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What are the differences between Deferreds, Promises and Futures?
Is there a generally approved theory behind all these three?
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A promise is a placeholder for a result which is initially unknown while a deferred represents the computation that results in the value. Every deferred has a promise which functions as a proxy for the future result.
A Promise is in one of these states: pending: initial state, neither fulfilled nor rejected. fulfilled: meaning that the operation was completed successfully. rejected: meaning that the operation failed.
The defer attribute tells the browser not to wait for the script. Instead, the browser will continue to process the HTML, build DOM. The script loads “in the background”, and then runs when the DOM is fully built.
A deferred object is an object that can create a promise and change its state to resolved or rejected . Deferreds are typically used if you write your own function and want to provide a promise to the calling code. You are the producer of the value. A promise is, as the name says, a promise about future value.
These answers, including the selected answer, are good for introducing promises conceptually, but lacking in specifics of what exactly the differences are in the terminology that arises when using libraries implementing them (and there are important differences).
Since it is still an evolving spec, the answer currently comes from attempting to survey both references (like wikipedia) and implementations (like jQuery):
Deferred: Never described in popular references, 1 2 3 4 but commonly used by implementations as the arbiter of promise resolution (implementing resolve and reject). 5 6 7
Sometimes deferreds are also promises (implementing then), 5 6 other times it's seen as more pure to have the Deferred only capable of resolution, and forcing the user to access the promise for using then. 7
Promise: The most all-encompasing word for the strategy under discussion.
A proxy object storing the result of a target function whose synchronicity we would like to abstract, plus exposing a then function accepting another target function and returning a new promise. 2
Example from CommonJS:
> asyncComputeTheAnswerToEverything() .then(addTwo) .then(printResult); 44
Always described in popular references, although never specified as to whose responsibility resolution falls to. 1 2 3 4
Always present in popular implementations, and never given resolution abilites. 5 6 7
Future: a seemingly deprecated term found in some popular references 1 and at least one popular implementation, 8 but seemingly being phased out of discussion in preference for the term 'promise' 3 and not always mentioned in popular introductions to the topic. 9
However, at least one library uses the term generically for abstracting synchronicity and error handling, while not providing then functionality. 10 It's unclear if avoiding the term 'promise' was intentional, but probably a good choice since promises are built around 'thenables.' 2
Difference between Promises/A and Promises/A+
(TL;DR, Promises/A+ mostly resolves ambiguities in Promises/A)
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