jQuery Deferred notify Method Explained

Calling the .notify() method on a jQuery Deferred object triggers any progress callbacks attached to that object, allowing you to broadcast progress updates during an asynchronous operation before the object is resolved or rejected. This overview covers the internal mechanics of .notify(), when it executes, how it passes data, and how it handles the Deferred object's lifecycle.

Execution of Progress Callbacks

When .notify() is invoked, jQuery executes all callbacks previously registered using deferred.progress() or the third argument of deferred.then(). The callbacks run in the order they were added.

var deferred = $.Deferred();

deferred.progress(function(message) {
    console.log("Progress: " + message);
});

deferred.notify("50% completed");
// Output: "Progress: 50% completed"

Passing Arguments

Any arguments passed to .notify() are forwarded directly to the progress handlers. You can pass single values, objects, or multiple arguments to convey state, percentage completion, or status messages:

deferred.notify(stepNumber, totalSteps, statusText);

To set a custom execution context (this) for the handlers, jQuery provides the companion method deferred.notifyWith(context, [args]).

State Restrictions

The .notify() method is state-dependent:

• Pending State: .notify() functions normally, immediately firing existing progress handlers and accepting new calls.
• Resolved or Rejected State: Once a Deferred transitions to either "resolved" (via .resolve()) or "rejected" (via .reject()), any subsequent calls to .notify() are ignored. Progress callbacks will no longer execute.

Multiple Invocations

Unlike .resolve() and .reject(), which permanently alter the Deferred's state and can only trigger completion handlers once, .notify() can be called repeatedly as long as the Deferred remains pending. This makes it ideal for streaming data, measuring file upload/download progress, or notifying the UI of multi-step task completion.