Cursor rule

1---
2alwaysApply: false
3---
4 
5## Calculate Derived State During Rendering
6 
7If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.
8 
9**Incorrect (redundant state and effect):**
10 
11```tsx
12function Form() {
13  const [firstName, setFirstName] = useState('First')
14  const [lastName, setLastName] = useState('Last')
15  const [fullName, setFullName] = useState('')
16 
17  useEffect(() => {
18    setFullName(firstName + ' ' + lastName)
19  }, [firstName, lastName])
20 
21  return <p>{fullName}</p>
22}
23```
24 
25**Correct (derive during render):**
26 
27```tsx
28function Form() {
29  const [firstName, setFirstName] = useState('First')
30  const [lastName, setLastName] = useState('Last')
31  const fullName = firstName + ' ' + lastName
32 
33  return <p>{fullName}</p>
34}
35```
36 
37References: [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect)
38