dcaaef1011
Format: YYYY-MM-DD-description.md - 2026-01-19-infrastructure-deployment.md - 2026-01-19-backend-api-implementation.md (in progress) Co-Authored-By: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>
59 lines
2.0 KiB
Markdown
59 lines
2.0 KiB
Markdown
---
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title: Use Lazy State Initialization
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impact: MEDIUM
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impactDescription: wasted computation on every render
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tags: react, hooks, useState, performance, initialization
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---
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## Use Lazy State Initialization
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Pass a function to `useState` for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.
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**Incorrect (runs on every render):**
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```tsx
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function FilteredList({ items }: { items: Item[] }) {
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// buildSearchIndex() runs on EVERY render, even after initialization
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const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
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const [query, setQuery] = useState('')
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// When query changes, buildSearchIndex runs again unnecessarily
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return <SearchResults index={searchIndex} query={query} />
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}
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function UserProfile() {
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// JSON.parse runs on every render
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const [settings, setSettings] = useState(
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JSON.parse(localStorage.getItem('settings') || '{}')
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)
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return <SettingsForm settings={settings} onChange={setSettings} />
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}
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```
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**Correct (runs only once):**
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```tsx
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function FilteredList({ items }: { items: Item[] }) {
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// buildSearchIndex() runs ONLY on initial render
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const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
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const [query, setQuery] = useState('')
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return <SearchResults index={searchIndex} query={query} />
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}
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function UserProfile() {
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// JSON.parse runs only on initial render
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const [settings, setSettings] = useState(() => {
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const stored = localStorage.getItem('settings')
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return stored ? JSON.parse(stored) : {}
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})
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return <SettingsForm settings={settings} onChange={setSettings} />
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}
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```
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Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.
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For simple primitives (`useState(0)`), direct references (`useState(props.value)`), or cheap literals (`useState({})`), the function form is unnecessary.
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