What to do if onFulfilled throws with no listeners attached to returned promise about promises-spec HOT 3 CLOSED

Yes, the spec defines that the error is captured as the rejection reason of the promise returned from then:

3.2.6.2 If either onFulfilled or onRejected throws an exception, promise2 must be rejected with the thrown exception as the reason.

You seem to be throwing away that returned promise (promise2 in the spec), which is the root of your problem.

As always, consider the sync analog.

promiseSomething().then(function(result){
  var s = new SomeObj();
  s.doSomething(); // this error is swallowed
}, function(){
  // this obviously won't catch the error
});

becomes

try {
  promiseSomething();
} catch {
  // this obviously won't catch the error
}

var s = new SomeObj();
s.doSomething(); // this error is not caught

where obviously the error will not be caught. However, if you add the .then(undefined, function errhandler(){}) as you suggest, it becomes

try {
  promiseSomething();
} catch {
  // this obviously won't catch the error
}

try {
  var s = new SomeObj();
  s.doSomething();
} catch (err) {
  errhandler(err);
}

See also this explanation over in the Q readme.

We are discussing how to fix the problem that, with promises, errors are swallowed instead of handled by the host platform, over in the unhandled rejections spec area. In particular, see this background thread.

from promises-spec.

Ah thanks.

Glad to hear it's being discussed, and thanks for pointing out the Q readme point. I'd read that before, but not while tackling my own implementation.

I just implemented my own console.unhandledRejection checking and it's working nicely.

Thanks again!

from promises-spec.

I just implemented my own console.unhandledRejection checking and it's working nicely.

That'll be sweet!

from promises-spec.