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https://github.com/sindresorhus/emittery

Simple and modern async event emitter
https://github.com/sindresorhus/emittery

async emitter event-emitter event-listener javascript nodejs npm-package promise

Last synced: 5 days ago
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Simple and modern async event emitter

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# 



> Simple and modern async event emitter



[![](https://badgen.net/bundlephobia/minzip/emittery)](https://bundlephobia.com/result?p=emittery)



It works in Node.js and the browser (using a bundler).



**Highlights**



- Async-first — listeners are deferred to the next microtask, keeping your code non-blocking

- TypeScript support with strongly typed events

- Async iteration and `for await...of` support

- [Lifecycle hooks](#initeventname-initfn) (`init`/`deinit`) for lazy resource setup and teardown

- [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) support for cancellation

- [`Symbol.dispose`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/dispose) / [`Symbol.asyncDispose`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/asyncDispose) support for automatic cleanup

- [Meta events](#custom-subscribable-events) for observing listener changes

- [Debug mode](#debugging) with customizable logging

- Zero dependencies



Emitting events asynchronously is important for production code where you want the least amount of synchronous operations. Since JavaScript is single-threaded, no other code can run while doing synchronous operations. For Node.js, that means it will block other requests, defeating the strength of the platform, which is scalability through async. In the browser, a synchronous operation could potentially cause lags and block user interaction.



## Install



```sh

npm install emittery

```



## Usage



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.on('🦄', ({data}) => {

	console.log(data);

});



const myUnicorn = Symbol('🦄');



emitter.on(myUnicorn, ({data}) => {

	console.log(`Unicorns love ${data}`);

});



emitter.emit('🦄', '🌈'); // Will trigger printing '🌈'

emitter.emit(myUnicorn, '🦋');  // Will trigger printing 'Unicorns love 🦋'

```



## API



### eventName



Emittery accepts strings, symbols, and numbers as event names.



Symbol event names are preferred given that they can be used to avoid name collisions when your classes are extended, especially for internal events.



### isDebugEnabled



Toggle debug mode for all instances.



Default: `true` if the `DEBUG` environment variable is set to `emittery` or `*`, otherwise `false`.



Example:



```js

import Emittery from 'emittery';



Emittery.isDebugEnabled = true;



const emitter1 = new Emittery({debug: {name: 'myEmitter1'}});

const emitter2 = new Emittery({debug: {name: 'myEmitter2'}});



emitter1.on('test', () => {

	// …

});



emitter2.on('otherTest', () => {

	// …

});



emitter1.emit('test');

//=> [16:43:20.417][emittery:subscribe][myEmitter1] Event Name: test

//	data: undefined



emitter2.emit('otherTest');

//=> [16:43:20.417][emittery:subscribe][myEmitter2] Event Name: otherTest

//	data: undefined

```



### emitter = new Emittery(options?)



Create a new instance of Emittery.



#### options?



Type: `object`



Configure the new instance of Emittery.



##### debug?



Type: `object`



Configure the debugging options for this instance.



###### name



Type: `string`\

Default: `undefined`



Define a name for the instance of Emittery to use when outputting debug data.



Example:



```js

import Emittery from 'emittery';



Emittery.isDebugEnabled = true;



const emitter = new Emittery({debug: {name: 'myEmitter'}});



emitter.on('test', () => {

	// …

});



emitter.emit('test');

//=> [16:43:20.417][emittery:subscribe][myEmitter] Event Name: test

//	data: undefined

```



###### enabled?



Type: `boolean`\

Default: `false`



Toggle debug logging just for this instance.



Example:



```js

import Emittery from 'emittery';



const emitter1 = new Emittery({debug: {name: 'emitter1', enabled: true}});

const emitter2 = new Emittery({debug: {name: 'emitter2'}});



emitter1.on('test', () => {

	// …

});



emitter2.on('test', () => {

	// …

});



emitter1.emit('test');

//=> [16:43:20.417][emittery:subscribe][emitter1] Event Name: test

//	data: undefined



emitter2.emit('test');

```



###### logger?



Type: `Function(string, string, EventName?, Record?) => void`



Default:



```js

(type, debugName, eventName, eventData) => {

	try {

		eventData = JSON.stringify(eventData);

	} catch {

		eventData = `Object with the following keys failed to stringify: ${Object.keys(eventData).join(',')}`;

	}



	if (typeof eventName === 'symbol' || typeof eventName === 'number') {

		eventName = eventName.toString();

	}



	const currentTime = new Date();

	const logTime = `${currentTime.getHours()}:${currentTime.getMinutes()}:${currentTime.getSeconds()}.${currentTime.getMilliseconds()}`;

	console.log(`[${logTime}][emittery:${type}][${debugName}] Event Name: ${eventName}\n\tdata: ${eventData}`);

