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[](http://travis-ci.org/kriskowal/q) <a href="http://promises-aplus.github.com/promises-spec"> <img src="http://promises-aplus.github.com/promises-spec/assets/logo-small.png" align="right" alt="Promises/A+ logo" /> </a> If a function cannot return a value or throw an exception without blocking, it can return a promise instead. A promise is an object that represents the return value or the thrown exception that the function may eventually provide. A promise can also be used as a proxy for a [remote object][Q-Connection] to overcome latency. [Q-Connection]: https://github.com/kriskowal/q-connection On the first pass, promises can mitigate the “[Pyramid of Doom][POD]”: the situation where code marches to the right faster than it marches forward. [POD]: http://calculist.org/blog/2011/12/14/why-coroutines-wont-work-on-the-web/ ```javascript step1(function (value1) { step2(value1, function(value2) { step3(value2, function(value3) { step4(value3, function(value4) { // Do something with value4 }); }); }); }); ``` With a promise library, you can flatten the pyramid. ```javascript Q.fcall(promisedStep1) .then(promisedStep2) .then(promisedStep3) .then(promisedStep4) .then(function (value4) { // Do something with value4 }) .catch(function (error) { // Handle any error from all above steps }) .done(); ``` With this approach, you also get implicit error propagation, just like `try`, `catch`, and `finally`. An error in `promisedStep1` will flow all the way to the `catch` function, where it’s caught and handled. (Here `promisedStepN` is a version of `stepN` that returns a promise.) The callback approach is called an “inversion of control”. A function that accepts a callback instead of a return value is saying, “Don’t call me, I’ll call you.”. Promises [un-invert][IOC] the inversion, cleanly separating the input arguments from control flow arguments. This simplifies the use and creation of API’s, particularly variadic, rest and spread arguments. [IOC]: http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript ## Getting Started The Q module can be loaded as: - A ``<script>`` tag (creating a ``Q`` global variable): ~2.5 KB minified and gzipped. - A Node.js and CommonJS module, available in [npm](https://npmjs.org/) as the [q](https://npmjs.org/package/q) package - An AMD module - A [component](https://github.com/component/component) as ``microjs/q`` - Using [bower](http://bower.io/) as ``q`` - Using [NuGet](http://nuget.org/) as [Q](https://nuget.org/packages/q) Q can exchange promises with jQuery, Dojo, When.js, WinJS, and more. ## Resources Our [wiki][] contains a number of useful resources, including: - A method-by-method [Q API reference][reference]. - A growing [examples gallery][examples], showing how Q can be used to make everything better. From XHR to database access to accessing the Flickr API, Q is there for you. - There are many libraries that produce and consume Q promises for everything from file system/database access or RPC to templating. For a list of some of the more popular ones, see [Libraries][]. - If you want materials that introduce the promise concept generally, and the below tutorial isn't doing it for you, check out our collection of [presentations, blog posts, and podcasts][resources]. - A guide for those [coming from jQuery's `$.Deferred`][jquery]. We'd also love to have you join the Q-Continuum [mailing list][]. [wiki]: https://github.com/kriskowal/q/wiki [reference]: https://github.com/kriskowal/q/wiki/API-Reference [examples]: https://github.com/kriskowal/q/wiki/Examples-Gallery [Libraries]: https://github.com/kriskowal/q/wiki/Libraries [resources]: https://github.com/kriskowal/q/wiki/General-Promise-Resources [jquery]: https://github.com/kriskowal/q/wiki/Coming-from-jQuery [mailing list]: https://groups.google.com/forum/#!forum/q-continuum ## Tutorial Promises have a ``then`` method, which you can use to get the eventual return value (fulfillment) or thrown exception (rejection). ```javascript promiseMeSomething() .then(function (value) { }, function (reason) { }); ``` If ``promiseMeSomething`` returns a promise that gets fulfilled later with a return value, the first function (the fulfillment handler) will be called with the value. However, if the ``promiseMeSomething`` function gets rejected later by a thrown exception, the second function (the rejection handler) will be called with the exception. Note that resolution of a promise is always asynchronous: that is, the fulfillment or rejection handler will always be called in the next turn of the event loop (i.e. `process.nextTick` in Node). This gives you a nice guarantee when mentally tracing the flow of your code, namely that ``then`` will always return before either handler is executed. In this tutorial, we begin with how to consume and work with promises. We'll talk about how to create them, and thus create functions like `promiseMeSomething` that return promises, [below](#the-beginning). ### Propagation The ``then`` method returns a promise, which in this example, I’m assigning to ``outputPromise``. ```javascript var outputPromise = getInputPromise() .then(function (input) { }, function (reason) { }); ``` The ``outputPromise`` variable becomes a new promise for the return value of either handler. Since a function can only either return a value or throw an exception, only one handler will ever be called and it will be responsible for resolving ``outputPromise``. - If you return a value in a handler, ``outputPromise`` will get fulfilled. - If you throw an exception in a handler, ``outputPromise`` will get rejected. - If you return a **promise** in a handler, ``outputPromise`` will “become” that promise. Being able to become a new promise is useful for managing delays, combining results, or recovering from errors. If the ``getInputPromise()`` promise gets rejected and you omit the rejection handler, the **error** will go to ``outputPromise``: ```javascript var outputPromise = getInputPromise() .then(function (value) { }); ``` If the input promise gets fulfilled and you omit the fulfillment handler, the **value** will go to ``outputPromise``: ```javascript var outputPromise = getInputPromise() .then(null, function (error) { }); ``` Q promises provide a ``fail`` shorthand for ``then`` when you are only interested in handling the error: ```javascript var outputPromise = getInputPromise() .fail(function (error) { }); ``` If you are writing JavaScript for modern engines only or using CoffeeScript, you may use `catch` instead of `fail`. Promises also have a ``fin`` function that is like a ``finally`` clause. The final handler gets called, with no arguments, when the promise returned by ``getInputPromise()`` either returns a value or throws an error. The value returned or error thrown by ``getInputPromise()`` passes directly to ``outputPromise`` unless the final handler fails, and may be delayed if the final handler returns a promise. ```javascript var outputPromise = getInputPromise() .fin(function () { // close files, database connections, stop servers, conclude tests }); ``` - If the handler returns a value, the value is ignored - If the handler throws an error, the error passes to ``outputPromise`` - If the handler returns a promise, ``outputPromise`` gets postponed. The eventual value or error has the same effect as an immediate return value or thrown error: a value would be ignored, an error would be forwarded. If you are writing JavaScript for modern engines only or using CoffeeScript, you may use `finally` instead of `fin`. ### Chaining There are two ways to chain promises. You can chain promises