TypeScript + React 19 Code Review Expert Skill

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 |

Agent Skills: TypeScript + React 19 Code Review Expert

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