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Refactoring Legacy Code with AI: Turning Spaghetti into Vibes
Legacy code is every developer's nightmare. It's the code written by someone who left the company three years ago. It has no tests, no documentation, and variable names like `x1` and `temp2`. It works, but nobody knows why.
Refactoring legacy code used to be a career-limiting move. It was risky, time-consuming, and often broke things. But with AI-powered Vibe Coding, refactoring becomes manageable—even enjoyable.
In this guide, we'll show you how to use Cursor and Windsurf to safely modernize a legacy codebase.
The Legacy Code Problem
Let's say you inherit a React project from 2018. It has:
* Class components (pre-Hooks era)
* Inline styles mixed with CSS files
* No TypeScript
* jQuery mixed with React (yes, this happens)
* A 2000-line `App.js` file
Your mission: Modernize it without breaking production.
Step 1: Set Up Your Safety Net
Before you touch anything, create a safety net.
1. Version Control
“`bash
git checkout -b refactor-legacy-code
git commit -m “Pre-refactor snapshot”
“`
2. Add Tests (If None Exist)
Prompt to Cursor:
> “Analyze this codebase and generate basic integration tests for the main user flows using Vitest and React Testing Library.”
This gives you a baseline to ensure nothing breaks.
3. Create a `.cursorrules` File
“`markdown
– This is a legacy React codebase being modernized.
– Convert all class components to functional components with hooks.
– Replace inline styles with Tailwind CSS.
– Add TypeScript types where possible.
– Keep the existing functionality intact.
“`
Step 2: The Incremental Refactor Strategy
Never refactor everything at once. Use the “Strangler Fig” pattern: gradually replace old code with new code.
Phase 1: Convert One Component
Pick the smallest, least critical component.
Prompt to Cursor:
> “Refactor `UserCard.jsx` from a class component to a functional component using hooks. Keep the exact same functionality.”
Review the diff. Make sure:
* Props are handled correctly.
* State is converted to `useState`.
* Lifecycle methods are converted to `useEffect`.
Test it. Run your tests. Click through the UI.
Phase 2: Repeat for All Components
Once you're confident, scale up.
Prompt to Windsurf (Flow Mode):
> “Refactor all components in the `src/components` folder from class components to functional components. Do them one at a time and show me the diff for each.”
Windsurf's Cascade will process them sequentially and pause for your approval after each one.
Step 3: Modernize the Styling
Legacy projects often have a mix of:
* Inline styles: `
* CSS files: `import ‘./App.css'`
* CSS-in-JS libraries: `styled-components`
Goal: Consolidate everything into Tailwind CSS.
Install Tailwind
“`bash
npm install tailwindcss postcss autoprefixer
npx tailwindcss init -p
“`
Prompt to Cursor:
> “Convert all inline styles and CSS classes in this project to Tailwind CSS. Start with the `Header.jsx` component.”
Review the output. Ensure the visual appearance hasn't changed.
Automate the Rest
Prompt:
> “Continue converting all components to use Tailwind CSS. Remove all `.css` files that are no longer needed.”
Step 4: Add TypeScript (Gradually)
You don't need to convert the entire codebase to TypeScript overnight. Start with new files.
Rename `.jsx` to `.tsx`
Start with one file:
“`bash
mv src/components/UserCard.jsx src/components/UserCard.tsx
“`
Prompt to Cursor:
> “Add TypeScript types to `UserCard.tsx`. Infer the prop types from usage.”
Cursor will analyze how the component is used and generate the correct types.
Expand Gradually
Over weeks (not days), convert more files. The AI makes this painless.
Step 5: Break Up the Monolith
That 2000-line `App.js` file? It needs to be split.
Prompt to Cursor:
> “Analyze `App.js` and suggest how to break it into smaller components. Then create those components and update `App.js` to import them.”
Cursor will:
1. Identify logical sections (Header, Sidebar, MainContent, Footer).
2. Extract them into separate files.
3. Update the imports.
Step 6: Remove Dead Code
Legacy codebases are full of unused functions and commented-out code.
Prompt to Cursor:
> “Scan the codebase for unused imports, functions, and variables. Remove them.”
Warning: Review the changes carefully. The AI might remove something that's used in a non-obvious way.
Step 7: Add Documentation
Future you (and your team) will thank you.
Prompt to Cursor:
> “Add JSDoc comments to all exported functions in this codebase. Include @param and @returns tags.”
Step 8: Deploy and Monitor
After refactoring, deploy to a staging environment.
Monitor for Errors
Use tools like:
* Sentry (for error tracking)
* LogRocket (for session replay)
If something breaks, you have your git history to roll back.
Common Refactoring Pitfalls (And How AI Helps)
Pitfall 1: Breaking Subtle Behaviors
Solution: Use AI to generate tests before refactoring. This catches regressions.
Pitfall 2: Inconsistent Styles
Solution: Use `.cursorrules` to enforce consistency across all AI-generated code.
Pitfall 3: Over-Refactoring
Solution: Refactor incrementally. One component per day is better than 50 components in one weekend.
Real-World Example: Refactoring a 5-Year-Old E-Commerce App
At BYS Marketing, we recently refactored a client's e-commerce platform:
* Before: 15,000 lines of jQuery and vanilla JS.
* After: Modern React with TypeScript and Tailwind.
* Time: 3 weeks (would have taken 3 months manually).
* Bugs Introduced: 2 (both caught in staging).
The AI handled 80% of the mechanical work. We focused on architecture and testing.
Conclusion
Refactoring legacy code is no longer a death march. With AI tools like Cursor and Windsurf, you can modernize codebases safely and efficiently. The key is to go slow, test often, and let the AI handle the tedious parts.
—
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