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

1. Scan for critical issues first - Check for the patterns in "Critical (Block Merge)" section
2. Check React 19 usage - See react19-patterns.md for new API patterns
3. Evaluate state management - Is state colocated? Server state vs client state separation?
4. Assess TypeScript safety - Generic components, discriminated unions, strict config
5. 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