Support return value on EventEmitter.once

[himself65]

Is your feature request related to a problem? Please describe.

I'm trying to code a Promise method which will be called after once and on have been called.

and It will resolve the params that are returned from once and on functions.

I use it on server, the project code like this

module.exports.emitThen = async function emitThen (event, ...args) {
  return Promise.all(
    this.rawListeners(event).map(
      listener => Promise.resolve()
        .then(() => {
            return listener.apply(this, args)
          }
        )
    )
  )
}

const res = await obj.emitThen(eventName, data).then(res=> { 
  // do something
})

but once just returns a null because it only just be called and return nothing.

node/lib/events.js

Lines 281 to 287 in 5e1d446

	function onceWrapper(...args) {
	if (!this.fired) {
	this.target.removeListener(this.type, this.wrapFn);
	this.fired = true;
	Reflect.apply(this.listener, this.target, args);
	}
	}

Describe the solution you'd like

function onceWrapper(...args) {
  if (!this.fired) {
    this.target.removeListener(this.type, this.wrapFn);
    this.fired = true;
    return Reflect.apply(this.listener, this.target, args);
  }
}

All in all, I think it should do the same behavior whatever on or once

[himself65]

Pull Request: #25818

[bnoordhuis]

@himself65 You closed the PR. Does that mean you're withdrawing this feature request?

[himself65]

@bnoordhuis

no

just I forgot to open it, and I open it now

[simonkcleung]

Do you want to do something like this?

const EventEmitter = require('events');
const ev = new EventEmitter();

EventEmitter.prototype.await = function(resolveEvent, rejectEvent) {

  var _resolve,
      _reject;
	  
  return new Promise( (resolve, reject)=>{
    _resolve = resolve;
    this.on(resolveEvent, resolve);
    if (rejectEvent) {
      _reject = reject;
      this.on(rejectEvent, reject);
	}
  }).finally( () => {
	  this.off(resolveEvent, _resolve);
	  if (rejectEvent){
		  this.off(rejectEvent, _reject);
	  }
	})
}

ev.await('ready')
  .then((v) => {console.log(v); return v+2;})
  .then((v) => {console.log(v); return v+3;})
  .then((v) => {console.log(v); return v+4;})
  .then((v) => {console.log(v); return v+5;})
  .then((v) => {console.log(v)})
ev.emit('ready', 1);
/*
1
3
6
10
15
*/

[himself65]

Do you want to do something like this?

const EventEmitter = require('events');
const ev = new EventEmitter();

EventEmitter.prototype.await = function(resolveEvent, rejectEvent) {

  var _resolve,
      _reject;
	  
  return new Promise( (resolve, reject)=>{
    _resolve = resolve;
    this.on(resolveEvent, resolve);
    if (rejectEvent) {
      _reject = reject;
      this.on(rejectEvent, reject);
	}
  }).finally( () => {
	  this.off(resolveEvent, _resolve);
	  if (rejectEvent){
		  this.off(rejectEvent, _reject);
	  }
	})
}

ev.await('ready')
  .then((v) => {console.log(v); return v+2;})
  .then((v) => {console.log(v); return v+3;})
  .then((v) => {console.log(v); return v+4;})
  .then((v) => {console.log(v); return v+5;})
  .then((v) => {console.log(v)})
ev.emit('ready', 1);
/*
1
3
6
10
15
*/

yeah

[himself65]

Add this support on #25818