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    p-event
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    5.0.1 • Public • Published

    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.

    It works with any event API in Node.js and the browser (using a bundler).

    If you want multiple individual events as they are emitted, you can use the pEventIterator() method. Observables can be useful too.

    Install

    npm install p-event

    Usage

    In Node.js:

    import {pEvent} from 'p-event';
    import emitter from './some-event-emitter';
    
    try {
    	const result = await pEvent(emitter, 'finish');
    
    	// `emitter` emitted a `finish` event
    	console.log(result);
    } catch (error) {
    	// `emitter` emitted an `error` event
    	console.error(error);
    }

    In the browser:

    import {pEvent} from 'p-event';
    
    await pEvent(document, 'DOMContentLoaded');
    console.log('😎');

    Async iteration:

    import {pEventIterator} from 'p-event';
    import emitter from './some-event-emitter';
    
    const asyncIterator = pEventIterator(emitter, 'data', {
    	resolutionEvents: ['finish']
    });
    
    for await (const event of asyncIterator) {
    	console.log(event);
    }

    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.

    Note: event is a string for a single event type, for example, 'data'. To listen on multiple events, pass an array of strings, such as ['started', 'stopped'].

    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 | string[]

    Name of the event or events to listen to.

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

    options

    Type: object

    rejectionEvents

    Type: string[]
    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:

    import {pEvent} from 'p-event';
    import emitter from './some-event-emitter';
    
    const [foo, bar] = await pEvent(emitter, 'finish', {multiArgs: true});
    timeout

    Type: number
    Default: Infinity

    Time in milliseconds before timing out.

    filter

    Type: Function

    A filter function for accepting an event.

    import {pEvent} from 'p-event';
    import emitter from './some-event-emitter';
    
    const result = await pEvent(emitter, '🦄', value => value > 3);
    // Do something with first 🦄 event with a value greater than 3

    pEventMultiple(emitter, event, options)

    Wait for multiple event emissions. Returns an array.

    This method has the same arguments and options as pEvent() with the addition of the following options:

    options

    Type: object

    count

    Required
    Type: number

    The number of times the event needs to be emitted before the promise resolves.

    resolveImmediately

    Type: boolean
    Default: false

    Whether to resolve the promise immediately. Emitting one of the rejectionEvents won't throw an error.

    Note: The returned array will be mutated when an event is emitted.

    Example:

    import {pEventMultiple} from 'p-event';
    
    const emitter = new EventEmitter();
    
    const promise = pEventMultiple(emitter, 'hello', {
    	resolveImmediately: true,
    	count: Infinity
    });
    
    const result = await promise;
    console.log(result);
    //=> []
    
    emitter.emit('hello', 'Jack');
    console.log(result);
    //=> ['Jack']
    
    emitter.emit('hello', 'Mark');
    console.log(result);
    //=> ['Jack', 'Mark']
    
    // Stops listening
    emitter.emit('error', new Error('😿'));
    
    emitter.emit('hello', 'John');
    console.log(result);
    //=> ['Jack', 'Mark']

    pEventIterator(emitter, event, options?)

    pEventIterator(emitter, event, filter)

    Returns an async iterator that lets you asynchronously iterate over events of event emitted from emitter. The iterator ends when emitter emits an event matching any of the events defined in resolutionEvents, or rejects if emitter emits any of the events defined in the rejectionEvents option.

    This method has the same arguments and options as pEvent() with the addition of the following options:

    options

    Type: object

    limit

    Type: number (non-negative integer)
    Default: Infinity

    The maximum number of events for the iterator before it ends. When the limit is reached, the iterator will be marked as done. This option is useful to paginate events, for example, fetching 10 events per page.

    resolutionEvents

    Type: string[]
    Default: []

    Events that will end the iterator.

    TimeoutError

    Exposed for instance checking and sub-classing.

    Example:

    import {pEvent} from 'p-event';
    
    try {
    	await pEvent(emitter, 'finish');
    } catch (error) {
    	if (error instanceof pEvent.TimeoutError) {
    		// Do something specific for timeout errors
    	}
    }

    Before and after

    import fs from 'node: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);
    });
    import fs from 'node:fs';
    import {pEvent} from 'p-event';
    
    async function getOpenReadStream(file) {
    	const stream = fs.createReadStream(file);
    	await pEvent(stream, 'open');
    	return stream;
    }
    
    (async () => {
    	const stream = await getOpenReadStream('unicorn.txt');
    	console.log('File descriptor:', stream.fd);
    	stream.pipe(process.stdout);
    })()
    	.catch(console.error);

    Tip

    Dealing with calls that resolve with an error code

    Some functions might use a single event for success and for certain errors. Promises make it easy to have combined error handler for both error events and successes containing values which represent errors.

    import {pEvent} from 'p-event';
    import emitter from './some-event-emitter';
    
    try {
    	const result = await pEvent(emitter, 'finish');
    
    	if (result === 'unwanted result') {
    		throw new Error('Emitter finished with an error');
    	}
    
    	// `emitter` emitted a `finish` event with an acceptable value
    	console.log(result);
    } catch (error) {
    	// `emitter` emitted an `error` event or
    	// emitted a `finish` with 'unwanted result'
    	console.error(error);
    }

    Related

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

    Install

    npm i p-event

    DownloadsWeekly Downloads

    7,131,019

    Version

    5.0.1

    License

    MIT

    Unpacked Size

    23.9 kB

    Total Files

    5

    Last publish

    Collaborators

    • sindresorhus