Skip to main content

Rerender Derived State No Effect

Source: .agents/references/coding-standard/vercel-react-best-practices/rules/rerender-derived-state-no-effect.md

Metadata​

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

Content​

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