JavaScript programmers are accustomed to the asynchronous programming model. Both a browser and the node.js execution environments are full of asynchronous events: XHR responses, DOM events, IO and timeouts, which can be triggered at any moment and in random order. Even if, we are all used to coping with the asynchronous nature of the execution environment the truth is that asynchronous programming might be perplexing, especially when it comes to synchronizing multiple asynchronous events.

In the synchronous world chaining function calls (invoking a function with a result of another function) and handling exceptions (with try/catch) is straightforward. In the asynchronous world, we can’t simply chain function calls; we need to rely on callbacks. Callbacks are fine when dealing with just one asynchronous event, but things start to get complicated as soon as we need to coordinate multiple asynchronous events. Exceptional situation handling is particularly tough in this case.

To make asynchronous programming easier the Promise API was recently adopted by several popular JavaScript libraries. The concepts behind the Promise API are not new, and were proposed in the late seventies, but only recently those concepts made it into the mainstream JavaScript programming.


The main idea behind the Promise API is to bring to the asynchronous world the same ease of functions calls chaining and error handling as we can enjoy in the synchronous programming world.

angularjs comes with the $q service a very lightweight Promise API implementation. A number of angularjs services (most notably $http, but also $timeout and others) heavily rely on the promise-style APIs. So we need to get familiar with $q in order to use those services effectively.


The $q service was inspired by the Kris Kowal’s Q Promise API library ( You might want to check out this library to gain a better understanding or promise concepts and compare angularjs lightweight implementation with the full featured Promise API library.