As our codebase grew in size, we were looking for ways to improve code quality. We chose TypeScript over Flow due to its rapid industry adoption and overall tools support.

We noticed how different open-source projects were migrating from Flow to TypeScript. Most notably, it was Jest, even though Jest and Flow were both developed by Facebook. See this HN thread if you want to dive into an interesting discussion around this move.

Additionally, at the beginning of 2019, both Babel and ESLint enabled seamless TypeScript support, which allowed easy migration path in a backward-compatible way.