Rejecting with a promise is not fully tested

[ziriax]

For learning promises, I made my own little implementation, see code below. I'm a Javascript newbie, so Douglas, don't be too hard on me please ;-)

For simplicity, it uses process.nextTick, so it only works in nodejs for now.

However, all 872 specs of your test suite pass, whether or not I 'adopt the state' of a 'thenable' passed as an argument to the reject method. See the single comment in the code.

What is the correct behavior?

Thanks a lot,
Peter Verswyvelen

var deferred = (function() {

    function Deferred() {
        this.promise = {
            then: this.then.bind(this),
        };
        this.thens = [];
    }

    function call_thens(state, value, thens) {
        thens.forEach(function (then) {
            var cfn = then[state],
                next_value = value,
                next_state = state;
            if (typeof cfn === "function") {
                try {
                    next_value = cfn(value);
                    next_state = "done";
                } catch (error) {
                    next_value = error;
                    next_state = "fail";
                }
            }

            then.next.transit(next_state, next_value);
        });
    }

    function then_transit(id, state, value) {
        if (this.state === id) {
            delete this.state;
            this.transit(state, value);
        }
    }

    var next_then_id = 0;

    Deferred.prototype = {
        asap: function () {
            process.nextTick(call_thens.bind(
                     this, this.state, this.value, this.thens));
            this.thens = [];
        },

        switchTo: function(state, value) {
            this.value = value;
            this.state = state;
            this.asap();
        },

        transit: function (state, value) {
            if (typeof this.state === "undefined") {
                // All tests succeeds with or without the state === "done" check?
                if (state === "done" && 
                     (typeof value === "function" || 
                         (typeof value === "object" && value !== null))) {
                    try {
                        if (value === this.promise)
                            throw new TypeError();

                        var then = value.then;
                        if (typeof then === "function") {
                            this.promise.then = then.bind(value);
                            var id = this.state = ++next_then_id;
                            try {
                                then.call(value,
                                    then_transit.bind(this, id, "done"),
                                    then_transit.bind(this, id, "fail"));
                            } catch (error) {
                                if (this.state === id)
                                    this.switchTo("fail", error);
                            } finally {
                                return;
                            }
                        }
                    } catch (error) {
                        value = error;
                        state = "fail";
                    }
                }

                this.switchTo(state, value);
            }
        },

        resolve: function (value) {
            this.transit("done", value);
        },

        reject: function (value) {
            this.transit("fail", value);
        },

        then: function (done, fail) {
            var then = {
                next: new Deferred(),
                done: done,
                fail: fail
            }

            this.thens.push(then);

            if (this.state)
                this.asap();

            return then.next.promise;
        }
    }

    return function () {
        return new Deferred();
    }
})();

var promisesAplusTests = require("promises-aplus-tests");

promisesAplusTests({
    deferred: deferred
}, {
    reporter: "spec"
});

[domenic]

I don't understand the state === "done" check enough to comment, really.

[briancavalier]

@ziriax It will probably be hard for us to comment, since we don't understand your code nearly as well as you do. If you could provide some sort of minimal test case that shows a potential problem, such as a particular test not matching a part of the Promises/A+ spec, that'd be extremely helpful.

[ziriax]

Sorry for not making my question clear enough. I don't understand my own code that much either ;-) I tweaked it until all the specs passed, some of them were really hard to get right.

Let me try to explain my question in detail, with an example.

Consider the following code:

var def1 = deferred();
var prom1 = def1.promise;

var def2 = deferred();
var prom2 = def2.promise;

prom1.then(function (x) {
    console.log("done:" + x);
}, function (x) {
    console.log("fail:" + x);
});

def1.reject(prom2);
def2.resolve("foo");

What should be the correct output?

If I keep my promise implementation as initially pasted, I get the output:

fail:[object Object]

So prom1 is rejected with prom2 as-is, not doing any further resolution.

However, if I modify my transit method, removing the state === "done" check, I get the output

done: foo

In this case prom1 is rejected with prom2, but prom1 adopts the state of prom2, so eventually gets fulfilled...

Only one of the two behaviors can be correct, but I don't know which one.

So this means that I have two different implementations, both of them passing all the tests, but with different behavior.

The test suite doesn't seem to dictate the correct behavior, or at least not in the context of my own simple promise implementation.

What confused me in the A+ spec was

2.3.3.3.2: If/when rejectPromise is called with a reason r, reject promise with r.

It's not clear to me what to do when the reason r is another promise?

Thanks a lot,
Peter Verswyvelen

[domenic]

So this means that I have two different implementations, both of them passing all the tests, but with different behavior.

That means you found a bug in the test coverage; nice!

It's not clear to me what to do when the reason r is another promise?

You should follow the spec, and reject promise with r.

[briancavalier]

It's not clear to me what to do when the reason r is another promise?

Reject is verbatim. IOW, you shouldn't attempt to resolve r, but rather use r as the reason verbatim, even if r is a promise.

[briancavalier]

jinx again! :)

[juandopazo]

Nice catch!