WinJS, for those who did not know, was first created for Windows 8 focusing on native Windows Runtime apps and eventually on other platforms, including the Xbox One. WinJS 2.0 was introduced with Windows 8.1 and added new UX patterns like the Hub control, and optimized performance for greater flexibility for designers and developers.
So why go cross platform? According to Microsoft, this allows WinJS to extend its reach and allow for a personality of Windows apps without having to rewrite code or learn a new skill.
Now that WinJS is open source, Microsoft wants your help to make it better. "The community is welcome to provide feedback on the check-ins, bug fixes, and new features through GitHub. You can build and test products on a daily basis using the most up-to-date version of the source code and corresponding tests. You can request bug fixes and new features by filing bugs. You can also submit pull requests for your contributions, all through GitHub. Your involvement can impact the quality and direction of the project significantly," Microsoft explained.