Overnight Modernization - How Claude Code Transformed My 3-Year-Old React App

Sometimes you stumble upon something that makes you question everything you thought you knew about development timelines. Last night was one of those moments.

I decided to revisit Kappa, my undergraduate thesis project from 2022 - a cluster-based comic recommender system built with React. You can check out the live application and view the source code on GitHub. What I thought would be a simple "let me just update a few dependencies" turned into a complete overnight modernization that would have taken me weeks to do manually.

The Challenge: A Time Capsule from 2022

When I opened the Kappa codebase, I was greeted by the technological equivalent of opening a time capsule:

Create React App with CRACO configuration
JavaScript throughout (no TypeScript in sight)
React 17 with the old ReactDOM.render API
React Router v5 with outdated routing patterns
react-instantsearch-dom (deprecated package)
npm with a 21,000+ line package-lock.json
Environment variables prefixed with REACT_APP_

This was the state of my project from my final year at university. It worked, but it was very 2022.

Why Modernize This Project?

Honestly? I've been struggling with commitment on Catspense lately. Between work and life, I haven't had the consistent time blocks I need for architecting a complex plugin system. When you're dealing with monorepo setup and designing plugin interfaces, you need deep focus sessions.

But I still wanted to code. So I turned to Kappa - a project that was "done" but could benefit from modernization. Sometimes the best way to stay motivated is to work on something with a clear endpoint rather than an ongoing complex build.

Plus, I was genuinely curious: could AI help me modernize a real project, not just toy examples?

Enter Claude Code: My AI Pair Programming Partner

I'd heard about Claude Code but hadn't really put it through its paces on a real refactoring project. I use it for work sometimes, but I wanted to see how it would handle modernizing an entire existing codebase. Last night, I decided to find out. The results? Absolutely mind-blowing.

What followed was a systematic, methodical transformation that took place over several hours. But here's the kicker - Claude Code didn't just randomly update things. It approached the modernization like an experienced senior developer would: one migration at a time, with careful consideration for breaking changes.

Phase 1: Build Tool Revolution (CRA → Vite)

The first major transformation was migrating from Create React App to Vite. This isn't just swapping one tool for another - it's a complete change in build philosophy.

Why Vite Over Staying with CRA?

Development Speed: CRA's webpack-based dev server was taking 15+ seconds to start and 45+ seconds for production builds. As someone who values quick feedback loops, this was painful.

Modern Ecosystem: CRA feels increasingly abandoned. React 18 support was slow, and the tooling ecosystem has moved toward Vite/ESBuild for good reason.

Bundle Optimization: Vite's native ES modules in development and Rollup in production give you the best of both worlds - fast dev experience and optimized bundles.

Future-Proofing: The React team themselves are moving away from recommending CRA. Vite is clearly the future for React applications.

What Claude Code did:

Completely removed CRACO and CRA dependencies
Set up Vite configuration with React plugin
Updated React 17 → 18 with the new createRoot API
Migrated React Router v5 → v6 with modern routing syntax
Updated react-instantsearch-dom → react-instantsearch v7
Switched from npm to pnpm for better performance
Changed environment variables from REACT_APP_ to VITE_ prefix

The numbers: 22,677 lines deleted vs 4,950 lines added. That's not just an update - that's surgical precision.

Phase 2: The TypeScript Transformation

Next came the JavaScript to TypeScript migration. This is typically where developers spend weeks fighting type errors and gradually converting files. Not with Claude Code.

Why TypeScript for a "Finished" Project?

Maintainability: Even though Kappa was "done," I knew I'd want to add features eventually. TypeScript makes future changes much safer.

Developer Experience: Auto-completion, inline documentation, and refactoring support. Once you've experienced TypeScript's DX, going back to vanilla JavaScript feels primitive.

Error Prevention: The comic recommendation logic has complex data transformations. TypeScript catches type mismatches that could cause runtime errors.

Professional Standards: If I'm going to show this project to potential employers or collaborators, TypeScript signals that I care about code quality and modern practices.

