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Rerender Memo

Source: .agents/references/coding-standard/vercel-react-best-practices/rules/rerender-memo.md

Metadata​

  • title: Extract to Memoized Components
  • impact: MEDIUM
  • impactDescription: enables early returns
  • tags: rerender, memo, useMemo, optimization

Content​

Extract to Memoized Components​

Extract expensive work into memoized components to enable early returns before computation.

Incorrect (computes avatar even when loading):

function Profile({ user, loading }: Props) {
  const avatar = useMemo(() => {
    const id = computeAvatarId(user)
    return <Avatar id={id} />
  }, [user])

  if (loading) return <Skeleton />
  return <div>{avatar}</div>
}

Correct (skips computation when loading):

const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
  const id = useMemo(() => computeAvatarId(user), [user])
  return <Avatar id={id} />
})

function Profile({ user, loading }: Props) {
  if (loading) return <Skeleton />
  return (
    <div>
      <UserAvatar user={user} />
    </div>
  )
}

Note: If your project has React Compiler enabled, manual memoization with memo() and useMemo() is not necessary. The compiler automatically optimizes re-renders.