once-promise

Create promises for events.

Contents

1. Breaking changes
2. Usage examples
3. JavaScript example
4. TypeScript example
5. Docs

Breaking changes

There are some incompatibilities between version 1.x and 2.x

• The once() method is no longer the default export. See the examples for more information.
• When providing multiple event names the array may no longer contain promises.

Usage examples

Await a single event

await once(process, 'exit')

This will await the "exit" event of the nodejs process.

Await multiple events

await once(readable, ['error', 'end'])

In this example we are awaiting the "error" or "end" event of a readable stream.

If the stream emits the "end" event, the promise will get resolved. If an "error" gets emitted first, the promise gets rejected with the error.

JavaScript example

const { once } = require('once-promise').default
const { EventEmitter } = require('events')
 
async function example () {
  const emitter = new EventEmitter()
 
  setTimeout(() => {
    emitter.emit('continue')
  }, 5000)
 
  console.log('start')
 
  // Create and await a promise for the "continue" event.
  // It should take 5s until the promise gets resolved.
  await once(emitter, 'continue')
 
  console.log('end :D')
}
 
example()

TypeScript example

import EventEmitter from 'events'
import { once } from 'once-promise'
 
async function example () {
  const emitter = new EventEmitter()
 
 
  setTimeout(() => {
    emitter.emit('continue')
  }, 5000)
 
  console.log('start')
 
  // Create and await a promise for the "continue" event.
  // It should take 5s until the promise gets resolved.
  await once(emitter, 'continue')
 
  console.log('end :D')
}
 
example()

Docs

once(emitter, event, arrayMode)

1. emitter <internal.EventEmitter> - This can be any EventEmitter (e.g. the process object, a stream,...)
2. event <string|string[]> - Either one eventname or an array containing the names of multiple events. If an array is passed, the returned promise will resolve to the first promise/event that gets resolved.
3. arrayMode? <true> - If you pass true as last parameter, the promise will resolve to an array containing all arguments provided by the (fastest) event. This can be useful if an event emits multiple values.

Note: When you are listening for an "error" event and the event gets emittet, the promise gets rejected instead of resolved.