Its good to know you are interesting in joining to the cause of creating native apps using JavaScript. You are in the right direction, both Nativescript and React Native will guide you to your goal: Build professional applications for iOS/Android using Javascript, however there are some differences you should know at the time you decide which framework to use.
React Native:
Its a framework developed by Facebook, using React it renders true native views. It uses Flexbox to decorate the apps so if you never used it that will be a new challenge for you, it is not hard to learn. My problem with this is that there is no direct support from the developer team, and only the community is from where you get the help, and sometimes it is not quite accurate. To create iOS apps you need a Mac computer, otherwise you can only create apps for Android. As far as I know, some basic information you might need in your app, such like platform, OS version, portrait/landscape it is developed by plugins from the community, and this information is not coming from the framework itself.
NativeScript:
Its created by Telerik, a very strong programming company who has high quality developers and strong support for its products. It uses Angular as a option to create your apps and its very well documented. If you are a good CSS developer you will be fine because they uses CSS to decorate your app. Nativescript community has developed tons of plugins. Nativescript core team is creating a lots of tools to help you through the process. Recently they launched a tool called Sidekick, which allows you to build/livesync your app from the cloud, which means you do not need a Mac computer for create iOS apps (isn't that cool?). With Nativescript you can choose Javascript, Typescript or Angular+Typescript, all of them will end up creating native apps. For support, you can contact the core team directly, and they will give you the best answer you can have, this is one of their goals.
I hope I have answered your question.
Thanks!
