Can React Native apps be tested in a browser?

No, React Native can be tested only in mobile simulators like IOS and Android

Update: Facebook has given a beautiful starter pack for React native.

Introducing Create React Native App

Using this you can run the app in your mobile using expo app(https://expo.io) .It syncs using QR code.

Yes! That's very much possible with react-native-web. Here are a couple of related, useful resources to get started:

• React Native Web Starter
• A guide to combine React and ReactNative into one codebase

You can use Expo Snack https://snack.expo.io/ to instantly test your code on a device (using QR code ) or in the browser. For more information, you can go thtrough this article. https://blog.expo.io/sketch-a-playground-for-react-native-16b2401f44a2