How We Migrated 200K Lines from JS to Strict TypeScript

We didn't mass-rename .js files to .ts and call it a day. We migrated 200,000 lines of production JavaScript to strict TypeScript — with zero downtime, over 4 months, while shipping features every sprint. Here's the playbook that actually worked. Why We Almost Didn't Do It The codebase was 6 years old. A Node.js monolith serving 50K daily active users, built by 12+ developers over the years. Some files had JSDoc types. Most had // @ts-ignore comments from a half-hearted migration attempt in 2021. The business case was clear: we were spending 30% of every sprint debugging type-related bugs that TypeScript would have caught at compile time. Null reference errors in production. API responses with unexpected shapes. Refactoring was terrifying because nobody knew what would break. But a big-bang migration was off the table. We couldn't freeze features for 4 months. So we built a system. The 5-Phase Strategy Phase 1: Set Up the Dual Runtime (Week 1) The first step wasn't touching any JavaScr