Synchronous promise resolution (bluebird vs. jQuery)

Question 1

Short version: I get why you want to do that, but the answer is no.

I think the underlying question being asked is whether a completed promise should immediately run a callback, if the promise has already completed. I can think of a lot of reasons that this might happen - for example, an asynchronous save procedure that only saves data if changes were made. It may be able to detect changes from the client side in a synchronous fashion without having to go through an external resource, but if changes are detected then and only then would an asynchronous operation be required.

In other environments that have asynchronous calls, the pattern seems to be that the developer is responsible for understanding that their work might complete immediately (for example, .NET framework's implementation of the async pattern accomodates this). This is not a design problem of the framework, it's the way it's implemented.

JavaScript's developers (and many of the commenters above) seem to have a different point of view on this, insisting that if something might be asynchronous, it must always be asynchronous. Whether this is "right" or not is immaterial - according to the specification I found at https://promisesaplus.com/, item 2.2.4 states that basically no callbacks can be called until you are out of what I'll refer to as "script code" or "user code"; that is, the specification says clearly that even if the promise is completed you can't invoke the callback immediately. I've checked a few other places and they either say nothing on the topic or agree with the original source. I don't know if https://promisesaplus.com/ could be considered a definitive source of information in this regard, but no other sources that I saw disagreed with it and it seems to be the most complete.

This limitation is somewhat arbitrary and I frankly prefer the .NET perspective on this one. I'll leave it up to others to decide if they consider it "bad code" to do something that might or might not be synchronous in a way that looks asynchronous.

Your actual question is whether or not Bluebird can be configured to do the non-JavaScript behavior. Performance-wise there may be a minor benefit to doing so, and in JavaScript anything's possible if you try hard enough, but as the Promise object becomes more ubiquitous across platforms you will see a shift to using it as a native component instead of custom written polyfills or libraries. As such, whatever the answer is today, reworking a promise in Bluebird is likely to cause you problems in the future, and your code should probably not be written to depend on or provide immediate resolution of a promise.

Question 2

You might think this is a problem, because there's no way to have

getSomeText('first').then(print);
print('second');

and to have getSomeText "first" printed before "second" when the resolution is synchronous.

But I think you have a logic problem.

If your getSomeText function may be synchronous or asynchronous, depending on the context, then it shouldn't impact the order of execution. You use promises to ensure it's always the same. Having a variable order of execution would likely become a bug in your application.

Use

getSomeText('first') // may be synchronous using cast or asynchronous with ajax
.then(print)
.then(function(){ print('second') });

In both cases (synchronous with cast or asynchronous resolution), you'll have the correct execution order.

Note that having a function being sometimes synchronous and sometimes not isn't a weird or unlikely case (think about cache handling, or pooling). You just have to suppose it's asynchronous, and all will be always fine.

But asking the user of the API to precise with a boolean argument if he wants the operation to be asynchronous doesn't seem to make any sense if you don't leave the realm of JavaScript (i.e. if you don't use some native code).

Question 3

The point of promises is to make asynchronous code easier, i.e. closer to what you feel when using synchronous code.

You're using synchronous code. Don't make it more complicated.

function print( text ){

    console.log( 'print -> %s', text );

    return text;
}

function getSomeTextSimpleCast( opt_text ){

    var text = opt_text || 'Some fancy text-value';

    return text;
}

print(getSomeTextSimpleCast('first'));
print('second');

And that should be the end of it.

If you want to keep the same asynchronous interface even though your code is synchronous, then you have to do it all the way.

getSomeTextSimpleCast('first')
    .then(print)
    .then(function() { print('second'); });

then gets your code out of the normal execution flow, because it's supposed to be asynchronous. Bluebird does it the right way there. A simple explanation of what it does:

function then(fn) {
    setTimeout(fn, 0);
}

Note that bluebird doesn't really do that, it's just to give you a simple example.

Try it!

then(function() {
    console.log('first');
});
console.log('second');

This will output the following:

second
first

Question 4

There are some good answers here already, but to sum up the crux of the matter very succinctly:

Having a promise (or other async API) that is sometimes asynchronous and sometimes synchronous is a bad thing.

