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I'm looking into BreezeJs and there samples are using Q.js for promises to handle asynchronous calls. John Papa is also using Q. JQuery has promises as well. What are the differences between the two?
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promise() method returns a dynamically generated Promise that is resolved once all actions of a certain type bound to the collection, queued or not, have ended. By default, type is "fx" , which means the returned Promise is resolved when all animations of the selected elements have completed.
jQuery objects can now return a Promise to observe when all animations on a collection have completed.” jQuery provides several methods that support custom animations (for example, the animate(), fadeIn(), and slideUp() methods). Their return value is the jQuery object.
If you are creating a Deferred, keep a reference to the Deferred so that it can be resolved or rejected at some point. Return only the Promise object via deferred. promise() so other code can register callbacks or inspect the current state. For more information, see the documentation for Deferred object.
The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.
Both are based on the Promises/A standard and implement a then method (though only current jQuery, they once had a incompatible pipe instead of then). However, there are a few differences:
then callbacks will be caught and reject the promise (and will only get re-thrown if you call .end()). Not sure whether I personally like that. It's the standardized way which jQuery does not follow, rejecting from then in jQuery deferreds is much more complicated.then), while jQuery allows multiple arguments in resolve/reject calls on its Deferreds..all and similiar, which are more complicated with jQuery ($.when.apply($, […])).… which is basically Promises/B. As you can see, the Q API is more powerful, and (imho) better designed. Depending on what you want to do, Q could be the better choice, but maybe jQuery (especially if already included) is enough.
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JQuery's promise implementation of the Promises/A spec has some real issues. The following link describes them far better than I can: missing-the-point-of-promises
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