js-hoist-regexp

---

name: js-hoist-regexp description: Hoist RegExp creation outside render or memoize with useMemo(). Apply when using regular expressions in React components or frequently called functions.

Hoist RegExp Creation

Don't create RegExp inside render. Hoist to module scope or memoize with useMemo().

Incorrect (new RegExp every render):

Copyfunction Highlighter({ text, query }: Props) {
  const regex = new RegExp(`(${query})`, 'gi')
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Correct (memoize or hoist):

Copyconst EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/

function Highlighter({ text, query }: Props) {
  const regex = useMemo(
    () => new RegExp(`(${escapeRegex(query)})`, 'gi'),
    [query]
  )
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Warning (global regex has mutable state):

Global regex (/g) has mutable lastIndex state:

Copyconst regex = /foo/g
regex.test('foo')  // true, lastIndex = 3
regex.test('foo')  // false, lastIndex = 0