Migrating 3.7 Million Lines of Flow Code to TypeScript

Pinterest successfully migrated 3.7 million lines of code from Flow to TypeScript over eight months, enhancing type safety, developer experience, and hiring practices. The process involved three key phases: setup, conversion, and integration, supported by various tools and community contributions. Daily automated testing, manual testing rounds, and static analysis ensured the migration's accuracy and stability. The migration culminated in a seamless rollout, with 97% of engineers positively rating the experience and improved developer education and library type coverage.