`Promise.join` calls callback synchronously

[overlookmotel]

Please answer the questions the best you can:

1. What version of bluebird is the issue happening on?

v2.10.2, v3.4.1

2. What platform and version? (For example Node.js 0.12 or Google Chrome 32)

Node v6.2.1, OS X 10.9.5

3. Did this issue happen with earlier version of bluebird?

Yes, consistent across v2.x and v3.x

---

Promise.join's behavior in when it calls the callback (final argument) depends on the state of the promises it's passed. If all promises are resolved, it runs the callback synchronously; but if any of the promises are pending, it runs callback async.

Promise.join(
    Promise.resolve(1),
    Promise.resolve(2),
    Promise.resolve(3),
    function(a, b, c) {
        console.log( {a: a, b: b, c: c} );
    }
);
console.log('next sync statement');

outputs:

{ a: 1, b: 2, c: 3 }
next sync statement

But:

Promise.join(
    Promise.resolve(1),
    Promise.resolve(2).tap( function() {} ),
    Promise.resolve(3),
    function(a, b, c) {
        console.log( {a: a, b: b, c: c} );
    }
);
console.log('next sync statement');

next sync statement
{ a: 1, b: 2, c: 3 }

I'm not completely sure of the intended behavior, but I tend to think that this unpredictability isn't ideal and the callback should never be called synchronously.

[overlookmotel]

Also, if no promises are provided, the callback doesn't run at all:

Promise.join( function(a, b, c) {
    console.log( {a: a, b: b, c: c} );
} );
console.log('next sync statement');

next sync statement

It would be pretty pointless for someone to call Promise.join with no promises to join! But still you'd expect the callback to be called or an error to be thrown.

[overlookmotel]

Any plans to backport this to bluebird v2.x? The issue remains in v2.10.2.

[petkaantonov]

Sure

[overlookmotel]

OK great. I've raised a separate issue for my second point about the callback not being called if no promises are provided. #1218

[benjamingr]

I'm not sure if we should fix this in 2.0, if there is someone in the wild who is doing this (for whatever weird use case) it'd be a behavior change for them.

[Venryx]

Just thought I'd mention myself as such a person -- and yes, it's a kind-of weird use case. (though it's only for debugging code, of course)

Anyway, for how to restore this synchronous behavior (in my case, for a stack-trace processing library with external source-maps), see here: stacktracejs/stacktrace.js#188