jquery - What are deferred objects?

Question

jQuery 1.5 adds "Deferred Objects". What are they, and what exactly do they do?

Answer 1

Deferred Object

As of jQuery 1.5, the Deferred object provides a way to register multiple callbacks into self-managed callback queues, invoke callback queues as appropriate, and relay the success or failure state of any synchronous or asynchronous function.

Deferred Methods:

• deferred.done()
  • Add handlers to be called when the Deferred object is resolved.
• deferred.fail()
  • Add handlers to be called when the Deferred object is rejected.
• deferred.isRejected()
  • Determine whether a Deferred object has been rejected.
• deferred.isResolved()
  • Determine whether a Deferred object has been resolved.
• deferred.reject()
  • Reject a Deferred object and call any failCallbacks with the given args.
• deferred.rejectWith()
  • Reject a Deferred object and call any failCallbacks with the given context and args.
• deferred.resolve()
  • Resolve a Deferred object and call any doneCallbacks with the given args.
• deferred.resolveWith()
  • Resolve a Deferred object and call any doneCallbacks with the given context and args.
• deferred.then()
  • Add handlers to be called when the Deferred object is resolved or rejected.

Deferred In Action:

$.get("test.php").done(
    function(){ alert("$.get succeeded"); }
);

$.get("test.php")
    .done(function(){ alert("$.get succeeded"); })
    .fail(function(){ alert("$.get failed!"); });

And it seems that the existing ajax() method callbacks can be chained rather than declared in the settings:

var jqxhr = $.ajax({ url: "example.php" })
    .success(function() { alert("success"); })
    .error(function() { alert("error"); })
    .complete(function() { alert("complete"); });

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Working Example From Eric Hynds blog post: http://jsfiddle.net/ehynds/Mrqf8/

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jqXHR

As of jQuery 1.5, the $.ajax() method returns the jXHR object, which is a superset of the XMLHTTPRequest object. For more information, see thejXHR section of the $.ajax entry

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From JQUERY 1.5 RELEASED:

DEFERRED OBJECTS

Along with the rewrite of the Ajax module a new feature was introduced which was also made publicly available: Deferred Objects. This API allows you to work with return values that may not be immediately present (such as the return result from an asynchronous Ajax request). Additionally it gives you the ability to attach multiple event handlers (something that wasn’t previously possible in the Ajax API).

Additionally you can make your own deferred objects using the exposed jQuery.Deferred. More information about this API can be found in the Deferred Object documentation.

Eric Hynds has written up a good tutorial on Using Deferreds in jQuery 1.5.