How We Adopted TypeScript 5.6 for Our React 19 App and Reduced Runtime Errors by 90% Introduction Our team maintains a large-scale React 19 e-commerce application serving 500k+ monthly active users. For 18 months, we ran the app in plain JavaScript, relying on manual testing and sporadic ESLint rules to catch issues. Runtime errors—undefined property accesses, mismatched prop types, invalid state updates—plagued our sprint cycles, with an average of 42 runtime-related bugs reported per month. We knew we needed a type system, but waited for React 19’s stable release and TypeScript 5.6’s new features to align our migration timeline.…