qcast-front/.agents/skills/vercel-react-best-practices/rules/rerender-derived-state-no-effect.md

title	impact	impactDescription	tags
Calculate Derived State During Rendering	MEDIUM	avoids redundant renders and state drift	rerender, derived-state, useEffect, state

Calculate Derived State During Rendering

If 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.

Incorrect (redundant state and effect):

function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const [fullName, setFullName] = useState('')

  useEffect(() => {
    setFullName(firstName + ' ' + lastName)
  }, [firstName, lastName])

  return <p>{fullName}</p>
}

Correct (derive during render):

function Form() {
  const [firstName, setFirstName] = useState('First')
  const [lastName, setLastName] = useState('Last')
  const fullName = firstName + ' ' + lastName

  return <p>{fullName}</p>
}

References: You Might Not Need an Effect