I modernized a legacy web application by transitioning it from a rigid, aging architecture to a modular and scalable system. The core of this initiative involved overhauling the build pipeline to support a complex, hybrid environment. By replacing the existing task runner with a more sophisticated bundling system, I enabled the application to simultaneously support legacy asynchronous module definitions and modern standard modules. This foundation was critical for introducing type safety across the codebase and managing a multi-stage migration without disrupting existing functionality. To bridge the gap between the old and new architectures, I engineered a custom synchronization adapter that linked the legacy data models with a modern, centralized state management system. This utility ensured two-way data consistency, allowing updates in the legacy models to reflect in the new state and vice versa. This seamless integration prevented data fragmentation and allowed the application to maintain a "single source of truth" while disparate parts of the system were being rewritten. For the interface migration, I implemented an "inside-out" integration strategy. Rather than a risky, all-at-once replacement, I established a pattern where new components were embedded directly within legacy views. This allowed the team to deliver modern user interface improvements immediately within the existing framework. By mounting independent instances of the new library into specific areas of the old application, we were able to systematically migrate the frontend piece-by-piece while maintaining a fully functional product for users. This modernization effort resulted in a significantly more maintainable codebase and a vastly improved developer experience. By establishing these architectural bridges, I moved the project away from technical debt and toward a future-proof environment focused on performance and long-term stability.