));
+	t.true(emitter.hasListeners('baz'));
+	t.false(emitter.hasListeners('qux'));
+});
+
+test('Emitter.hasListeners returns true when there are listeners for any event', t => {
+	const emitter = new Emitter();
+
+	emitter.on('foo', () => {});
+	emitter.once('bar', () => {});
+	emitter.once('baz', () => {});
+
+	t.true(emitter.hasListeners());
+});