`Promise.join` calls callback synchronously

Please answer the questions the best you can:

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

v2.10.2, v3.4.1

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

Node v6.2.1, OS X 10.9.5

1. 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.

[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: https://github.com/stacktracejs/stacktrace.js/issues/188

[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.