# `$q` (service in module `ng` )

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## Description

A promise/deferred implementation inspired by Kris Kowal's Q.

The CommonJS Promise proposal describes a promise as an interface for interacting with an object that represents the result of an action that is performed asynchronously, and may or may not be finished at any given point in time.

From the perspective of dealing with error handling, deferred and promise APIs are to asynchronous programming what `try`, `catch` and `throw` keywords are to synchronous programming.

      // for the purpose of this example let's assume that variables `$q` and `scope` are
      // available in the current lexical scope (they could have been injected or passed in).

      function asyncGreet(name) {
        var deferred = $q.defer();

        setTimeout(function() {
          // since this fn executes async in a future turn of the event loop, we need to wrap
          // our code into an $apply call so that the model changes are properly observed.
          scope.$apply(function() {
            if (okToGreet(name)) {
              deferred.resolve('Hello, ' + name + '!');
            } else {
              deferred.reject('Greeting ' + name + ' is not allowed.');
            }
          });
        }, 1000);

        return deferred.promise;
      }

      var promise = asyncGreet('Robin Hood');
      promise.then(function(greeting) {
        alert('Success: ' + greeting);
      }, function(reason) {
        alert('Failed: ' + reason);
      });

At first it might not be obvious why this extra complexity is worth the trouble. The payoff comes in the way of guarantees that promise and deferred APIs make.

Additionally the promise api allows for composition that is very hard to do with the traditional callback (CPS) approach. For more on this please see the Q documentation especially the section on serial or parallel joining of promises.

### The Deferred API

A new instance of deferred is constructed by calling `$q.defer()`.

The purpose of the deferred object is to expose the associated Promise instance as well as APIs that can be used for signaling the successful or unsuccessful completion of the task.

Methods

- `resolve(value)` – resolves the derived promise with the `value`. If the value is a rejection constructed via `$q.reject`, the promise will be rejected instead.
- `reject(reason)` – rejects the derived promise with the `reason`. This is equivalent to resolving it with a rejection constructed via `$q.reject`.

Properties

- promise – `{Promise}` – promise object associated with this deferred.

### The Promise API

A new promise instance is created when a deferred instance is created and can be retrieved by calling `deferred.promise`.

The purpose of the promise object is to allow for interested parties to get access to the result of the deferred task when it completes.

Methods

- `then(successCallback, errorCallback)` – regardless of when the promise was or will be resolved or rejected calls one of the success or error callbacks asynchronously as soon as the result is available. The callbacks are called with a single argument the result or rejection reason.

  This method returns a new promise which is resolved or rejected via the return value of the `successCallback` or `errorCallback`.

### Chaining promises

Because calling `then` api of a promise returns a new derived promise, it is easily possible to create a chain of promises:

      promiseB = promiseA.then(function(result) {
        return result + 1;
      });

      // promiseB will be resolved immediately after promiseA is resolved and its value will be
      // the result of promiseA incremented by 1

It is possible to create chains of any length and since a promise can be resolved with another promise (which will defer its resolution further), it is possible to pause/defer resolution of the promises at any point in the chain. This makes it possible to implement powerful apis like $http's response interceptors.

### Differences between Kris Kowal's Q and $q

There are three main differences:

- $q is integrated with the `ng.$rootScope.Scope` Scope model observation mechanism in angular, which means faster propagation of resolution or rejection into your models and avoiding unnecessary browser repaints, which would result in flickering UI.
- $q promises are recognized by the templating engine in angular, which means that in templates you can treat promises attached to a scope as if they were the resulting values.

- Q has many more features that $q, but that comes at a cost of bytes. $q is tiny, but contains all the important functionality needed for common async tasks.

  ### Testing

         it('should simulate promise', inject(function($q, $rootScope) {
           var deferred = $q.defer();
           var promise = deferred.promise;
           var resolvedValue;

           promise.then(function(value) { resolvedValue = value; });
           expect(resolvedValue).toBeUndefined();

           // Simulate resolving of promise
           deferred.resolve(123);
           // Note that the 'then' function does not get called synchronously.
           // This is because we want the promise API to always be async, whether or not
           // it got called synchronously or asynchronously.
           expect(resolvedValue).toBeUndefined();

           // Propagate promise resolution to 'then' functions using $apply().
           $rootScope.$apply();
           expect(resolvedValue).toEqual(123);
         });

## Dependencies

- `$rootScope`

## Methods

- ### all(promises)

  Combines multiple promises into a single promise that is resolved when all of the input promises are resolved.

  #### Parameters

  - `promises – {Array.<Promise>|Object.<Promise>} –`

    An array or hash of promises.

  #### Returns

  `{Promise}` –

  Returns a single promise that will be resolved with an array/hash of values, each value corresponding to the promise at the same index/key in the `promises` array/hash. If any of the promises is resolved with a rejection, this resulting promise will be resolved with the same rejection.

- ### defer()

  Creates a `Deferred` object which represents a task which will finish in the future.

  #### Returns

  `{Deferred}` –

  Returns a new instance of deferred.

- ### reject(reason)

  Creates a promise that is resolved as rejected with the specified `reason`. This api should be used to forward rejection in a chain of promises. If you are dealing with the last promise in a promise chain, you don't need to worry about it.

  When comparing deferreds/promises to the familiar behavior of try/catch/throw, think of `reject` as the `throw` keyword in JavaScript. This also means that if you "catch" an error via a promise error callback and you want to forward the error to the promise derived from the current promise, you have to "rethrow" the error by returning a rejection constructed via `reject`.

        promiseB = promiseA.then(function(result) {
          // success: do something and resolve promiseB
          //          with the old or a new result
          return result;
        }, function(reason) {
          // error: handle the error if possible and
          //        resolve promiseB with newPromiseOrValue,
          //        otherwise forward the rejection to promiseB
          if (canHandle(reason)) {
           // handle the error and recover
           return newPromiseOrValue;
          }
          return $q.reject(reason);
        });

  #### Parameters

  - `reason – {*} –`

    Constant, message, exception or an object representing the rejection reason.

  #### Returns

  `{Promise}` –

  Returns a promise that was already resolved as rejected with the `reason`.

- ### when(value)

  Wraps an object that might be a value or a (3rd party) then-able promise into a $q promise. This is useful when you are dealing with an object that might or might not be a promise, or if the promise comes from a source that can't be trusted.

  #### Parameters

  - `value – {*} –`

    Value or a promise

  #### Returns

  `{Promise}` –

  Returns a single promise that will be resolved with an array of values, each value corresponding to the promise at the same index in the `promises` array. If any of the promises is resolved with a rejection, this resulting promise will be resolved with the same rejection.