You will always keep separate code base for Android and iOS due to difference in the interaction paradigms between the platforms. Doesn't matter if the code is

You will always keep separate code base for Android and iOS due to difference in the interaction paradigms between the platforms. Doesn't matter if the code is in Javascript/CSS or shared via Swift between Android and iOS. For native apps. For web it's different but you see more and more webapps following the Android's Material Design which for me feels awkward on Windows or macOS.