Nativescript is different from react native because it includes a native call translation layer -- you can write JS that actually uses native runtime objects. It's also got excellent support for cross platform applications that reach down to native on both platforms, and can meld right in with a fully native app as well, and is thoroughly F/OSS (they take the marketplace approach to monetization).
Nativescript different from Flutter in that it has nothing to do with the tirefire that is Dart. Nativescript is OSS and is taking the marketplace approach to capitalizing and recently they got Hot Module Reloading as a result of integrating more closely with webpack/other capable bundlers.
I've recently done projects (one larger client app, another a small personal project) in both and kept some notes on what I liked about Nativescript vs Flutter (I'm pretty biased for Nativescript now), not sure if I'll make a blog post about it but here are some hopefully interesting points:
- Performance tradeoffs are roughly the same for dart vs nativescript, do hard work in an isolate (dart) or webworker(nativescript)
- Flutter's reactivity is 10x more tedious than Nativescript-Vue. Making an entire other class and doing all these overrides for what should be a basic feature is pretty ridiculous, though everyone's fine with it because they just right click in their $IDE.
- Flutter's data management is not great -- everything they've made so far seems like a hack. I'm uncomfortable with the build function being on the state object conceptually, InheritedWidgets are just using context as a grab-bag, the BLoC pattern is so complicated.
- State<MyPage> & StatefulWidget being separate classes and the State<MyPage> having the build function unsettles me. I don't know if it had to be this way because of dart 2's type system (which is subpar in my experience, Typescript is better because it at least offers duck-typed interfaces)
- Dart is terrible. It is a modern language yet it lacks almost none of the advances of modern languages, Dart 2 was a near rewrite and it it still manages to be less compelling than Typescript, and nowhere near languages like Go, Rust, Crystal, etc. I am kind of suspicious google is pushing this language just because they invented it and can control it -- I don't know a single redeeming quality of dart, especially if you consider dart 1 before you consider dart 2.
- Flutter subtly forces you into either Material Design or Cupertino -- this surprised me. Flutter subtly pushes you towards material design, which I have no interest in, and I think most people actually trying to create unique app experiences wouldn't either. This is a double edged sword, in Nativescript you have to seek out native-looking components, but in Flutter they're forced upon you especially at the beginning, and I prefer the former.
- Automatic sending of crash logs to Google -- who thought this was a good idea? And to make it opt-out instead of opt-in?
- Flutter probably has the better longer term viability because it's being embedded into Fuschia and other things google is doing.
- I expect Nativescript to eventually take on WASM and really open up the choices for languages to use with it.