Workflow
Refactoring
Modernize legacy code without breaking existing behavior.
Overview
Refactoring legacy code is risky without the right tools and process. Claude Code helps you modernize outdated patterns, improve code structure, and eliminate technical debt while maintaining backward compatibility. Instead of manually rewriting code and risking breaking changes, you can leverage Claude to analyze dependencies, suggest safe transformations, and execute changes incrementally.
Use this workflow when modernizing legacy codebases, eliminating code duplication, adopting new language features, restructuring for better maintainability, or preparing code for new requirements.
Step-by-step process
Describe the refactor target and constraints
Clearly define what needs to change and what must stay the same. Include information about existing tests, deployment constraints, and any breaking change concerns.
Ask for a plan and staged rollout
Request a detailed refactoring plan that breaks the work into safe, reviewable stages. This minimizes risk and makes it easier to catch issues early.
Apply the refactor in small, reviewable chunks
Execute one stage at a time, reviewing each change before proceeding. Commit after each successful stage to create clear rollback points.
Run tests and check performance
Validate that functionality remains unchanged and performance hasn't degraded. Look for edge cases that tests might not cover.
Example prompts
"Convert all class components in src/components to functional components with hooks. Preserve all existing behavior and prop types. Create a checklist of components to migrate."
"I see similar authentication logic in both api/auth.ts and middleware/auth.ts. Extract the common code into a shared utility and update both files to use it."
"Enable TypeScript strict mode in this project. Create a migration plan that fixes each strict mode violation one directory at a time, starting with utility functions."
"Refactor this monolithic service.ts file into separate modules organized by domain. Maintain all existing exports so consumers don't break."
Expected outputs
Refactor plan
Step-by-step roadmap showing which files change in each stage, dependencies between stages, and validation checkpoints
Code changes
Transformed code with improved structure, modern patterns, and better maintainability while preserving functionality
Validation checklist
List of tests to run, edge cases to verify, and performance benchmarks to check before considering the refactor complete
Migration notes
Documentation of what changed, why, and any follow-up work needed for consumers of the refactored code
Best practices
Common pitfalls
Refactoring too much at once
Solution: Break the work into 3-5 reviewable stages. If a stage touches more than 10 files, split it further.
Not running tests between stages
Solution: Run the test suite after every stage. If tests fail, fix them before proceeding to the next stage.
Assuming tests cover all edge cases
Solution: Ask Claude to identify edge cases not covered by tests and validate them manually or write new tests first.
Pro tips
Use git blame to understand history
""Before refactoring user.service.ts, show me the git blame and recent commits to understand why the current pattern exists.""
Start with high-test-coverage modules
""Identify which modules have the best test coverage. We'll refactor those first since they're safest to change.""
Create a deprecation path for breaking changes
""If we need to change the API, keep the old version as deprecated with a warning, and add the new version alongside it.""