Promises are a programming pattern for dealing with asynchronous operations. The pattern could be applied to other languages, but they are most commonly encountered in JS libraries (like jQuery and WinJS).
Kraig Brockschmidt has a really good blog post about how they work (in general) and in WinJS here:
http://blogs.msdn.com/b/windowsappdev/archive/2013/06/11/all-about-promises-for-windows-store-apps-written-in-javascript.aspx
I’ve written a blog post comparing jQuery promises and promises in WinJS. The short answer: they’re interoperable.
http://blogs.windows.com/windows/b/appbuilder/archive/2013/07/10/jquery-and-winjs-working-together-in-windows-store-apps.aspx
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solved JQuery and WinJS – Promise [closed]