You may think it's fine because the initial call to your API takes a boolean to switch off between sync/async. But what if that's buried in some wrapper code and the person using that code doesn't know about these shenanigans? They've just wound up with some unpreditable behavior through no fault of their own.

The bottom line: Don't try to do this. If you want synchronous behavior, don't return a promise.

With that, I'll leave you with this quotation from You Don't Know JS:

Another trust issue is being called "too early." In application-specific terms, this may actually involve being called before some critical task is complete. But more generally, the problem is evident in utilities that can either invoke the callback you provide now (synchronously), or later (asynchronously).

This nondeterminism around the sync-or-async behavior is almost always going to lead to very difficult to track down bugs. In some circles, the fictional insanity-inducing monster named Zalgo is used to describe the sync/async nightmares. "Don't release Zalgo!" is a common cry, and it leads to very sound advice: always invoke callbacks asynchronously, even if that's "right away" on the next turn of the event loop, so that all callbacks are predictably async.

Note: For more information on Zalgo, see Oren Golan's "Don't Release Zalgo!" (https://github.com/oren/oren.github.io/blob/master/posts/zalgo.md) and Isaac Z. Schlueter's "Designing APIs for Asynchrony" (http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony).

Consider:

function result(data) {
    console.log( a );
}

var a = 0;

ajax( "..pre-cached-url..", result );
a++;`

Will this code print 0 (sync callback invocation) or 1 (async callback invocation)? Depends... on the conditions.

You can see just how quickly the unpredictability of Zalgo can threaten any JS program. So the silly-sounding "never release Zalgo" is actually incredibly common and solid advice. Always be asyncing.

Question 5

What about this case, also CrmFetchKit related which in latest version uses Bluebird. I have upgraded from version 1.9 that was based on jQuery. Still the old app code that uses CrmFetchKit has methods the prototypes of which I can't or won't change.

Existing App Code

CrmFetchKit.FetchWithPaginationSortingFiltering(query.join('')).then(
    function (results, totalRecordCount) {
        queryResult = results;

        opportunities.TotalRecords = totalRecordCount;

        done();
    },
    function err(e) {
        done.fail(e);
    }
);

Old CrmFetchKit implementation (a custom version of fetch())

function fetchWithPaginationSortingFiltering(fetchxml) {

    var performanceIndicator_StartTime = new Date();

    var dfd = $.Deferred();

    fetchMore(fetchxml, true)
        .then(function (result) {
            LogTimeIfNeeded(performanceIndicator_StartTime, fetchxml);
            dfd.resolve(result.entities, result.totalRecordCount);
        })
        .fail(dfd.reject);

    return dfd.promise();
}

New CrmFetchKit implementation

function fetch(fetchxml) {
    return fetchMore(fetchxml).then(function (result) {
        return result.entities;
    });
}

My problem is that the old version had the dfd.resolve(...) where I was able to pass any number of params that I need.

The new implementation just returns, the parent seems to call the callback, I can't call it directly.

I went and made a custom version of the fetch() in the new implementation

function fetchWithPaginationSortingFiltering(fetchxml) {
    var thePromise = fetchMore(fetchxml).then(function (result) {
        thePromise._fulfillmentHandler0(result.entities, result.totalRecordCount);
        return thePromise.cancel();
        //thePromise.throw();
    });

    return thePromise;
}

But the problem is that the callback gets called two times, once when I do it explicitly and second by the framework but it passes it one parameter only. To trick it and "tell" not to call anything because I do it explicitly I try to call .cancel() but it is ignored. I understood why but still how do you do the "dfd.resolve(result.entities, result.totalRecordCount);" in the new version with out having to changes prototypes in the app that uses this library ?

Question 6

You can in fact do this, yes.

Modify the bluebird.js file (for npm: node_modules/bluebird/js/release/bluebird.js), with the following change:

[...]

    target._attachExtraTrace(value);
    handler = didReject;
}

- async.invoke(settler, target, {
+ settler.call(target, {
    handler: domain === null ? handler
        : (typeof handler === "function" &&

[...]

For more info, see here: https://github.com/stacktracejs/stacktrace.js/issues/188