Small recommendation for folks looking to use JS for Windows development (I understand this is just a runtime, but people may be inclined to combine these two things): do not use WinJS for any reason. It's undoubtably the worst UI "framework" I've ever used. Completely inflexible (HTML and styles defined in private and inaccessible methods, swallows events, monolithic functions) and riddled with bugs and incomplete features. Really surprised it's an official Microsoft project. I don't mean to disparage the developers of it, but it's really bad. Don't use it if you value your time or peace of mind and plan on customizing it in any meaningful way.
Do yourself a favor and use C# or even C++\Cx instead. They're both a much better fit for XAML.<p>Full disclosure: I work at Microsoft on XAML UI in C++. It's nice. Opinions are my own.
I have started to develop mobile applications with NativeScript and I am absolute in love with it. It has an Angular feel to it (I despise all things React). Before, I had only been doing backend work and I still have only taken care of the Web API portion of our NS app, but I hope to do some GUI soon. I am extremely impressed by NS.