js-hoist-regexp

title: Hoist RegExp Creation
impact: LOW-MEDIUM
impactDescription: avoids recreation
tags: javascript, regexp, optimization, memoization

Hoist RegExp Creation

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

**Incorrect (new RegExp every render):**

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

**Correct (memoize or hoist):**

const 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:

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