Learn all about the jQuery function deferred.progress().
deferred.progress() method accepts one or more arguments, all of which can be either a single function or an array of functions. When the Deferred generates progress notifications by calling
progressCallbacks are called. Since
deferred.progress() returns the Deferred object, other methods of the Deferred object can be chained to this one. When the Deferred is resolved or rejected, progress callbacks will no longer be called, with the exception that any
progressCallbacks added after the Deferred enters the resolved or rejected state are executed immediately when they are added, using the arguments that were passed to the
notifyWith() call. For more information, see the documentation for jQuery.Deferred().