Allow for once to return a promise

[benjamingr]

To start some discussion regarding nodejs/node#20909 (comment)

[lpinca]

I'm not a fan of this, apart from the simplified usage offered by await, what is the real benefit?
cc: @3rd-Eden

[3rd-Eden]

I've been following the discussion around this, and while I applaud the effort modernize the EventEmitter pattern to become fully async aware I don't think that this is way to go about it. It is weird, that a single method of the EventEmitter is upgraded to support async, especially if this be solved in userland in 5~ LOC as well:

function wonce(emitter, event) {
  return new Promise(function (resolve) {
    emitter.once(event, resolve)
  })
}

const data = await wonce(server, 'connected');

So for me this, the particular change would be -1, unless Node.js decides to implement it, then we will add support for it as well. Id be much more interested to see if we can make a fully async aware EventEmitter, instead of just spicing up a single method. Maybe work can be done around async iterators/generators for the on method. e.g:

for await (const data = stream.on('data')) {
  ...
}

Or otherways to make EventEmitter fully async, instead of just a single method.

[benjamingr]

especially if this be solved in userland in 5~ LOC as well:

The reason I am proposing this is because I've seen this seemingly simple implementation done wrong several times - when people have to write new Promise they end up having to put logic there.

What do you think about my example here: nodejs/node#20909 (comment) and the discussion here: nodejs/node#20909 (comment) ?

Maybe work can be done around async iterators/generators for the on method. e.g:

Node.js 10 already ships with Symbol.asyncIterator for streams - you can for await streams in Node.js today. If this excites you - we need help!

Quoting our docs:

const fs = require('fs');

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const k of readable) {
    data += k;
  }
  console.log(data);
}

print(fs.createReadStream('file')).catch(console.log);

This also has full backpressure support.

[lpinca]

I'm closing this, but we should add https://nodejs.org/api/events.html#events_events_once_emitter_name.