Migrating 200K Lines of JavaScript to TypeScript: A Survival Guide
The two-year journey of migrating our entire frontend from JavaScript to TypeScript — the strategy, the tooling, and the bugs we caught.
We did not start with TypeScript. Our frontend was 200,000 lines of plain JavaScript with JSDoc comments pretending to be type annotations. Migration was inevitable — here is how we did it without stopping feature development.
The Strategy
- Started with strict: false and gradually tightened
- Migrated shared libraries and utilities first
- New code must be TypeScript — no exceptions
- Automated codemods for common patterns (PropTypes → interfaces)
- Dedicated "TypeScript Friday" — one day per sprint for migration work
What TypeScript Caught
During migration we found and fixed over 400 latent bugs — null reference errors, wrong function argument orders, impossible state combinations, and stale API response types. Many of these had been causing intermittent production issues for months.
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