rejection turns into fulfillment

[brandf]

This has me puzzled, and I can't find justification for it in the Promise/A+ spec.

var r = Promise.pending();
r.promise.catch(Promise.CancellationError,
               function () { console.log("cancel"); })
         .then(function () { console.log("fulfilled"); });
r.cancel();

When I do the above I was expecting that I'd get "cancel" in the output, but I got "cancel" and "fulfilled".

It turns out this is because my rejection handler didn't return anything, so the return value of undefined was promoted into a fulfilled promise! I can't find anything in Promise/A+ that indicates this should happen. I would argue that if you don't return anything, the .then promise should take the state of the promise that triggered the .catch.

This would prevent a rejection from unexpectedly being converted to a fulfillment.

To work around the issue, I explicitly return Promise.rejected(reason) form the .catch callback, but this requires allocating a new promise. What I really want to say is 'just use the existing promise', but I don't see any way to do that.

[domenic]

If you don't re-throw the error in the catch handler, it is … handled. This is just like how sync try/catch works. It's not an unexpected conversion; it's a conversion.

Promises/A+ covers this fully; it makes no exception for undefined.

[petkaantonov]

Your code parallels:

try {
    throw new CancellationError();
}
//Imagine this syntax working
catch(CancellationError e) {
    console.log("cancel");
}
console.log("fulfilled");

It looks like you want:

try {
    throw new CancellationError();
}
//Imagine this syntax working
catch(CancellationError e) {
    console.log("cancel");
    throw e;
}
console.log("fulfilled");

Which can be done:

var r = Promise.pending();
r.promise.catch(Promise.CancellationError, function (e) {
    console.log("cancel");
    throw e;
})
.then(function () {
    console.log("fulfilled");
});
r.cancel();

And gives:

cancel
Possibly unhandled CancellationError: cancellation error
    at Promise$cancel [as cancel] (<anonymous>:1200:19)
From previous event:
    at <anonymous>:2:17
    at Object.InjectedScript._evaluateOn (<anonymous>:580:39)
    at Object.InjectedScript._evaluateAndWrap (<anonymous>:539:52)
    at Object.InjectedScript.evaluate (<anonymous>:458:21)

[brandf]

Yes, I agree this makes sense, and I was simply confused.