Best practices for using Claude Code to refactor legacy React components

Claude Code handles large-scale refactoring differently than the standard Claude.ai chat because it actually sees the file system and can run tests. After spending a few days pushing it through a 5-year-old legacy React codebase—mostly class components with messy componentDidUpdate logic and prop-drilling nightmares—I’ve noticed a massive gap in how it performs compared to GPT-4o or DeepSeek-V3 when tasked with "cleaning up" code.

The biggest mistake people make is giving it a generic "refactor this to hooks" prompt. That usually results in a hallucinated API or a component that looks clean but breaks edge cases. The secret is leveraging its terminal access to create a safety loop.

The Workflow that actually works

I’ve found that a "Test-Drive" approach is the only way to ensure legacy logic isn't lost. Before asking for a refactor, I force it to write a snapshot or a basic Vitest suite for the existing component.

# I start by telling Claude Code:
"Analyze ComponentX.tsx and write a test suite in ComponentX.test.tsx that covers all current render states and side effects. Run the tests and ensure they pass before we touch the source code."

Once the baseline is established, I move to a phased refactor. Instead of one giant leap, I break it down: (1) Extracting logic to custom hooks, (2) Converting Class to Function, (3) Optimizing re-renders.

Performance Comparison: Claude Code vs. The Rest

Claude 3.5 Sonnet (via Claude Code)

• Pros: Incredible at maintaining architectural consistency. It doesn't just swap this.state for useState; it recognizes patterns across the folder and suggests shared hooks. It feels "aware" of the project structure.
• Cons: Can be overly cautious, sometimes suggesting a "safe" refactor that doesn't actually simplify the code as much as it could.

GPT-4o

• Pros: Faster at generating the initial boilerplate for a new component.
• Cons: Terrible at context window management for large files. It often forgets a crucial prop or a weird legacy dependency when rewriting a 500-line file, leading to a "broken build" cycle.

DeepSeek-V3

• Pros: Extremely aggressive and efficient with logic optimization. It often finds a more concise way to write the logic than Claude.
• Cons: Lacks the integrated tool-use loop of Claude Code. You spend more time copy-pasting and manually verifying that the imports didn't break.

Key Technical Tips for React Refactoring

Avoid the "Rewrite All" trap. If you have a 1,000-line legacy file, don't tell it to "refactor the file." It will truncate code or miss nuances. Instead, use specific scope prompts:

"Refactor the data fetching logic in ComponentX.tsx into a separate useFetchData hook. Ensure the loading and error states are handled exactly as they are now. Update the component to use this hook."

Enforce TypeScript strictness. Legacy React is often a mess of any types. I’ve had great luck telling Claude Code to "fix types as you refactor." It’s surprisingly good at inferring the correct types from the actual usage in the codebase, which actually makes the refactoring safer because the compiler catches the mistakes Claude might make.

Handle CSS-in-JS carefully. If you're moving from legacy styled-components or inline styles to something like Tailwind, do that in a separate pass. Mixing structural refactoring (Class → Function) with styling changes usually leads to a debugging nightmare where you can't tell if a UI bug was caused by a hook mistake or a CSS class mistake.