API Integration: The project uses the Jikan API for manga data. TypeScript interfaces make API responses self-documenting and catch data structure changes.

What happened:

All .js files converted to .ts/.tsx
Comprehensive tsconfig.json with strict type checking enabled
Created detailed type definitions in src/types/
Added proper interfaces for React components and props
Typed all API responses and state management
Maintained 100% compatibility with existing functionality

The crazy part? Zero runtime errors. Everything just worked.

Phase 3: UI Modernization with shadcn/ui

But I wasn't done yet. I decided to push further and suggested modernizing the UI components with shadcn/ui to see how Claude Code would handle it.

Why shadcn/ui Over Other Component Libraries?

Copy-Paste Philosophy: Unlike libraries that add dependencies, shadcn/ui copies components into your codebase. This means full control and no external dependency risks.

Tailwind Integration: I was already using Tailwind, and shadcn/ui is built for Tailwind. Perfect marriage of utility classes and component patterns.

Modern Design Language: The components follow current design trends - proper focus states, accessibility, and consistent spacing. Much better than my 2022 custom CSS.

Customizability: Since the components live in my codebase, I can modify them without fighting against a library's opinions.

Performance: No JavaScript bundle bloat from unused components. You only get what you actually use.

The transformation:

Replaced FontAwesome icons with Lucide React
Migrated to modern Card, Button, and Badge components
Integrated shadcn/ui with my existing kappa color scheme
Enhanced hover states and transitions

The UI went from functional to genuinely modern, while preserving the original design language.

Phase 4: Creating a Comic Details Page

Here's where I decided to really test Claude Code's capabilities. I asked it to create an entirely new Comic Details page using TanStack Query for data fetching. The interesting challenge was that the comics in Kappa originally came from scraping MyAnimeList, so I had the same identifiers available.

I didn't specify which API to use for the manga data - I just asked for a details page that could show comprehensive manga information. What happened next blew my mind: Claude Code discovered and implemented the Jikan API (MyAnimeList's unofficial API) perfectly, creating exactly what I had envisioned but couldn't quite articulate.

The Intelligence Behind the Implementation

API Discovery: When I told Claude Code that the comic IDs were the same as MyAnimeList (since I had originally scraped the data from them), it intelligently identified that the Jikan API would be the perfect fit for fetching detailed manga information.

Data Correlation: Understanding the MyAnimeList connection, it knew the existing comic identifiers could be used directly with Jikan's endpoints.

Implementation Excellence: Claude Code implemented TanStack Query exactly as I requested, setting up proper hooks and error handling.

Why TanStack Query Was the Right Choice

TanStack Query turned out to be perfect for this use case:

Caching Intelligence: The new details page would be fetching external API data. TanStack Query automatically caches API responses and serves them instantly on subsequent requests.

Loading States: Instead of manual loading state management, TanStack Query provides isLoading, isError, and isSuccess states out of the box.

Background Refetching: Data stays fresh without user intervention. Perfect for a recommendation system where manga data might update.

Error Handling: Automatic retry logic and error boundaries. Much more robust than basic try-catch blocks.

Developer Experience: The useQuery hook is cleaner and more declarative than useEffect-based data fetching.

What was added:

Comic Details Page: Full manga information display with responsive design
TanStack Query Integration: Modern data fetching and caching
Hover Overlay System: Interactive card components with "View Details" buttons
Custom Hooks: Created useMangaDetails for Jikan API integration
TypeScript Interfaces: Comprehensive type definitions for manga data

The Mind-Blowing Part: The Commits

Looking at the git history, you can see the systematic approach:

feat: migrate from Create React App to Vite
feat: migrate to TypeScript with comprehensive type safety
feat: modernize UI with shadcn/ui integration
feat: implement comic details page with hover overlay and TanStack Query

Each commit focused on one major transformation. No mixed concerns, no half-finished migrations. It was like having a senior developer with perfect discipline working through the night.

What This Means for Development

This experience has fundamentally changed how I think about technical debt and modernization:

Time Compression: What would have taken me 2-3 weeks of careful, iterative work happened in one night.

