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p-event

Promisify an event by waiting for it to be emitted

Useful when you need only one event emission and want to use it with promises or await it in an async function.

If you want multiple individual events as they are emitted, this module is not for you, as a Promise is a single value. Instead, just continue using event callback or use Observables.

Install

$ npm install --save p-event

Usage

const pEvent = require('p-event');
const emitter = require('./some-event-emitter');

pEvent(emitter, 'finish')
	// Called when `emitter` emits a `finish` event
	.then(result => {
		console.log(result);
	})
	// Called when `emitter` emits an `error` event
	.catch(error => {
		console.error(error);
	});

const pEvent = require('p-event');

pEvent(document, 'DOMContentLoaded').then(() => {
	console.log('😎');
});

API

pEvent(emitter, event, [options])

pEvent(emitter, event, filter)

Returns a Promise that is fulfilled when emitter emits an event matching event, or rejects if emitter emits any of the events defined in the rejectionEvents option.

The returned promise has a .cancel() method, which when called, removes the event listeners and causes the promise to never be settled.

emitter

Type: Object

Event emitter object.

Should have either a .on()/.addListener()/.addEventListener() and .off()/.removeListener()/.removeEventListener() method, like the Node.js EventEmitter and DOM events.

event

Type: string

Name of the event to listen to.

If the same event is defined both here and in rejectionEvents, this one takes priority.

options

Type: Object

rejectionEvents

Type: Array
Default: ['error']

Events that will reject the promise.

multiArgs

Type: boolean
Default: false

By default, the promisified function will only return the first argument from the event callback, which works fine for most APIs. This option can be useful for APIs that return multiple arguments in the callback. Turning this on will make it return an array of all arguments from the callback, instead of just the first argument. This also applies to rejections.

Example:

const pEvent = require('p-event');
const emitter = require('./some-event-emitter');

pEvent(emitter, 'finish', {multiArgs: true}).then(result => {
	const [id, type] = result;
});

timeout

Type: Number
Default: Infinity

Time in milliseconds before timing out.

filter

Type: Function

Filter function for accepting an event.

const pEvent = require('p-event');
const emitter = require('./some-event-emitter');

pEvent(emitter, '🦄', value => value > 3).then(result => {
	// Do something with first 🦄 event with a value greater than 3
});

Before and after

const fs = require('fs');

function getOpenReadStream(file, callback) {
	const stream = fs.createReadStream(file);

	stream.on('open', () => {
		callback(null, stream);
	});

	stream.on('error', error => {
		callback(error);
	});
}

getOpenReadStream('unicorn.txt', (error, stream) => {
	if (error) {
		console.error(error);
		return;
	}

	console.log('File descriptor:', stream.fd);
	stream.pipe(process.stdout);
});

const fs = require('fs');
const pEvent = require('p-event');

async function getOpenReadStream(file) {
	const stream = fs.createReadStream(file);
	await pEvent(stream, 'open');
	return stream;
}

getOpenReadStream('unicorn.txt')
	.then(stream => {
		console.log('File descriptor:', stream.fd);
		stream.pipe(process.stdout);
	})
	.catch(console.error);

pify - Promisify a callback-style function
p-map - Map over promises concurrently
More…

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