Gradual JavaScript to TypeScript Migration: Strategies for Large Codebases Over Extended Periods

Introduction: The Challenge of a Decade-Long Migration Imagine a sprawling metropolis built over decades, its infrastructure a patchwork of styles and eras. Now, picture replacing every brick, beam, and wire with a modern alternative—while the city remains fully operational. This was the essence of migrating 11,000 files (1M+ LOC) from JavaScript to TypeScript over 7 years. The scale was immense, the timeline unforgiving, and the risks tangible: technical debt accumulation, plummeting developer productivity, and code quality erosion. The Initial Fracture: Voluntary Adoption’s Double-Edged Sword The migration began as a grassroots movement, with developers voluntarily adopting TypeScript in isolated modules. This approach, while fostering buy-in, introduced a critical flaw: inconsistent TypeScript usage. Without centralized coordination, teams implemented TypeScript in silos, creating a fragmented codebase. The mechanical analogy? A machine where each gear is upgraded independently—some