}

```



Function that handles debug data.



Example:



```js

import Emittery from 'emittery';



const myLogger = (type, debugName, eventName, eventData) => {

	console.log(`[${type}]: ${eventName}`);

};



const emitter = new Emittery({

	debug: {

		name: 'myEmitter',

		enabled: true,

		logger: myLogger

	}

});



emitter.on('test', () => {

	// …

});



emitter.emit('test');

//=> [subscribe]: test

```



#### on(eventName | eventName[], listener, options?: {signal?: AbortSignal})



Subscribe to one or more events.



Returns an unsubscribe method (which is also [`Disposable`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/dispose), so it can be used with `using`).



Using the same listener multiple times for the same event will result in only one method call per emitted event.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.on('🦄', ({data}) => {

	console.log(data);

});



emitter.on(['🦄', '🐶'], ({name, data}) => {

	console.log(name, data);

});



emitter.emit('🦄', '🌈'); // log => '🌈' and '🦄 🌈'

emitter.emit('🐶', '🍖'); // log => '🐶 🍖'

```



You can pass an [abort signal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) to unsubscribe too:



```js

import Emittery from 'emittery';



const emitter = new Emittery();

const abortController = new AbortController();



emitter.on('🐗', ({data}) => {

	console.log(data);

}, {signal: abortController.signal});



abortController.abort();

emitter.emit('🐗', '🍞'); // Nothing happens

```



Or use `using` for automatic cleanup when leaving scope:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



{

	using off = emitter.on('🦄', ({data}) => {

		console.log(data);

	});

	await emitter.emit('🦄', '🌈'); // Logs '🌈'

}



await emitter.emit('🦄', '🌈'); // Nothing happens

```



##### Custom subscribable events



Emittery exports some symbols which represent "meta" events that can be passed to `Emittery.on` and similar methods.



- `Emittery.listenerAdded` - Fires when an event listener was added.

- `Emittery.listenerRemoved` - Fires when an event listener was removed.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.on(Emittery.listenerAdded, ({data: {listener, eventName}}) => {

	console.log(listener);

	//=> ({data}) => {}



	console.log(eventName);

	//=> '🦄'

});



emitter.on('🦄', ({data}) => {

	// Handle data

});

```



###### Listener data



- `listener` - The listener that was added.

- `eventName` - The name of the event that was added or removed if `.on()` or `.off()` was used, or `undefined` if `.onAny()` or `.offAny()` was used.



Only events that are not of this type are able to trigger these events.



##### listener({name, data?})



#### off(eventName | eventName[], listener)



Remove one or more event subscriptions.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



const listener = ({data}) => {

	console.log(data);

};



emitter.on(['🦄', '🐶', '🦊'], listener);

await emitter.emit('🦄', 'a');

await emitter.emit('🐶', 'b');

await emitter.emit('🦊', 'c');

emitter.off('🦄', listener);

emitter.off(['🐶', '🦊'], listener);

await emitter.emit('🦄', 'a'); // Nothing happens

await emitter.emit('🐶', 'b'); // Nothing happens

await emitter.emit('🦊', 'c'); // Nothing happens

```



##### listener({name, data?})



#### once(eventName | eventName[], predicateOrOptions?)



Subscribe to one or more events only once. It will be unsubscribed after the first event that matches the predicate (if provided).



The second argument can be a predicate function or an options object with `predicate` and/or `signal`.



Returns a promise for the event data when `eventName` is emitted and predicate matches (if provided). This promise is extended with an `off` method.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



const {data} = await emitter.once('🦄');

console.log(data);

//=> '🌈'

```



```js

// With multiple event names

const {name, data} = await emitter.once(['🦄', '🐶']);

console.log(name, data);

```



```js

// With predicate

const event = await emitter.once('data', ({data}) => data.ok === true);

console.log(event.data);

//=> {ok: true, value: 42}

```



You can pass an [abort signal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) to cancel the subscription. If the signal is aborted before the event fires, the returned promise rejects with the signal's reason. This is useful for timeouts:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



// Reject if 'ready' doesn't fire within 5 seconds

await emitter.once('ready', {signal: AbortSignal.timeout(5000)});

```



The returned promise has an `off` method to cancel the subscription without rejecting:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



const promise = emitter.once('🦄');

// Cancel the subscription (the promise will never resolve)

promise.off();

```



#### events(eventName, options?: {signal?: AbortSignal})



Get an async iterator which buffers data each time an event is emitted.



