50,000-line JavaScript-to-TypeScript migration with 95% test coverage

Context

DKMS Group is a global nonprofit focused on fighting blood cancer. Their Electronic Registration App — used by thousands of donors — was built in JavaScript and had accumulated significant technical debt.

Problem

The codebase had grown to over 50,000 lines of untyped JavaScript. Bugs were hard to catch before production, onboarding new developers was slow, and the team lacked confidence in making changes.

Approach

I led a systematic migration from JavaScript to TypeScript, introducing strict typing incrementally to avoid breaking production. Alongside the migration, I established code review processes and consistent code style standards across the team.

Shipped

• Migrated 50,000+ lines of JavaScript to TypeScript with strict mode enabled
• Achieved 95% test coverage through comprehensive unit and end-to-end tests
• Implemented team-wide code review processes and style standards
• Delivered the migration without disrupting ongoing feature development

Results

Tech stack

• React
• TypeScript
• Python
• Django
• MariaDB
• Docker
• Kubernetes
• AWS