typescript-react-reviewer

SKILL.md

TypeScript + React 19 Code Review Expert

Expert code reviewer with deep knowledge of React 19's new features, TypeScript best practices, state management patterns, and common anti-patterns.

Review Priority Levels

🚫 Critical (Block Merge)

These issues cause bugs, memory leaks, or architectural problems:

Issue
Why It's Critical

useEffect for derived state
Extra render cycle, sync bugs

Missing cleanup in useEffect
Memory leaks

Direct state mutation (.push(), .splice())
Silent update failures

Conditional hook calls
Breaks Rules of Hooks

key={index} in dynamic lists
State corruption on reorder

any type without justification
Type safety bypass

useFormStatus in same component as <form>
Always returns false (React 19 bug)

Promise created inside render with use()
Infinite loop

⚠️ High Priority

Issue
Impact

Incomplete dependency arrays
Stale closures, missing updates

Props typed as any
Runtime errors

Unjustified useMemo/useCallback
Unnecessary complexity

Missing Error Boundaries
Poor error UX

Controlled input initialized with undefined
React warning

📝 Architecture/Style

Issue
Recommendation

Component > 300 lines
Split into smaller components

Prop drilling > 2-3 levels
Use composition or context

State far from usage
Colocate state

Custom hooks without use prefix
Follow naming convention

Quick Detection Patterns

useEffect Abuse (Most Common Anti-Pattern)

// ❌ WRONG: Derived state in useEffect
const [firstName, setFirstName] = useState('');
const [fullName, setFullName] = useState('');
useEffect(() => {
  setFullName(firstName + ' ' + lastName);
}, [firstName, lastName]);

// ✅ CORRECT: Compute during render
const fullName = firstName + ' ' + lastName;

// ❌ WRONG: Event logic in useEffect
useEffect(() => {
  if (product.isInCart) showNotification('Added!');
}, [product]);

// ✅ CORRECT: Logic in event handler
function handleAddToCart() {
  addToCart(product);
  showNotification('Added!');
}

React 19 Hook Mistakes

// ❌ WRONG: useFormStatus in form component (always returns false)
function Form() {
  const { pending } = useFormStatus();
  return <form action={submit}><button disabled={pending}>Send</button></form>;
}

// ✅ CORRECT: useFormStatus in child component
function SubmitButton() {
  const { pending } = useFormStatus();
  return <button type="submit" disabled={pending}>Send</button>;
}
function Form() {
  return <form action={submit}><SubmitButton /></form>;
}

// ❌ WRONG: Promise created in render (infinite loop)
function Component() {
  const data = use(fetch('/api/data')); // New promise every render!
}

// ✅ CORRECT: Promise from props or state
function Component({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise);
}

State Mutation Detection

// ❌ WRONG: Mutations (no re-render)
items.push(newItem);
setItems(items);

arr[i] = newValue;
setArr(arr);

// ✅ CORRECT: Immutable updates
setItems([...items, newItem]);
setArr(arr.map((x, idx) => idx === i ? newValue : x));

TypeScript Red Flags

// ❌ Red flags to catch
const data: any = response;           // Unsafe any
const items = arr[10];                // Missing undefined check
const App: React.FC<Props> = () => {}; // Discouraged pattern

// ✅ Preferred patterns
const data: ResponseType = response;
const items = arr[10]; // with noUncheckedIndexedAccess
const App = ({ prop }: Props) => {};  // Explicit props

Review Workflow

Scan for critical issues first - Check for the patterns in "Critical (Block Merge)" section

Check React 19 usage - See react19-patterns.md for new API patterns

Evaluate state management - Is state colocated? Server state vs client state separation?

Assess TypeScript safety - Generic components, discriminated unions, strict config

Review for maintainability - Component size, hook design, folder structure

Reference Documents

For detailed patterns and examples:

react19-patterns.md - React 19 new hooks (useActionState, useOptimistic, use), Server/Client Component boundaries

antipatterns.md - Comprehensive anti-pattern catalog with fixes

checklist.md - Full code review checklist for thorough reviews

State Management Quick Guide

Data Type
Solution

Server/async data
TanStack Query (never copy to local state)

Simple global UI state
Zustand (~1KB, no Provider)

Fine-grained derived state
Jotai (~2.4KB)

Component-local state
useState/useReducer

Form state
React 19 useActionState

TanStack Query Anti-Pattern

// ❌ NEVER copy server data to local state
const { data } = useQuery({ queryKey: ['todos'], queryFn: fetchTodos });
const [todos, setTodos] = useState([]);
useEffect(() => setTodos(data), [data]);

// ✅ Query IS the source of truth
const { data: todos } = useQuery({ queryKey: ['todos'], queryFn: fetchTodos });

TypeScript Config Recommendations

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "exactOptionalPropertyTypes": true
  }
}

noUncheckedIndexedAccess is critical - it catches arr[i] returning undefined.

Immediate Red Flags

When reviewing, flag these immediately:

Pattern
Problem
Fix

eslint-disable react-hooks/exhaustive-deps
Hides stale closure bugs
Refactor logic

Component defined inside component
Remounts every render
Move outside

useState(undefined) for inputs
Uncontrolled warning
Use empty string

React.FC with generics
Generic inference breaks
Use explicit props

Barrel files (index.ts) in app code
Bundle bloat, circular deps
Direct imports