Windows® 8.1 Apps with HTML5 and JavaScript Unleashed

Using Promises

The CommonJS website defines a promise like this (http://wiki.commonjs.org/wiki/Promises):

“Promises provide a well-defined 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. By utilizing a standard interface, different components can return promises for asynchronous actions and consumers can utilize the promises in a predictable manner.”

A promise provides a standard pattern for specifying callbacks. In the WinJS library, when you create a promise, you can specify three callbacks: a complete callback, a failure callback, and a progress callback (both the failure and progress callbacks are optional).

Promises ...