Develop Universal App that runs across Windows, Windows Phone and FirefoxOS

With this release of the Windows developer platform of universal Windows apps, Microsoft sets out to accomplish three major goals: 1) Reach customers across phones, tablets, and PCs; 2) Deliver innovation that supports developer investments; 3) Make cross-platform technology easier and more capable.
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You can build an app for Windows and Windows Phone at the same time, and share code, user controls, styles, strings, and other assets between the two projects in Visual Studio. This reduces the expense associated with building and maintaining an app for each type of device. The universal app templates in Visual Studio is currently available in JavaScript, C#. VB and C++.

If you already have a Windows Store app, you can easily add a Windows Phone Store app to the same solution. Similarly, if you started by creating a Windows Phone app, you can easily add a Windows Store app.

With the open sourcing of WinJS, it might be interesting to see if we can build an app that runs across phone, Windows and another platform such as Firefox OS. Martin Beeby did an interesting demo on building a JavaScript app and see how far we can take it:

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