Cursor rule

1---
2alwaysApply: false
3---
4 
5## Use Functional setState Updates
6 
7When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.
8 
9**Incorrect (requires state as dependency):**
10 
11```tsx
12function TodoList() {
13  const [items, setItems] = useState(initialItems)
14  
15  // Callback must depend on items, recreated on every items change
16  const addItems = useCallback((newItems: Item[]) => {
17    setItems([...items, ...newItems])
18  }, [items])  // ❌ items dependency causes recreations
19  
20  // Risk of stale closure if dependency is forgotten
21  const removeItem = useCallback((id: string) => {
22    setItems(items.filter(item => item.id !== id))
23  }, [])  // ❌ Missing items dependency - will use stale items!
24  
25  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
26}
27```
28 
29The first callback is recreated every time `items` changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial `items` value.
30 
31**Correct (stable callbacks, no stale closures):**
32 
33```tsx
34function TodoList() {
35  const [items, setItems] = useState(initialItems)
36  
37  // Stable callback, never recreated
38  const addItems = useCallback((newItems: Item[]) => {
39    setItems(curr => [...curr, ...newItems])
40  }, [])  // ✅ No dependencies needed
41  
42  // Always uses latest state, no stale closure risk
43  const removeItem = useCallback((id: string) => {
44    setItems(curr => curr.filter(item => item.id !== id))
45  }, [])  // ✅ Safe and stable
46  
47  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
48}
49```
50 
51**Benefits:**
52 
531. **Stable callback references** - Callbacks don't need to be recreated when state changes
542. **No stale closures** - Always operates on the latest state value
553. **Fewer dependencies** - Simplifies dependency arrays and reduces memory leaks
564. **Prevents bugs** - Eliminates the most common source of React closure bugs
57 
58**When to use functional updates:**
59 
60- Any setState that depends on the current state value
61- Inside useCallback/useMemo when state is needed
62- Event handlers that reference state
63- Async operations that update state
64 
65**When direct updates are fine:**
66 
67- Setting state to a static value: `setCount(0)`
68- Setting state from props/arguments only: `setName(newName)`
69- State doesn't depend on previous value
70 
71**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.
72