Validates against assignment/mutation of globals during render, part of ensuring that side effects must run outside of render.

Rule Details

Global variables exist outside React’s control. When you modify them during render, you break React’s assumption that rendering is pure. This can cause components to behave differently in development vs production, break Fast Refresh, and make your app impossible to optimize with features like React Compiler.

Invalid

Examples of incorrect code for this rule:

// ❌ Global counter
let renderCount = 0;
function Component() {
  renderCount++; // Mutating global
  return <div>Count: {renderCount}</div>;
}

// ❌ Modifying window properties
function Component({userId}) {
  window.currentUser = userId; // Global mutation
  return <div>User: {userId}</div>;
}

// ❌ Global array push
const events = [];
function Component({event}) {
  events.push(event); // Mutating global array
  return <div>Events: {events.length}</div>;
}

// ❌ Cache manipulation
const cache = {};
function Component({id}) {
  if (!cache[id]) {
    cache[id] = fetchData(id); // Modifying cache during render
  }
  return <div>{cache[id]}</div>;
}

Valid

Examples of correct code for this rule:

// ✅ Use state for counters
function Component() {
  const [clickCount, setClickCount] = useState(0);

  const handleClick = () => {
    setClickCount(c => c + 1);
  };

  return (
    <button onClick={handleClick}>
      Clicked: {clickCount} times
    </button>
  );
}

// ✅ Use context for global values
function Component() {
  const user = useContext(UserContext);
  return <div>User: {user.id}</div>;
}

// ✅ Synchronize external state with React
function Component({title}) {
  useEffect(() => {
    document.title = title; // OK in effect
  }, [title]);

  return <div>Page: {title}</div>;
}
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