State Architecture Decisions

The Three Types of State

• Local UI state — belongs in the component that uses it or its nearest parent. Examples: form input values, dropdown open/closed, loading spinner for a specific button. Use useState or useReducer
• Global client state — shared across many unrelated components and changes over time. Examples: authenticated user, shopping cart, theme/locale. Use Zustand or Redux Toolkit
• Server state — data that lives on the server, needs to be fetched, cached, and synchronized. Examples: product list, user profile, comments. Use TanStack Query or RTK Query — NOT useState+useEffect
• URL state — state encoded in the URL (search params, route params). Use React Router's useSearchParams — it's automatically shareable and bookmarkable
• The #1 mistake is treating server state as client state — putting API responses in Zustand or Context forces you to manage caching, invalidation, and synchronization manually; TanStack Query does this for free

State Co-location Principle

// RULE: State should live as close as possible to where it's used
// Move state UP only when genuinely needed by multiple components

// ❌ Over-lifted state — everything at the top
function App() {
  const [isDropdownOpen, setIsDropdownOpen] = useState(false); // used only by Header
  const [inputValue, setInputValue] = useState('');            // used only by SearchBar
  const [hoveredCard, setHoveredCard] = useState(null);        // used only by ProductCard
  // App re-renders on every micro-interaction — performance disaster
}

// ✅ Co-located state — each component owns its own UI state
function Header() {
  const [isDropdownOpen, setIsDropdownOpen] = useState(false); // lives HERE
}

function SearchBar() {
  const [inputValue, setInputValue] = useState(''); // lives HERE
}

function ProductCard({ product }) {
  const [isHovered, setIsHovered] = useState(false); // lives HERE
}

// Only lift state when two sibling components need the same data
// And they have a common parent that can hold it

Decision Matrix — Which State Tool to Use

• useState: single component, simple values (boolean, string, number, small object). Default choice
• useReducer: single component, complex state with multiple sub-values that update together, or state machine logic
• Context API: low-frequency global state (theme, auth user, locale). Max 2-3 values per context. Split by update frequency
• Zustand: frequently updating shared state, many consumers across the tree, devtools support needed, simpler than Redux
• Redux Toolkit: existing Redux codebase, very complex action flows, large team needing strict conventions, RTK Query for API layer
• TanStack Query: ALL server state — anything fetched from an API. Handles caching, background refresh, optimistic updates, pagination

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