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Simple and modern async event emitter
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readme.md

Simple and modern async event emitter

Build Status codecov

It's only ~200 bytes minified and gzipped. I'm not fanatic about keeping the size at this level though.

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

Emitting events asynchronously is important for production code where you want the least amount of synchronous operations.

Install

$ npm install emittery

Usage

const Emittery = require('emittery');
const emitter = new Emittery();

emitter.on('🦄', data => {
	console.log(data);
	// '🌈'
});

emitter.emit('🦄', '🌈');

Node.js 6

The above only works in Node.js 8 or newer. For older Node.js versions you can use require('emittery/legacy').

API

emitter = new Emittery()

on(eventName, listener)

Subscribe to an event.

Returns an unsubscribe method.

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

listener(data)

off(eventName, listener)

Remove an event subscription.

listener(data)

once(eventName)

Subscribe to an event only once. It will be unsubscribed after the first event.

Returns a promise for the event data when eventName is emitted.

emitter.once('🦄').then(data => {
	console.log(data);
	//=> '🌈'
});

emitter.emit('🦄', '🌈');

emit(eventName, [data])

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

Returns a promise for 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 of the listeners throw/reject, the returned promise will be rejected with the error, but the other listeners will not be affected.

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.

onAny(listener)

Subscribe to be notified about any event.

Returns a method to unsubscribe.

listener(eventName, data)

offAny(listener)

Remove an onAny subscription.

clearListeners()

Clear all event listeners on the instance.

If eventName is given, only the listeners for that event are cleared.

listenerCount([eventName])

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

TypeScript

Definition for emittery and emittery/legacy are included. Use import Emittery = require('emittery') or import Emittery = require('emittery/legacy') to load the desired implementation.

The default Emittery class does not let you type allowed event names and their associated data. However you can use Emittery.Typed with generics:

import Emittery = require('emittery');

const ee = new Emittery.Typed<{value: string}, 'open' | 'close'>();

ee.emit('open');
ee.emit('value', 'foo\n');
ee.emit('value', 1); // TS compilation error
ee.emit('end'); // TS compilation error

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.

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

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:

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 multiple arguments for emit()?

No, just use destructuring:

emitter.on('🦄', ([foo, bar]) => {
	console.log(foo, bar);
});

emitter.emit('🦄', [foo, bar]);

Related

  • p-event - Promisify an event by waiting for it to be emitted

License

MIT © Sindre Sorhus

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