Both of those platforms have perfectly reasonable languages available (Java & Swift or ObjC), why would you use JavaScript over them? Cross platform UI has AFA

Both of those platforms have perfectly reasonable languages available (Java & Swift or ObjC), why would you use JavaScript over them? Cross platform UI has AFAICT never worked, so you won't be able to reuse code from one platform on another, or is that what you intend to do? In that case you could still transpile from either Java or Swift...