What are deferred objects?

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.