Note: This is for just getting a brand new react-native app working using react-native init. I haven’t tried it with CRNA. You might have to do lots of other fun things if you have an existing project.
So, if you’re trying to get react-native working with XCode 10 and encountering a bunch of mysterious errors, here’s what you do.
Change the build setting to legacy
Fix babel runtime
Just install @babel/runtime as a dev dependency (https://github.com/facebook/react-native/issues/21310).
npm install @babel/runtime --save-dev
yarn add @babel/runtime --dev
For folks with an existing app who upgraded to XCode 10, you probably have other issues and this might serve as a good starting point.