Subchapter 3.53

Rerender Derived State No Effect

rules/rerender-derived-state-no-effect.mdMarkdown1 KBView on GitHub

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.

Rerender Derived State No Effect · Vercel React Best Practices · vercel-labs/agent-skills · Skills Docs

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>
}

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

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