I doubt it would happen at runtime in iOS/Android, though? They would need to ship a JS interpreter that gets run on application startup, parse the React files, instantiate the native controls and create a bridge between them back to the JS interpreter?

Well on second thought I guess Appcelerator Titanium does something similar. So it's native UI only, but with the added overhead (JS interpreter + bridge, larger binary size, slower startup, etc.).

>They would need to ship a JS interpreter that gets run on application startup

Um, why? They already have JavaScriptCore (it's an iOS API).

Depends. There could be a build step that compiles all that logic. Or there could indeed by a JS runtime.