Add `Bluebird.on()` for promisifying an event emitter

It’s incredibly common to need to promisify event emitters of various types, particularly streams. I’ve written something like this a million times (approximately):

function promisifyEmitter (emitter, success = "data", error = "error") {
  return new Promise((resolve, reject) => {
    emitter.on(success, resolve).on(error, reject);
  });
}

It would be really nice to have this as part of bluebird, perhaps in the form of Bluebird.on():

Bluebird.on = function (emitter, success, error) {
  if (success === undefined) success = "data";
  if (error === undefined) error = "error";
  return new Promise(function (resolve, reject) {
    if (Array.isArray(success)) {
      for (var i = 0; i < success.length; i++) {
        emitter.once(success[i], resolve);
      }
    }
    else {
      emitter.once(success, resolve);
    }
    if (Array.isArray(error)) {
      for (var i = 0; i < error.length; i++) {
        emitter.once(error[i], reject);
      }
    }
    else {
      emitter.once(error, reject);
    }
  });
};

[kirly-af]

Sorry to dig out this old ticket, but I did not understand @petkaantonov’s last comment.

It is the developer choice to properly choose what event he wants to await on. Especially nowadays, since we have async/await available. For example, being able to await on the “end” event of a stream is quite useful. If one needs to wait for another event, she can just use Promise.all.

try {
    await Promise.all([
        Promise.on('end', readbleStream),
        Promise.on('finish', writableStream),
        Promise.on('someEvent', writableStream)
    ]);
} catch (err) {
    // convenient error handling
}

[kirly-af]

@benjamingr I just created https://github.com/nodejs/node/issues/20893