From Legacy to Modern: My Strategy for Refactoring Large React Codebases

Modernizing legacy React doesn't require a full rewrite. I break down my approach to incrementally refactor large codebases — improving testability, structure, and developer experience without slowing delivery.

Every engineering team eventually faces it: the bloated legacy React codebase. Years of iteration, evolving styles, inconsistent patterns, and a jungle of props passed down like family heirlooms. The problem isn't just technical debt, it's velocity debt. When changes take longer than they should, onboarding new engineers is painful, and testing becomes fragile, it’s time to refactor.

Here’s how I approach modernizing React codebases without stopping the world.

1. Why Refactoring Was Necessary

Our legacy codebase had grown organically for years, resulting in deeply nested props, outdated class components, and tightly coupled business logic. It slowed delivery and made onboarding new engineers harder than it should be. Refactoring wasn't about polishing code, it was about unblocking the team and creating a maintainable foundation.

2. Define Clear Goals and Constraints

Before refactoring, we set clear objectives:

  • Migrate from class components to modern function components with hooks
  • Replace Enzyme with React Testing Library for better test reliability
  • Decouple business logic from UI components

We had to do this incrementally, feature work couldn’t stop. The solution had to balance improvement with ongoing delivery.

3. Adopt a “Refactor-as-You-Touch” Strategy

We avoided big rewrites. Instead, we refactored as we touched components for feature updates or bug fixes. During each touchpoint, we:

  • Broke down large components into smaller pieces
  • Introduced custom hooks to encapsulate business logic
  • Improved type definitions and prop validation

This approach allowed us to modernize without risking disruption.

4. Modernizing Our Testing Stack

Switching from Enzyme to React Testing Library was a major win. RTL encouraged tests that better reflected user behavior and reduced false positives. We prioritized test migrations for:

  • Shared components
  • Bug-prone features
  • Business-critical workflows

We created helpers like renderComponent to simplify setup and keep tests DRY.

5. Extracting Business Logic with Custom Hooks

We made a deliberate effort to remove business logic from UI layers. For example:

  • Task filtering and feature flag checks were moved into hooks
  • Components focused solely on rendering and user interaction
  • Result: testable, predictable, and reusable logic

Hooks became a powerful tool for enforcing separation of concerns.

6. Cross-Team Collaboration and Documentation

Refactoring only works if it’s embraced by the whole team. We:

  • Created internal guides and shared refactor examples
  • Paired with teammates during early migrations
  • Standardized patterns to avoid fragmentation

This helped scale the effort and build consensus around best practices.

7. Developer Experience and Productivity Wins

Small changes added up:

  • Reduced boilerplate in tests and components
  • Better error messages from cleaner data flows
  • Clearer component responsibilities made debugging faster

Feedback from the team was consistently positive, the codebase became easier to navigate and contribute to.

8. Results and Reflections

The impact wasn’t just technical, we shipped faster, with fewer regressions and higher confidence. The migration wasn’t perfect, but it proved that refactoring can be sustainable, incremental, and aligned with product delivery.

Modernizing a React codebase isn’t about perfection, it’s about enabling progress. With clear goals, a collaborative approach, and thoughtful tradeoffs, you can turn a legacy system into one that grows with your team, not against it.

About the Author

Miguel Osuna

Software Engineer with 5+ years of experience developing scalable applications, improving system performance, and automating workflows. Skilled in full-stack development, cloud technologies, and CI/CD. Passionate about optimizing processes, enhancing user experiences, and building reliable software. A strong collaborator, dedicated to delivering high-quality solutions in fast-paced environments.

Expertise

Management 5+ yrs

Javascript 5+ yrs

Spring framework 5+ yrs

Data model 5+ yrs

Express 5+ yrs