What's the syntax for a no-op Promise in Parse?

Question

Often I find myself writing beforeSave and afterSave using promises in this form:

beforeSavePromise: function (request) {

    var recipe = request.object;

    var promises = [
        doFooIfNeeded(recipe),
        doBarIfNeeded(recipe),
        doQuuxIfNeeded(recipe)
    ];

    return Parse.Promise.when(promises)
},

Each of these are conditional actions that perform an action only if a particular field or fields are dirty. So for example, doFooIfNeeded might look something like:

if (recipe.dirty('imageFile')) {
    return /* some Promise that updates thumbnails of this image */; 
} else {
    return Parse.Promise.as();  // The no-op Promise. Do nothing!
}

My question is, is Parse.Promise.as() really the no-op Promise? or is new Parse.Promise() more correct?

Answer 1

With all "dirty" outcomes contributing a resolved promise to the aggregation, you can choose for each "clean" outcome to contribute in any of the following ways :

1. not to put anything in the array,
2. put a value in the array,
3. put a resolved promise in the array,
4. put a rejected promise in the array.

(1), (2) and (3) will guarantee that the aggregated promise resolves regardless of the clean/dirty outomes (except some unpredicted error).

(4) will cause the aggregared promise to resolve only if all outcomes are "dirty", or to reject as soon as any one "clean" outcome arises.

Realistically, the choice is between (2) and (4), depending on how you want the aggregated promise to behave. (1) would complicate the aggregation process, and (3) would be unnecessarily expensive.

It would seem appropriate for the aggregated promise to resolve when everything is either already "clean" or has been cleaned up, therefore I would suggest (2), in which case your foo()/bar()/quux() functions could be written as follows :

function foo() {
    return recipe.dirty('imageFile') ? updates_thumbnails() : true; // doesn't have to be `true` or even truthy - could be almost anything except a rejected promise. 
}

And aggregate the outcomes as in the question :

$.when([ foo(recipe), bar(recipe), quux(recipe) ]).then(function() {
    //all thumbnails were successfully updated.
}).fail(function() {
    //an unexpected error occurred in foo(), bar() or quux().
});