Why callback, errback?

[Raynos]

Why not

when(eventual, function (err, value) {
    // do stuff
})

It's idiomatic to node and trivially composes with callback-reduce

[Gozala]

No, that requires handling errors at every step and is not idiomatic promise API either. There is lower level await if you want to handle things at each step.

[Raynos]

What about adding a new method.

goodName(eventual, function (err, value) {

})

So that you can use promises as an implementation detail of a library and then still support a node style callback interface

[Gozala]

Alternatives to existing API will only do harm. I also don't see any specific reasons for adding this alternative.