Migrating 160,000 Lines of Production Banking JavaScript to TypeScript with Zero Downtime

Migrating a 160,000-line production JavaScript codebase to TypeScript was challenging but beneficial for WorkMade, a seed-stage startup. Despite the complexities, including zero downtime and multiple environments, the migration improved developer experience, maintainability, and stability. Success was achieved through a strategic approach, including a separate migration branch, incremental typing, continuous integration validation, and extensive testing. Essential tools and extensions, such as ESLint with TypeScript support and Checkly for monitoring, played crucial roles.