Our team recently undertook the migration of a 5,000-line React component from JavaScript to TypeScript. Initially, the task seemed straightforward, but it quickly evolved into a multi-day effort filled with unexpected type errors, subtle runtime issues, and edge cases that only manifested in production. To provide transparency and practical insights, we documented the migration as a live refactor diary, including key commits, timestamps, and obstacles. This approach allowed us to capture the real challenges of a large-scale TypeScript migration rather than presenting an oversimplified narrative of success.

Throughout the process, we found that utility types, including mapped and conditional types, were invaluable in managing repetitive prop transformations. Incrementally migrating smaller sections of the component enabled rapid feedback and safe rollback of changes. The migration also uncovered hidden bugs that had persisted undetected in the original JavaScript code, highlighting the added value of type safety in long-term maintainability.

We have compiled a public repository containing the full commit history and diffs, illustrating what broke, how it was resolved, and the lessons learned in context. For teams planning similar migrations, we also prepared a safe migration checklist covering preparation, incremental strategies, and testing approaches. We invite other developers to contribute additional insights or share their experiences to make this a collaborative resource.