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)

typescript

// ❌ 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;

typescript

// ❌ 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

typescript

// ❌ 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>;
}

typescript

// ❌ 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

typescript

// ❌ 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

typescript

// ❌ 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

typescript

// ❌ 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

json

{
  "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