Fulfill (don't resolve) promise with another promise

Question

I want to fulfill a promise with some other promise. The point is that I really want to get access to the (still pending) second promise as soon as the first promise is fulfilled. Unfortunately, I only seem to be able to get the second promise's resolution value once both both promises are fulfilled.

Here's the use case that I have in mind:

var picker = pickFile();

picker.then(  // Wait for the user to pick a file.
    function(downloadProgress) {
        // The user picked a file. The file may not be available just yet (e.g.,
        // if it has to be downloaded over the network) but we can already ask
        // the user some more questions while the file is being obtained in the
        // background.

        ...do some more user interaction...

        return downloadProgress;
    }
).then( // Wait for the download (if any) to complete.
    function(file) {
        // Do something with the file.
    }
)

The function pickFile displays a file picker where the user may pick a file either from their own hard drive or from a URL. It returns a promise picker that is fulfilled as soon as the user has picked a file. At this point, we may still have to download the selected file over the network. Therefore, I cannot fulfill picker with the selected file as resolution value. Instead, picker should be fulfilled with another promise, downloadProgress, which in turn will eventually be fulfilled with the selected file.

For completenes, here's a mock implementation of the pickFile function:

function pickFile() {
    ...display the file picker...

    var resolveP1 = null;

    var p1 = new Promise(
        function(resolve, reject) {
            resolveP1 = resolve;
        }
    );

    // Mock code to pretend the user picked a file
    window.setTimeout(function() {
        var p2 = Promise.resolve('thefile');
        resolveP1(p2);  // <--- PROBLEM: I actually want to *fulfill* p1 with p2
    }, 3000);

    return p1;
}

The problem in the marked line is that I would like to fulfill the promise p1 with the new promise p2, but I only know how to resolve it. The difference between fulfilling and resolving is that resolving first checks if the supplied value p2 is again a promise. If it is, then fulfillment of p1 will be deferred until p2 is fulfilld, and then p1 will be fulfilled with p2's resolution value instead of p2 itself.

I could work around this issue by building a wrapper around p2, i.e. by replacing the line

        resolveP1(p2);  // <--- PROBLEM: I actually want to *fulfill* p1 with p2

from the second code example by

        resolveP1({promise: p2});

Then, in the first code example, I'd have to replace the line

        return downloadProgress;

by

        return downloadProgress.promise;

But this seems like a bit of a hack when all I really want to do is just fulfill (instead of resolve) a promise.

I'd appreciate any suggestions.

Answer 1

There doesn't seem to be a solution apart from the workaround I already described in the question. For future reference, if you want to fulfill (rather than resolve) a promise p with a value val, where val is another promise, then just calling the promise resolution function for p with argument val won't work as expected. It would cause p to be "locked in" on the state of val, such that p will be fulfilled with val's resolution value once val is fulfilled (see spec).

Instead, wrap val in another object and resolve p with that object:

var resolveP;  // Promise resolution function for p

var p = new Promise(
    function(resolve, reject) {
        resolveP = resolve;
    }
);

function fulfillPwithPromise(val) {  // Fulfills p with a promise val
    resolveP({promise: val});
}

p.then(function(res) {
    // Do something as soon as p is fulfilled...

    return res.promise;
}).then(function(res) {
    // Do something as soon as the second promise is fulfilled...
});

This solution works if you already know that val is a promise. If you cannot make any assumptions about val's type, then you seem to be out of luck. Either you have to always wrap promise resolution values in another object, or you can try to detect whether val has a field then of type "function" and wrap it conditionally.

That said, in some cases the default behavior of promise resolution may actually have the desired effect. So only use the workaround described above if you are sure that you want to fulfill instead of resolve the first promise with the second one.

Answer 2

Although different people use different terms, in common terminology, "fulfill" means to put a promise in the "success" state (as opposed to "reject")--the state that will trigger then then handlers hanging off it.

In other words, you cannot "fulfill" a promise with a promise. You can fulfill it with a value. (By the way, the term "resolve" is usually meant as either of fulfilling or rejecting.)

What you can do is return a promise from a .then handler and that will have the effect of essentially replacing the original promise with the returned promise.

Here is a simple example of doing that:

asyncTask1 . then(asyncTask2) . then(processData)

where asyncTask1 is a promise, and asyncTask2 is a function which returns a promise. So when asyncTask1 is fulfilled (done successfully), then asyncTask2 runs, and the promise returned by the .then is "taken over" by the promise asyncTask2 returns, so that when it finishes, the data can be processed.

