Chaining Vow.Promise methods (then, fail) works strangely

[edoroshenko]

If I chain calls of method then like this

var promise = new Vow.Promise();
promise.then(onSuccess1).then(onSuccess2);
promise.fulfill();

both onSuccess1 and onSuccess2 are called. And it's an expected behaviour. If I chain calls of method fail like this

promise.fail(onError1).fail(onError2);
promise.reject();

or do something like this

promise.then(onSuccess, onError1).fail(onError2);
promise.reject();

then only onError1 is called. And it's an unexpected behaviour. This hurts, when you return Vow.Promise from function. You can't write something like this

var magic = function() {
  ...
  return promise.fail(function() {
    ...
  });
}

magic().fail(function() {
  ...
})

So, while chaining works expectedly only for method then, you should not use chaining at all to prevent remembering complicated rules.

[dfilatov]

If you need to propagate error through chain, you should either throw exception from onError handler or return rejected promise. Otherwise it's assumed that onError handler has processed error.

See specification: https://github.com/promises-aplus/promises-spec

[edoroshenko]

So, am I correct, if I say, that i shouldn't want to chain calls of promise methods?

[dfilatov]

No, you aren't.

promise
    .fail(function(e) {
        ...
        throw e; // or return vow.reject(e);
    })
    .fail(function(e) {
        // this callback will be invoked
    });

[edoroshenko]

If I want to chain fail calls, I must throw an exception in each of them. True?

[dfilatov]

Yes, you should propagate error in each callback if you need. The point is that you can handle error and turn chain to the success case.

[edoroshenko]

ok, understood. Thanks!