What are the differences between Deferred, Promise and Future in JavaScript?

In light of apparent dislike for how I've attempted to answer the OP's question. The literal answer is, a promise is something shared w/ other objects, while a deferred should be kept private. Primarily, a deferred (which generally extends Promise) can resolve itself, while a promise might not be able to do so.

If you're interested in the minutiae, then examine Promises/A+.

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So far as I'm aware, the overarching purpose is to improve clarity and loosen coupling through a standardized interface. See suggested reading from @jfriend00:

Rather than directly passing callbacks to functions, something which can lead to tightly coupled interfaces, using promises allows one to separate concerns for code that is synchronous or asynchronous.

Personally, I've found deferred especially useful when dealing with e.g. templates that are populated by asynchronous requests, loading scripts that have networks of dependencies, and providing user feedback to form data in a non-blocking manner.

Indeed, compare the pure callback form of doing something after loading CodeMirror in JS mode asynchronously (apologies, I've not used jQuery in a while):

/* assume getScript has signature like: function (path, callback, context) 
   and listens to onload && onreadystatechange */
$(function () {
   getScript('path/to/CodeMirror', getJSMode);

   // onreadystate is not reliable for callback args.
   function getJSMode() {
       getScript('path/to/CodeMirror/mode/javascript/javascript.js', 
           ourAwesomeScript);
   };

   function ourAwesomeScript() {
       console.log("CodeMirror is awesome, but I'm too impatient.");
   };
});

To the promises formulated version (again, apologies, I'm not up to date on jQuery):

/* Assume getScript returns a promise object */
$(function () {
   $.when(
       getScript('path/to/CodeMirror'),
       getScript('path/to/CodeMirror/mode/javascript/javascript.js')
   ).then(function () {
       console.log("CodeMirror is awesome, but I'm too impatient.");
   });
});

Apologies for the semi-pseudo code, but I hope it makes the core idea somewhat clear. Basically, by returning a standardized promise, you can pass the promise around, thus allowing for more clear grouping.

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

References

1. Wikipedia on Promises & Futures
2. Promises/A+ spec
3. DOM Standard on Promises
4. DOM Standard Promises Spec WIP
5. DOJO Toolkit Deferreds
6. jQuery Deferreds
7. Q
8. FutureJS
9. Functional Javascript section on Promises
10. Futures in AngularJS Integration Testing

Misc potentially confusing things

• Difference between Promises/A and Promises/A+

  (TL;DR, Promises/A+ mostly resolves ambiguities in Promises/A)