What Is It Like for a Core React Engineer at Facebook?

At Facebook, we have over 50,000 React components in our codebase. We also use a single version of React across all of them. This means that it's a lot easier to make sweeping changes to the codebase because you can update all of it in a single commit. In order for us to upgrade React, we need to make sure that nobody is relying on something that doesn't work in the new version. The biggest thing we've done actually is automating as much as we can.