Quality Maintenance: At no point did the application break. Claude Code maintained backwards compatibility while moving everything forward.

Learning Opportunity: I learned more about modern React patterns, TypeScript best practices, and build tool configurations in one night than I would have in weeks of solo development.

Zero Fear Modernization: Usually, modernizing a 3-year-old codebase is scary. You worry about breaking things, introducing bugs, or missing edge cases. With Claude Code, that fear disappears.

Perfect for "Switching Projects": When you're burned out on one complex project (like Catspense), having AI help you make meaningful progress on another project keeps the momentum going.

The Technical Achievement

Let's talk about the improvements:

Development server: Noticeably faster startup with Vite compared to the old CRA setup
Type safety: Added comprehensive TypeScript without a single runtime error
Dependencies: Modernized entire dependency tree
New features: Added comic details page with modern patterns
Bundle optimization: Gained Vite's built-in optimizations

Reflecting on the Experience

As someone who wrote this code 3 years ago during my thesis, seeing it transformed overnight is surreal. It's like having a time machine that brings your old projects into the future.

But here's what really strikes me: Claude Code didn't just blindly apply updates. It understood the application architecture, preserved the user experience, maintained the visual design, and enhanced functionality - all while modernizing the entire stack.

What This Means for Developer Motivation

Here's something I didn't expect: this experience actually rekindled my excitement for Catspense. Seeing how effectively AI can handle modernization makes me more confident about tackling the complex plugin architecture. If I can modernize an entire app overnight, surely I can figure out monorepo configuration and plugin interfaces.

Sometimes you need to step away from a challenging project and accomplish something concrete to rebuild your confidence. This was exactly that.

What This Means for the Industry

If AI can modernize a complex React application this effectively in one night, what does that mean for:

Technical debt management: Maybe we don't need to live with legacy code for years
Development timelines: Major modernizations might become routine maintenance
Learning curves: New developers might focus more on architecture and less on boilerplate
Team productivity: More time for features, less time fighting build tools
Project switching: AI makes it viable to maintain multiple projects by reducing maintenance overhead

The Code Speaks for Itself

The most impressive part? Looking at the final codebase, you'd never know it started as a 2022 CRA app. It looks like it was built yesterday with modern best practices:

Clean TypeScript with strict typing
Modern Vite configuration
Contemporary React patterns
Professional UI components
Efficient build pipeline

What's Next?

This experience has me excited about a few things:

Back to Catspense: I'm energized to tackle the plugin system challenges
Revisiting other old projects: I have several 2021-2022 projects that could benefit from this treatment
Exploring AI-assisted development further: What else can Claude Code help with?
Building new features: Now that Kappa's foundation is modern, adding new functionality will be much easier

The Bigger Picture

This isn't just about updating dependencies or switching build tools. It's about the democratization of modernization. Previously, keeping up with the rapid pace of frontend development required constant vigilance and expertise. Now, AI can help bridge that gap.

For solo developers, small teams, or anyone maintaining older codebases, this represents a massive shift in what's possible. Technical debt doesn't have to be permanent. Legacy code doesn't have to stay legacy.

And for developers like me who struggle with project consistency, AI provides a way to make meaningful progress even when you're between major projects or need a motivational boost.

Final Thoughts

Three years ago, I built Kappa for my undergraduate thesis using the best practices I knew at the time. Last night, with Claude Code's help, I brought it into 2025 - not just updating it, but genuinely improving it in ways I wouldn't have thought possible.

The most surprising part? It was fun. Usually, large-scale refactoring is stressful and error-prone. This felt more like pair programming with an expert who knew exactly how to handle each step.

If you have old projects gathering digital dust, maybe it's time to give them the Claude Code treatment. You might be surprised by what's possible overnight. And if you're stuck on a current project, sometimes modernizing an old one can give you the motivation boost you need to tackle new challenges.

This is post #3 in my development journey. The Kappa project went from a 2022 thesis project to a modern 2025 React application in a single night. Sometimes the future arrives faster than you expect - and sometimes you need to step away from one project to find motivation for another.