Call `return()` on the iterator to remove the subscription. You can also pass an [abort signal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) to cancel the subscription externally, or use `await using` for automatic cleanup.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



for await (const {data} of emitter.events('🦄')) {

	console.log(data);



	if (data === '🌈2') {

		break; // Revoke the subscription when we see the value '🌈2'.

	}

}

```



It accepts multiple event names:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



for await (const {name, data} of emitter.events(['🦄', '🦊'])) {

	console.log(name, data);

}

```



You can use `await using` for automatic cleanup when leaving scope:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



{

	await using iterator = emitter.events('🦄');

	for await (const {data} of iterator) {

		console.log(data);

	}

} // Subscription is automatically revoked

```



Since Emittery requires Node.js 22+, you can use the built-in [async iterator helpers](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncIterator#iterator_helpers) to transform events:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



for await (const {data} of emitter.events('🦄').filter(event => event.data > 3).take(5)) {

	console.log(data);

}

```



#### emit(eventName, data?)



Trigger an event asynchronously, optionally with some data. Listeners are called in the order they were added, but executed concurrently.



Returns a promise that resolves when all the event listeners are done. *Done* meaning executed if synchronous or resolved when an async/promise-returning function. You usually wouldn't want to wait for this, but you could for example catch possible errors. If any listeners throw/reject, the returned promise rejects with an `AggregateError` — all listener errors are collected in `error.errors`, so no errors are silently lost. All listeners always run to completion, even if some throw or reject.



#### emitSerial(eventName, data?)



Same as above, but it waits for each listener to resolve before triggering the next one. This can be useful if your events depend on each other. Although ideally they should not. Prefer `emit()` whenever possible.



If any of the listeners throw/reject, the returned promise will be rejected with the error and the remaining listeners will *not* be called.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.on('🦄', async () => {

	console.log('listener 1 start');

	await new Promise(resolve => setTimeout(resolve, 100));

	console.log('listener 1 done');

});



emitter.on('🦄', () => {

	console.log('listener 2'); // Only runs after listener 1 is done

});



await emitter.emitSerial('🦄');

```



#### onAny(listener, options?: {signal?: AbortSignal})



Subscribe to be notified about any event.



Returns a method to unsubscribe (which is also [`Disposable`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/dispose)). Abort signal is respected too.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



const off = emitter.onAny(({name, data}) => {

	console.log(name, data);

});



emitter.emit('🦄', '🌈'); // log => '🦄 🌈'

emitter.emit('🐶', '🍖'); // log => '🐶 🍖'



off();

```



##### listener({name, data?})



#### offAny(listener)



Remove an `onAny` subscription.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



const listener = ({name, data}) => {

	console.log(name, data);

};



emitter.onAny(listener);

emitter.emit('🦄', '🌈'); // log => '🦄 🌈'

emitter.offAny(listener);

emitter.emit('🦄', '🌈'); // Nothing happens

```



#### anyEvent(options?: {signal?: AbortSignal})



Get an async iterator which buffers an event object each time an event is emitted.



Call `return()` on the iterator to remove the subscription. You can also pass an [abort signal](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) to cancel the subscription externally, or use `await using` for automatic cleanup.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



for await (const {name, data} of emitter.anyEvent()) {

	console.log(name, data);

}

```



You can use `await using` for automatic cleanup when leaving scope:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



{

	await using iterator = emitter.anyEvent();

	for await (const {name, data} of iterator) {

		console.log(name, data);

	}

} // Subscription is automatically revoked

```



#### clearListeners(eventNames?)



Clear all event listeners on the instance.



If `eventNames` is given, only the listeners for those events are cleared. Accepts a single event name or an array.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.on('🦄', listener);

emitter.on('🐶', listener);

emitter.on('🦊', listener);



// Clear a single event

emitter.clearListeners('🦄');



// Clear multiple events

emitter.clearListeners(['🐶', '🦊']);



// Clear all events

emitter.clearListeners();

```



#### init(eventName, initFn)



Register a function to be called when the first `.on()` listener subscribes to `eventName`. The `initFn` can optionally return a cleanup (deinit) function, which is called when the last `.on()` listener unsubscribes (or when `clearListeners()` removes all listeners for that event).



If `.on()` listeners already exist when `init()` is called, `initFn` is called immediately.



Returns an unsubscribe function (which is also [`Disposable`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/dispose)). Calling it removes the init/deinit hooks, and if the init is currently active, it calls deinit immediately.



