// create a deferred object by
var dfd = jQuery.Deferred();
// create a promise object by
var promise = dfd.promise();
the differences between them:
The Promise exposes only the Deferred methods needed to attach additional handlers or determine the state(then, done, fail, always, pipe, progress, state, promise),
NOT the ones that change the state (resolve, reject, notify, resolveWith, rejectWith and notifyWith)
the AJAX function $.ajax() returns a jqXHR object, it implements the Promise interface, so you can add callbacks to it by .done(), .fail() and .always()
do something only when multiple actions are completed:
$.when(promise1, promise2).done(function() {
// do something here, it will only be executed when both the promises have been resolved
})