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):
tsx
function Highlighter({ text, query }: Props) {
const regex = new RegExp(`(${query})`, 'gi')
const parts = text.split(regex)
return <>{parts.map((part, i) => ...)}</>
}
Correct (memoize or hoist):
tsx
const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
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:
typescript
const regex = /foo/g
regex.test('foo') // true, lastIndex = 3
regex.test('foo') // false, lastIndex = 0