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46 lines
1.0 KiB
Markdown
46 lines
1.0 KiB
Markdown
---
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title: Hoist RegExp Creation
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impact: LOW-MEDIUM
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impactDescription: avoids recreation
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tags: javascript, regexp, optimization, memoization
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---
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## Hoist RegExp Creation
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Don't create RegExp inside render. Hoist to module scope or memoize with `useMemo()`.
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**Incorrect (new RegExp every render):**
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```tsx
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function Highlighter({ text, query }: Props) {
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const regex = new RegExp(`(${query})`, 'gi')
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const parts = text.split(regex)
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return <>{parts.map((part, i) => ...)}</>
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}
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```
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**Correct (memoize or hoist):**
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```tsx
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const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
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function Highlighter({ text, query }: Props) {
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const regex = useMemo(
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() => new RegExp(`(${escapeRegex(query)})`, 'gi'),
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[query]
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)
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const parts = text.split(regex)
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return <>{parts.map((part, i) => ...)}</>
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}
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```
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**Warning (global regex has mutable state):**
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Global regex (`/g`) has mutable `lastIndex` state:
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```typescript
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const regex = /foo/g
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regex.test('foo') // true, lastIndex = 3
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regex.test('foo') // false, lastIndex = 0
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```
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