/refactor-legacy-code

name: refactor-legacy-code
description: >
  Modernize and improve legacy codebases while maintaining functionality. Use
  when you need to refactor old code, reduce technical debt, modernize
  deprecated patterns, or improve code maintainability without breaking existing
  behavior.

Refactor Legacy Code

Table of Contents

• [Overview](#overview)
• [When to Use](#when-to-use)
• [Quick Start](#quick-start)
• [Reference Guides](#reference-guides)
• [Best Practices](#best-practices)

Overview

This skill helps you systematically refactor legacy code to improve maintainability, readability, and performance while preserving existing functionality. It follows industry best practices for safe refactoring with comprehensive testing.

When to Use

• Modernizing outdated code patterns or deprecated APIs
• Reducing technical debt in existing codebases
• Improving code readability and maintainability
• Extracting reusable components from monolithic code
• Upgrading to newer language features or frameworks
• Preparing code for new feature development

Quick Start

First, analyze the legacy code to understand:

# Review the codebase structure
tree -L 3 -I 'node_modules|dist|build'

# Check for outdated dependencies
npm outdated  # or pip list --outdated, composer outdated, etc.

# Identify code complexity hotspots
# Use tools like:
# - SonarQube for code smells
# - eslint for JavaScript
# - pylint for Python
# - RuboCop for Ruby

Reference Guides

Detailed implementations in the `references/` directory:

| Guide | Contents | |---|---| | [Code Assessment](references/code-assessment.md) | Code Assessment | | [Establish Safety Net](references/establish-safety-net.md) | Establish Safety Net | | [Incremental Refactoring](references/incremental-refactoring.md) | Incremental Refactoring | | [Modernize Patterns](references/modernize-patterns.md) | Modernize Patterns | | [Reduce Dependencies](references/reduce-dependencies.md) | Reduce Dependencies, Documentation | | [Complete Refactoring Example](references/complete-refactoring-example.md) | Complete Refactoring Example | | [Benefits Achieved](references/benefits-achieved.md) | Benefits Achieved |

Best Practices

✅ DO

• **Refactor incrementally**: Small, testable changes
• **Run tests frequently**: After each refactoring step
• **Commit often**: Create logical, atomic commits
• **Keep existing tests passing**: Don't break functionality
• **Use IDE refactoring tools**: Safer than manual edits
• **Review code coverage**: Ensure tests cover refactored code
• **Document decisions**: Why, not just what
• **Seek peer review**: Fresh eyes catch issues

❌ DON'T

• **Mix refactoring with new features**: Separate concerns
• **Refactor without tests**: Recipe for breaking changes
• **Change behavior**: Refactoring should preserve functionality
• **Refactor large chunks**: Increases risk and review difficulty
• **Ignore code smells**: Address them systematically
• **Skip documentation**: Future maintainers need context