Does Q.allSettled ever fail?

Question

I've seen some places in unit tests where a function returns a q.allSettled() promise from a function call and there's a .fail attached to the promise. But .allSettled will call .then even if some/all promises are rejected.

Here's an example:

function someFunctionToTest(){
  var aRejectedDeferred = Q.defer();
  var anotherRejectedDeferred = Q.defer();
  var aResolvedDeferred = Q.defer();

  aRejectedDeferred.reject(new Error("I'm aRejectedPromise"));
  anotherRejectedDeferred.reject(new Error("I'm anotherRejectedPromise"));
  aResolvedDeferred.resolve({awesome:"I'm aResolvedPromise"});

  return Q.allSettled([aRejectedDeferred.promise,anotherRejectedDeferred.promise, aResolvedDeferred.promise])
}

it('should never fail', function(done) {

  someFunctionToTest()
      .then(function (data) {
        should.equal(data[0].state,'rejected');
        should.equal(data[1].state,'rejected');
        should.equal(data[2].state,'fulfilled');
        // throw(new Error('I will trigger fail!!!'));
        done();
      })
      .fail(function (err) {
        done(err);
      });
});

Can you provide some examples where .fail would be triggered?

Answer 1

Q.allSettled cannot fail. See Q API which says

Returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected.

This is quite clear that whatever promises in allSettled do you will still get a resolved promise.

In your particular case the .then statement may fail because of the e.g. following line

should.equal(data[0].state,'rejected');

This may throw an assertion exception which will lead to the fail and you can catch it and run done(); which is the way to say to the testing tool that asynchronous request has been finished. But this is just a guess. Q.allSettled itself cannot fail.