React Fundamentals

Core Philosophy

Effects are escape hatches. They synchronize React with external systems. If no external system is involved, you likely don't need one.

Performance optimization is targeted. Don't memoize everything. Profile first, optimize measurable bottlenecks.

useEffect Decision Tree

Do I need useEffect?

Is there an external system involved?
├── No → Don't use useEffect
│   ├── Derived state? → Calculate during render
│   ├── Event response? → Handle in event handler
│   └── Data fetching? → Use TanStack Query
└── Yes → Maybe use useEffect
    ├── Browser APIs (focus, scroll, localStorage)
    ├── Third-party widgets (maps, charts)
    ├── Network connections (WebSockets)
    └── Analytics/logging

Derived State (No Effect Needed)

// ❌ Bad - useEffect for derived state
const [fullName, setFullName] = useState('');
useEffect(() => {
  setFullName(`${firstName} ${lastName}`);
}, [firstName, lastName]);

// ✅ Good - calculate during render
const fullName = `${firstName} ${lastName}`;

// ✅ With memoization (only if expensive)
const sortedItems = useMemo(
  () => items.toSorted((a, b) => a.name.localeCompare(b.name)),
  [items],
);

Event Handlers (No Effect Needed)

// ❌ Bad - effect chain for event response
useEffect(() => {
  if (submitted) {
    navigate('/success');
  }
}, [submitted]);

// ✅ Good - handle in event
const handleSubmit = async () => {
  await submitForm();
  navigate('/success');
};

Performance Patterns

useMemo - Expensive Calculations Only

// ❌ Bad - memoizing cheap operations
const doubled = useMemo(() => value * 2, [value]);

// ✅ Good - memoizing expensive operations
const sortedItems = useMemo(
  () => items.toSorted((a, b) => a.price - b.price),
  [items],
);

// ✅ Good - maintaining referential equality for deps
const filterFn = useMemo(() => (item) => item.active, []);

useCallback - For Memoized Children Only

// ❌ Bad - useCallback without memoized child
const handleClick = useCallback(() => doSomething(), []);
return <button onClick={handleClick}>Click</button>;

// ✅ Good - useCallback for memoized child
const handleClick = useCallback(() => doSomething(id), [id]);
return <MemoizedChild onClick={handleClick} />;

Avoiding Re-renders

// ❌ Bad - object literal creates new reference every render
<Child style={{ color: 'red' }} />;

// ✅ Good - stable reference
const style = useMemo(() => ({ color: 'red' }), []);
<Child style={style} />;

// ✅ Better - define outside component if static
const style = { color: 'red' };
function Parent() {
  return <Child style={style} />;
}

Async Patterns

Parallel Fetches

// ❌ Bad - sequential (waterfall)
const user = await getUser(id);
const posts = await getPosts(id);
const comments = await getComments(id);

// ✅ Good - parallel with Promise.all
const [user, posts, comments] = await Promise.all([
  getUser(id),
  getPosts(id),
  getComments(id),
]);

useTransition for Non-Urgent Updates

const [isPending, startTransition] = useTransition();

const handleSearch = (query: string) => {
  // Urgent: update input immediately
  setQuery(query);

  // Non-urgent: can be interrupted
  startTransition(() => {
    setFilteredResults(filterItems(query));
  });
};

Lazy State Initialization

// ❌ Bad - expensive function runs every render
const [items, setItems] = useState(parseExpensiveData(raw));

// ✅ Good - lazy initialization runs once
const [items, setItems] = useState(() => parseExpensiveData(raw));

Set/Map for O(1) Lookups

// ❌ Bad - O(n) on every render
const isSelected = (id: string) => selectedIds.includes(id);

// ✅ Good - O(1) lookups with Set
const selectedSet = useMemo(() => new Set(selectedIds), [selectedIds]);
const isSelected = (id: string) => selectedSet.has(id);

Common Mistakes

Delegation

• Data fetching: For queries and caching, see [tanstack-query](../tanstack-query/SKILL.md) skill
• Form state: For form handling, see [tanstack-form](../tanstack-form/SKILL.md) skill
• URL state: For routing and params, see [tanstack-router](../tanstack-router/SKILL.md) skill
• Error handling: For error boundaries, see [error-boundaries](../error-boundaries/SKILL.md) skill
• Full-stack flows: For integrated patterns, see [integration-patterns](../integration-patterns/SKILL.md) skill
• Code review: After optimizing components, delegate to code-reviewer agent