> [!NOTE]

> Lifecycle hooks only apply to `.on()` listeners. Subscriptions via `.events()` async iterators do not trigger the init or deinit functions.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.init('mouse', () => {

	terminal.grabInput({mouse: 'button'});



	terminal.on('mouse', (name, data) => {

		emitter.emit('mouse', data);

	});



	// Optional: return cleanup (deinit) function

	return () => {

		terminal.releaseInput();

	};

});



// Init is called when the first listener subscribes

const off = emitter.on('mouse', handler);



// Adding more listeners does not call init again

emitter.on('mouse', anotherHandler);



// Removing one listener does not call deinit yet

off();



// Deinit is called when the last listener unsubscribes

emitter.off('mouse', anotherHandler);

```



You can use `using` for automatic cleanup of the init/deinit hooks:



```js

import Emittery from 'emittery';



const emitter = new Emittery();



{

	using removeInit = emitter.init('mouse', () => {

		startListening();

		return () => stopListening();

	});

} // init/deinit hooks are automatically removed

```



#### listenerCount(eventNames?)



The number of listeners for the `eventNames` or all events if not specified.



```js

import Emittery from 'emittery';



const emitter = new Emittery();



emitter.on('🦄', listener);

emitter.on('🐶', listener);



emitter.listenerCount('🦄'); // 1

emitter.listenerCount(); // 2

```



#### bindMethods(target, methodNames?)



Bind the given `methodNames`, or all `Emittery` methods if `methodNames` is not defined, into the `target` object.



```js

import Emittery from 'emittery';



const object = {};



new Emittery().bindMethods(object);



object.emit('event');

```



## TypeScript



The default `Emittery` class has generic types that can be provided by TypeScript users to strongly type the list of events and the data passed to their event listeners.



```ts

import Emittery from 'emittery';



const emitter = new Emittery<

	// Pass `{[eventName]: undefined | }` as the first type argument for events that pass data to their listeners.

	// A value of `undefined` in this map means the event listeners should expect no data, and a type other than `undefined` means the listeners will receive one argument of that type.

	{

		open: string,

		close: undefined

	}

>();



// Typechecks just fine because the data type for the `open` event is `string`.

emitter.emit('open', 'foo\n');



// Typechecks just fine because `close` is present but points to undefined in the event data type map.

emitter.emit('close');



// TS compilation error because `1` isn't assignable to `string`.

emitter.emit('open', 1);



// TS compilation error because `other` isn't defined in the event data type map.

emitter.emit('other');

```



### Emittery.mixin(emitteryPropertyName, methodNames?)



A decorator which mixins `Emittery` as property `emitteryPropertyName` and `methodNames`, or all `Emittery` methods if `methodNames` is not defined, into the target class.



```ts

import Emittery from 'emittery';



@Emittery.mixin('emittery')

class MyClass {}



const instance = new MyClass();



instance.emit('event');

```



## Scheduling details



Listeners are not invoked for events emitted *before* the listener was added. Removing a listener will prevent that listener from being invoked, even if events are in the process of being (asynchronously!) emitted. This also applies to `.clearListeners()`, which removes all listeners. Listeners will be called in the order they were added. So-called *any* listeners are called *after* event-specific listeners.



Listeners always fire asynchronously — they are deferred to the next microtask, so any synchronous code after an unawaited `emit()` call runs first. If ordering matters, use `await emit()`.



Note that when using `.emitSerial()`, a slow listener will delay invocation of subsequent listeners. It's possible for newer events to overtake older ones.



## Debugging



Emittery can collect and log debug information.



To enable this feature set the DEBUG environment variable to `'emittery'` or `'*'`. Additionally you can set the static `isDebugEnabled` variable to true on the Emittery class, or `myEmitter.debug.enabled` on an instance of it for debugging a single instance.



See [API](#api) for more details on how debugging works.



## FAQ



### How is this different than the built-in `EventEmitter` in Node.js?



There are many things to not like about `EventEmitter`: its huge API surface, synchronous event emitting, magic error event, flawed memory leak detection. Emittery has none of that.



### Isn't `EventEmitter` synchronous for a reason?



Mostly backwards compatibility reasons. The Node.js team can't break the whole ecosystem.



It also allows silly code like this:



```js

let unicorn = false;



emitter.on('🦄', () => {

	unicorn = true;

});



emitter.emit('🦄');



console.log(unicorn);

//=> true

```



But I would argue doing that shows a deeper lack of Node.js and async comprehension and is not something we should optimize for. The benefit of async emitting is much greater.



### Can you support synchronous `emit()`?



No. Async emission is Emittery's core design principle. If you need synchronous event emission (for example, proxying DOM events like React's `onChange`), use a synchronous event emitter.



### Can you support multiple arguments for `emit()`?



No, just use [destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment):



```js

emitter.on('🦄', ({data: [foo, bar]}) => {

	console.log(foo, bar);

});



emitter.emit('🦄', [foo, bar]);

```



## Related



- [p-event](https://github.com/sindresorhus/p-event) - Promisify an event by waiting for it to be emitted