Yelp migrated its million‑line Flow monorepo to TypeScript package‑by‑package
Over three years, Yelp transformed a monorepo of more than a million lines from Flow to TypeScript by converting one package at a time, keeping both type systems running side‑by‑side in production. Using the open‑source flowts tool plus custom codemods, they avoided massive freezes, preserved type‑checking trust, and saved tens of thousands of engineer hours.