The way that UWP APIs are exposed in each language is called projection. To make projection work, each language has a special piece of code that knows how to take a UWP API and make it appear correctly to programs written in that language. To turn on that special code in Chakra requires some calls that were not available to JsRT hosts in the past, but which have now been exposed in Edge JsRT. The main one is called JsProjectWinRTNamespace. (WinRT is yet another legacy name for the UWP.)
JsProjectWinRTNamespace will take a root UWP namespace (such as “Windows”) and project it into the current context, making that namespace available in the global JS namespace. Great, right! Well, there’s one catch: unless your host is a UWP application (i.e. a Store app), you can only project the Windows namespace. That’s because projecting anything outside of the Windows namespace requires loading some UWP component that supports that namespace. And UWP components can only be loaded by UWP applications. This is a somewhat entirely arbitrary restriction, but since the UWP platform is built that way, oh well. It does mean that non-UWP hosts can still get access to some useful UWP system APIs.
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