managing-local-vs-global-state

name	managing-local-vs-global-state
description	Teaches when to use local state vs global state (Context) in React 19. Use when deciding state management strategy, avoiding prop drilling, or architecting component state. Use when this capability is needed.
metadata	{"author":"djankies"}

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 onClick={() => 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 onClick={() => 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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