React Expert

Core Philosophy

Components consume external state, contain no logic:

• External hooks at top (Zustand, XState, TanStack Query)
• No useState/useReducer/useEffect for complex logic
• Inline actions unless repeated 2+ times
• Test stores/machines (unit tests), not components (E2E only)

State Management Stack

State Type	Solution
Remote (REST)	TanStack Query
Remote (GraphQL)	Apollo Client
Application state	Zustand
Complex machines	XState
Local UI state	useState (rare, last resort)

Component Pattern

// ✅ External hooks → Early returns → JSX
function UserProfile({ userId }: { userId: string }) {
  const { user, isLoading } = useUser(userId);
  const { updateProfile } = useUserActions();

  if (isLoading) return <Spinner />;

  return (
    <div>
      <h1>{user.name}</h1>
      <button onClick={() => updateProfile({ email: 'new@example.com' })}>
        Update
      </button>
    </div>
  );
}

Testing: Stores, Not Components

What	Tool	Why
Zustand stores	Vitest	Test without React
XState machines	Vitest	Deterministic transitions
Critical flows	Playwright	Real browser
Components	Never	Logic should be in stores

// Test store directly
const { login } = useAuthStore.getState();
await login({ email: "test@example.com", password: "pass" });
expect(useAuthStore.getState().user).toBeDefined();

Unique Patterns

Prop Ordering: Simple → Complex

<Table
  data={items}
  loading={isLoading}
  sortable
  onSort={handleSort}
  renderRow={(item) => <Row>{item.name}</Row>}
/>

Inline Props for Type Inference

// ✅ Inline - TypeScript infers types
<SearchableList
  items={budgets}
  renderItem={(budget) => <Card name={budget.name} />}
/>;

// ❌ Extract only if repeated 2+ times
const renderItem = (budget: Budget) => <Card name={budget.name} />;

Pattern Matching for Variants

import { match } from "ts-pattern";

{
  match(state)
    .with({ _tag: "loading" }, () => <Spinner />)
    .with({ _tag: "success" }, (s) => <Data value={s.data} />)
    .exhaustive();
}

Performance: Profile First

Technique	When
useMemo	Profiled as slow
useCallback	Repeated 2+ times
React.memo	Props rarely change
Code splitting	Route-level

Styled-Components: Consolidate with CSS Selectors

// ✅ Single component with CSS selectors
const Table = styled.table`
  thead {
    background: ${(p) => p.theme.colors.header};
  }
  tbody tr:hover {
    background: ${(p) => p.theme.colors.hover};
  }
  td {
    padding: ${(p) => p.theme.space.md};
  }
`;

// ❌ Separate components for each element
const TableHeader = styled.thead`...`;
const TableRow = styled.tr`...`;
const TableCell = styled.td`...`;

Version 1.0.1