I can do something similar by calling Promise.resolve with a promise as parameter. It's a bit of a misnomer, because I'm not resolving the promise in the technical sense. Instead, the new promise created is "inhabited" by the promise I passed in. It's also useless, because using the result is exactly the same as using the promise I passed in:

Promise.resolve(asyncTask2)

behaves exactly the same as

asyncTask2

(assuming asyncTask2 is already a promise; otherwise Promise.resolve has the effect of creating a promise which is immediately fulfilled with the passed in value.)

Just as you can pass a promise to Promise.resolve, you can pass a promise to the resolve function provided to you as a parameter of the promise constructor callback. If the parameter you pass to resolve is a non-promise, the promise immediately fulfills with that value. However, if the parameter you pass to resolve is another promise, that promise "takes over the body" of the promise you are constructing. To put it another way, the promise you are constructing starts to behave exactly as the the promise passed to resolve.

By "behave exactly" I mean, if the promise you pass in to resolve is already fulfilled, the promise you are constructing is instantly fulfilled with the same value. If the promise you pass in to resolve is already rejected, the promise you are constructing is instantly rejected with the same reason. If the promise you pass in to resolve is not resolved yet, then any then handlers you hang off the promise you are constructing will be invoked if and when the promise you pass to resolve is resolved.

Just as it is confusing that Promise.resolve may result in a promise which is not actually resolved, it is similarly confusing that calling the resolve function handed to you as a parameter to the promise constructor may not actually resolve the promise being constructed if you call it with an unresolved promise. Instead, as I've said a couple of times now, it has the effect of putting the promise being constructed in a state of total congruence with the promise passed to resolve.

Therefore, unless I am missing the point of your question, pickfile could be written as

function pickFile() {
  return new Promise(function(resolve, reject) {
    ...display the file picker...

    // Mock code to pretend the user picked a file
    window.setTimeout(function() {
        resolve('thefile');
    });
}

I didn't really understand your question clearly, so this might not be what you want. Please clarify if you care to.

Answer 3

Found a similar solution in the process of moving away from Angular's $q to the native Promise feature. Promise.all could be an option (in cases of independent parallel async tasks) by passing around an appropriate object, or something decorated with the state, passing it off to whatever is ready when appropriate. In the Promise.all sample below note how it recovers in one of the promises--took me awhile to realize how to redirect the result of a chain. The result of the all is just the last promise's return. While this doesn't answer the question's title, using return Promise.reject(<an-object-including-a-promise>) (or resolve) gives a series and/or group of async tasks shared access and control along the way. In the case of picking, downloading then working with a file I'd take out the progress-event handling then do: pickFile.then(download,orFailGracefully) with downloadProgress handled within the download onResolve handler (download-progress doesn't appear to be an async task). Below are related experiments in the console.

var q = {
defer: function _defer(){
    var deferred = { };
    deferred.promise = new Promise(function(resolve, reject){
        deferred.resolve = resolve;
        deferred.reject = reject;
    });
    return deferred;
    }
};

var communityThatCares = q.defer();
communityThatCares.promise.then(function(someGood){
    console.log('someGood', someGood);
    return someGood;
}, function(someBad){
    console.warn('someBad', someBad);
    return someBad;
});

(new Promise(function(resolve, reject){ communityThatCares.about = 'communityThatCares'; setTimeout(resolve,1000, communityThatCares); }))
.then(
function(e){
    console.log(3,e); return e.resolve(e);
}, function(e){
    console.warn(3, e); return e.reject(e);
});

var todo = {
    find:'swan'
};

var greaterGood = [(
(new Promise(function(res,rej){ res(todo); })).then(function(e){ e.stuff = 'things'; return e; }),
(new Promise(function(res,reject){ 
    reject(todo);
})).then(function(e){ return e; }
,function(e){
    console.warn(1,e);
    e.recover = 'uh oh';
    return Promise.resolve(e);
}).then(function(e){ console.log(2,e); return e; }),
(new Promise(function(res,rej){ res(todo); })).then(function(e){ console.log(1,e); e.schedule = 'today'; return e; },function(e){ console.warn(1,e); return e; }).then(function(e){ console.log(2,e); return e; }))
];

var nkay = Promise.all( greaterGood )
.then(function(todo){
    console.log('all',todo[0]); return todo;
}, function(todo){
    console.warn('all',todo[0]); return todo;
});