1---
2title: Calculate Derived State During Rendering
3impact: MEDIUM
4impactDescription: avoids redundant renders and state drift
5tags: rerender, derived-state, useEffect, state
6---
7 
8## Calculate Derived State During Rendering
9 
10If 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.
11 
12**Incorrect (redundant state and effect):**
13 
14```tsx
15function Form() {
16  const [firstName, setFirstName] = useState('First')
17  const [lastName, setLastName] = useState('Last')
18  const [fullName, setFullName] = useState('')
19 
20  useEffect(() => {
21    setFullName(firstName + ' ' + lastName)
22  }, [firstName, lastName])
23 
24  return <p>{fullName}</p>
25}
26```
27 
28**Correct (derive during render):**
29 
30```tsx
31function Form() {
32  const [firstName, setFirstName] = useState('First')
33  const [lastName, setLastName] = useState('Last')
34  const fullName = firstName + ' ' + lastName
35 
36  return <p>{fullName}</p>
37}
38```
39 
40References: [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect)
41