Promises are an exciting jQuery feature that make it a breeze to manage async events. They allow you to write clearer, shorter callbacks and keep high-level application logic separate from low-level behaviors.

Once you understand Promises, you'll want to use them for everything from AJAX calls to UI flow. That's a promise!

Understanding Promises

Once a Promise is resolved or rejected, it'll remain in that state forever.

A Promise is an object that represents a one-time event, typically the outcome of an async task like an AJAX call. At first, a Promise is in a pending state. Eventually, it's either resolved (meaning the task is done) or rejected (if the task failed). Once a Promise is resolved or rejected, it'll remain in that state forever, and its callbacks will never fire again.

You can attach callbacks to the Promise, which will fire when the Promise is resolved or rejected. And you can add more callbacks whenever you want - even after the Promise has been resolved/rejected! (In that case, they'll fire immediately.)

Plus, you can combine Promises logically into new Promises. That makes it trivially easy to write code that says, "When all of these things have happened, do this other thing."

And that's all you need to know about Promises in the abstract. There are several JavaScript implementations to choose from. The two most notable are Kris Kowal's q, based on the CommonJS Promises/A spec, and jQuery Promises (added in jQuery 1.5). Because of jQuery's ubiquity, we"ll use its implementation in this tutorial.

Making Promises With $.Deferred

Every jQuery Promise begins with a Deferred. A Deferred is just a Promise with methods that allow its owner to resolve or reject it. All other Promises are "read-only" copies of a Deferred; we'll talk about those in the next section. To create a Deferred, use the $.Deferred() constructor:

A Deferred is just a Promise with methods that allow its owner to resolve or reject it.

(Version note: state() was added in jQuery 1.7. In 1.5/1.6, use isRejected() and isResolved() .)

We can get a "pure" Promise by calling a Deferred's promise() method. The result is identical to the Deferred, except that the resolve() and reject() methods are missing.

The promise() method exists purely for encapsulation: If you return a Deferred from a function, it might be resolved or rejected by the caller. But if you only return the pure Promise corresponding to that Deferred, the caller can only read its state and attach callbacks. jQuery itself takes this approach, returning pure Promises from its AJAX methods:

Using the - ing tense in the name of a Promise makes it clear that it represents a process.

Modeling a UI Flow With Promises

Once you have a Promise, you can attach as many callbacks as you like using the done() , fail() , and always() methods:

Version Note: always() was referred to as complete() before jQuery 1.6.

There's also a shorthand for attaching all of these types of callbacks at once, then() :

Callbacks are guaranteed to run in the order they were attached in.

One great use case for Promises is representing a series of potential actions by the user. Let's take a basic AJAX form, for example. We want to ensure that the form can only be submitted once, and that the user receives some acknowledgement when they submit the form. Furthermore, we want to keep the code describing the application's behavior separate from the code that touches the page's markup. This will make unit testing much easier, and minimize the amount of code that needs to be changed if we modify our page layout.

(We're taking advantage of the fact that arguments passed to resolve() / reject() are forwarded verbatim to each callback.)

Borrowing Promises From the Future

pipe() returns a new Promise that will mimic any Promise returned from one of the pipe() callbacks.

Our feedback form code looks good, but there's room for improvement in the interaction. Rather than optimistically assuming that our POST call will succeed, we should first indicate that the form has been sent (with an AJAX spinner, say), then tell the user whether the submission succeeded or failed when the server responds.

We can do this by attaching callbacks to the Promise returned by $.post . But therein lies a challenge: We need to manipulate the DOM from those callbacks, and we've vowed to keep our DOM-touching code out of our application logic code. How can we do that, when the POST Promise is created within an application logic callback?

A solution is to "forward" the resolve/reject events from the POST Promise to a Promise that lives in the outer scope. But how do we do that without several lines of bland boilerplate ( promise1.done(promise2.resolve); ...)? Thankfully, jQuery provides a method for exactly this purpose: pipe() .

pipe() has the same interface as then() ( done() callback, reject() callback, always() callback; each callback is optional), but with one crucial difference: While then() simply returns the Promise it's attached to (for chaining), pipe() returns a new Promise that will mimic any Promise returned from one of the pipe() callbacks. In short, pipe() is a window into the future, allowing us to attach behaviors to a Promise that doesn't even exist yet.

Here's our new and improved form code, with our POST Promise piped to a Promise called savingFeedback :

Finding the Intersection of Promises

Part of the genius of Promises is their binary nature. Because they have only two eventual states, they can be combined like booleans (albeit booleans whose values may not yet be known).

The Promise equivalent of the logical intersection ( AND ) is given by $.when() . Given a list of Promises, when() returns a new Promise that obeys these rules:

When all of the given Promises are resolved, the new Promise is resolved. When any of the given Promises is rejected, the new Promise is rejected.

Any time you're waiting for multiple unordered events to occur, you should consider using when() .

Simultaneous AJAX calls are an obvious use case:

Another use case is allowing the user to request a resource that may or may not have already be available. For example, suppose we have a chat widget that we're loading with YepNope (see Easy Script Loading with yepnope.js)

Conclusion

Promises have proven themselves to be an indispensable tool in the ongoing fight against async spaghetti code. By providing a binary representation of individual tasks, they clarify application logic and cut down on state-tracking boilerplate.