Synchronous promise resolution (bluebird vs. jQuery)

Question

I have developed a small lib for the Dynamics CRM REST/ODATA webservice (CrmRestKit). The lib dependes on jQuery and utilizes the promise-pattern, repectivly the promise-like-pattern of jQuery.

Now I like to port this lib to bluebird and remove the jQuery dependency. But I am facing a problem because bluebird does not support the synchronous resolution of promise-objects.

Some context information:

The API of the CrmRestKit excepts an optional parameter that defines if the web-service call should be performed in sync or async mode:

CrmRestKit.Create( 'Account', { Name: "foobar" }, false ).then( function ( data ) {
   ....
} );

When you pass "true" or omit the last parameter, will the method created the record in sync. mode.

Sometimes it is necessary to perform a operation in sync-mode, for instance you can write JavaScript code for Dynamics CRM that is involed for the save-event of an form and in this event-handler you need to perform sync-operation for validation (e.g. validate that a certain number of child-records exist, in case the right number of records exist, cancel the save-operation and show an error message).

My problem now is the following: bluebird does not support the resolution in sync-mode. For instance when I do the following, the "then" handler is invoked in async fashion:

function print( text ){

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

    return text;
}

///
/// 'Promise.cast' cast the given value to a trusted promise. 
///
function getSomeTextSimpleCast( opt_text ){

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

    return Promise.cast( text );
}

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

The output is the following:

print -> second
print -> first

I would expect that the "second" appears after the "first" because the promise is already resolved with an value. So I would assume that an then-event-handler is immediately invoked when applied on an already resolved promise-object.

When I do the same (use then on an already resolved promise) with jQuery I will have my expected result:

function jQueryResolved( opt_text ){

    var text = opt_text || 'jQuery-Test Value',
    dfd =  new $.Deferred();

    dfd.resolve(text);

        // return an already resolved promise
    return dfd.promise();
}

jQueryResolved('third').then(print);
print('fourth');

This will generate the following output:

print -> third
print -> fourth

Is there a way to make bluebird work in the same fashion?

Update: The provided code was just to illustrate the problem. The idea of the lib is: Regardless of the execution-mode (sync, async) the caller will always deal with an promise-object.

Regarding "... asking the user... doesn't seems to make any sense": When you provide two methods "CreateAsync" and "CreateSync" it is also up to the user to decide how the operation is executed.

Anyway with the current implementation the default behavior (last parameter is optional) is a async execution. So 99% of the code requires a promise-object, the optional parameter is only use for the 1% cases where you simply need a sync execution. Furthermore I developed to lib for myself and I use in 99,9999% of the case the async mode but I thought it is nice to have the option to go the sync-road as you like.

But I thinks I got the point an sync method should simply return the value. For the next release (3.0) I will implement "CreateSync" and "CreateAsync".

Thanks for your input.

Update-2 My intension for the optional parameter was to ensure a consistend behavior AND prevent logic error. Assume your as a consumer of my methode "GetCurrentUserRoles" that uses lib. So the method will alway return an promise, that means you have to use the "then" method to execute code that depends on the result. So when some writes code like this, I agree it is totally wrong:

var currentUserRoels = null;

GetCurrentUserRoles().then(function(roles){

    currentUserRoels = roles;
});

if( currentUserRoels.indexOf('foobar') === -1 ){

    // ...
}

I agree that this code will break when the method "GetCurrentUserRoles" changes from sync to async.

But I understand that this I not a good design, because the consumer should now that he deals with an async method.

Answer 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.

Answer 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).