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Rerender Simple Expression In Memo

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

Metadata​

  • title: Do not wrap a simple expression with a primitive result type in useMemo
  • impact: LOW-MEDIUM
  • impactDescription: wasted computation on every render
  • tags: rerender, useMemo, optimization

Content​

Do not wrap a simple expression with a primitive result type in useMemo​

When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in useMemo. Calling useMemo and comparing hook dependencies may consume more resources than the expression itself.

Incorrect:

function Header({ user, notifications }: Props) {
  const isLoading = useMemo(() => {
    return user.isLoading || notifications.isLoading
  }, [user.isLoading, notifications.isLoading])

  if (isLoading) return <Skeleton />
  // return some markup
}

Correct:

function Header({ user, notifications }: Props) {
  const isLoading = user.isLoading || notifications.isLoading

  if (isLoading) return <Skeleton />
  // return some markup
}