If the message wasn't clear enough, one of the main strengths of React Native is being able to leverage Web technologies.
These obviously include JavaScript, the Browser and its Developer Tools.

Joshua Sierles has made it even clearer by hacking the

React Native Playground

Think JSFiddle, CodePen or JS Bin for Mobile Apps.

All of this is possible thanks to the Appetize.io's iOS & Android Simulators in the Browser.