Managing Local Vs Global State

Local vs Global State

Decision Flow

Use Local State (useState) when:

• State only needed in one component
• State only needed by component + direct children
• State changes frequently (avoid Context re-renders)
• State is UI-specific (form input, toggle, etc.)

Use Lifted State when:

• Two sibling components need to share state
• Parent coordinates between children
• Still contained to component subtree

Use Context when:

• Many components at different nesting levels need state
• Prop drilling through 3+ levels
• Global configuration (theme, locale, auth)
• State changes infrequently

Examples

Local State:

function Counter() {
  const [count, setCount] = useState(0);

  return <button => setCount(count + 1)}>Count: {count}</button>;
}

Lifted State:

function Parent() {
  const [filter, setFilter] = useState('all');

  return (
    <>
      <FilterButtons filter={filter} setFilter={setFilter} />
      <ItemList filter={filter} />
    </>
  );
}

Context (React 19 with use()):

import { createContext, use } from 'react';

const ThemeContext = createContext('light');

function App() {
  const [theme, setTheme] = useState('light');

  return (
    <ThemeContext value={{ theme, setTheme }}>
      <Layout />
    </ThemeContext>
  );
}

function DeepComponent() {
  const { theme, setTheme } = use(ThemeContext);

  return <button => setTheme(theme === 'light' ? 'dark' : 'light')}>
    Toggle Theme
  </button>;
}

Anti-Patterns

❌ Context for frequently changing state:

const MousePositionContext = createContext();

function App() {
  const [position, setPosition] = useState({ x: 0, y: 0 });

  useEffect(() => {
    const handler = (e) => setPosition({ x: e.clientX, y: e.clientY });
    window.addEventListener('mousemove', handler);
    return () => window.removeEventListener('mousemove', handler);
  }, []);

  return (
    <MousePositionContext value={position}>
      <App />
    </MousePositionContext>
  );
}

This causes re-render of entire tree on every mouse move!

✅ Local state or ref instead:

function Component() {
  const [position, setPosition] = useState({ x: 0, y: 0 });

  useEffect(() => {
    const handler = (e) => setPosition({ x: e.clientX, y: e.clientY });
    window.addEventListener('mousemove', handler);
    return () => window.removeEventListener('mousemove', handler);
  }, []);

  return <div>Mouse: {position.x}, {position.y}</div>;
}

For comprehensive state management patterns, see: research/react-19-comprehensive.md lines 1293-1342.

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