Promisify function that returns some value and takes callback for result/error

[thesebas]

I think that promisifying function that returns some value and takes callback for result/error makes result of this function lost, lets say there is some function that returns object for controlling this async process:

function doAsync(params, cb){
  ///...
  return {
    abort: function(){ /* ... */},
    // ....
  }
}

var process = doAsync({/* some params */}, function(err, result){ 
 // handle result or err 
});

// something happens
process.abort();

now after promisifying this function I can't access this process object

var doAsyncPromised = Promise.promisify(doAsync);
doAsyncPromised({/* some params */})
  .then(function(result){
    // handle result
  }, function(err){
    // handle err
  });

so what I'm missing here is some callback that I could intercept this returned value

doAsyncPromised({/* some params */})
  .intercept(function(process){
    // something happens
    process.abort();
  })
  .then(function(result){
    // ...
  });

Maybe creating functions that returns value and calls callback is not the best pattern but some libraries doing that already exists (for example request).

[phpnode]

@thesebas yikes, that is pretty horrible, almost a reinvention of promises. In such cases it's better to write your own promisifier I think, this cannot be a common scenario.

[thesebas]

Well, as I said there is request library that has those methods request.post , request.get that returns request object you can manipulate and/or inspect after calling

var request = require('request');
var req = request.post( params, callback ); // returns req object I can abort with `req.abort()`

but after promisifying this lib

var request = require('request');
Promise.promisifyAll(request)

request.postAsync(params) // ->returns Promise, can't inspect request in progress, 
                          // can't abort it, etc...

and I think I could find more examples, but request is the one I'm working with now.

[benjamingr]

As stated in the documentation and previous issues automatic promisifcation only works for method explicitly abiding the nodeback contract (including no return value).

Indeed the correct way to address this is to write your own promisifier for promisifyAll (it accepts it as an option) or your own promisification method. For most of these modules (request included) there is already a package doing this on NPM.

[petkaantonov]

The default promisifier is for node callback convention, if you have something else, use a custom promisifier with the promisifier option https://github.com/petkaantonov/bluebird/blob/master/API.md#option-promisifier.

[thesebas]

Ok, promisifier and this module looks "promising" and understand that this issue in general is not place of improvement bluebird, thx.

[spion]

I'd love it if bluebird were somehow able to handle cases like this. Unfortunately there are many variants and its pretty hard to cover all of them, so custom promisifiers is probably the best solution for now.

This could be fixed pretty easily by node's core team though. They'd just add methods like readAll(function(err, data) { }) or .ended(function(err) { ... }) to streams. Then you could promisify streams and not pass any callbacks to request:

var req = request(url);
req.readAllAsync().then(...)
// req is available for progress etc.

[benjamingr]

If you suggest it on io.js I'd support it

[spion]

Done here, nodejs/node#153 - came up with a basic draft proposal.

[spion]

The proposal was rejected - its not going to happen. Maybe we could build a standard replacement for request that has the same API plus additional stream methods such as collectResponse(), or a module that extends (Readable)Stream.prototype to make it actually useful

[benjamingr]

@spion dat edit :D

[spacemonkey92]

Tried to use promisifyAll on sign.

jwt.sign({ foo: 'bar' }, cert, { algorithm: 'RS256' }, function(token) {
  console.log(token);
});

I end up getting the token value inside the error. ( because it doesn't have any error param)
Im quite new to promises. is it just me or is it really strange ?

[phpnode]

@spacemonkey92 sounds like you're not actually using the promisified version.

[spacemonkey92]

@phpnode That was the actual method.

jwt.signAsync({ foo: 'bar' }, cert, { algorithm: 'RS256' })
      .then((token)=>{
      console.log(token);
      })
      .catch((err)=>{
      console.log(err);
      });

This is going to the error block.
gives me this
{ [Error: eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJzZXNzaW9uSWQiOiJibGI1SEI1VWNqZElzNVR6c1ViR0RrOGxTWVJWQU5jWGh6QXkiLCJpYXQiOjE0NTcwOTkxNDAsImV4cCI6MTQ1NzEwMjc0MH0.V-ClT-in0dzyl1lGZ8vJh-F0Nn5fWxbXOqchhfUXOi-nsfkidN034FP-X7p-9qCl9s1WqE6_75d0WrTH5qVt-5mpKR310FJW0crf2ZDF3chXYf17_BpcqM72XotKmxi4lIaDOUlNyPxQcJeg4LX2HSKVLsGD1Aw62SsoClfGv3s]
cause: [Error: eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJzZXNzaW9uSWQiOiJibGI1SEI1VWNqZElzNVR6c1ViR0RrOGxTWVJWQU5jWGh6QXkiLCJpYXQiOjE0NTcwOTkxNDAsImV4cCI6MTQ1NzEwMjc0MH0.V-ClT-in0dzyl1lGZ8vJh-F0Nn5fWxbXOqchhfUXOi-nsfkidN034FP-X7p-9qCl9s1WqE6_75d0WrTH5qVt-5mpKR310FJW0crf2ZDF3chXYf17_BpcqM72XotKmxi4lIaDOUlNyPxQcJeg4LX2HSKVLsGD1Aw62SsoClfGv3s],
isOperational: true }