JL Kruger
5422131782
Homepage (site/index.html): integration-v14 promoted, Writings section integrated with 33 pieces clustered by type (stories/essays/miscellany), Writings welcome lightbox, content frame at 98% opacity. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
66 lines
1.7 KiB
Markdown
66 lines
1.7 KiB
Markdown
---
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title: Minimize State Variables and Derive Values
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impact: MEDIUM
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impactDescription: fewer re-renders, less state drift
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tags: state, derived-state, hooks, optimization
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---
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## Minimize State Variables and Derive Values
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Use the fewest state variables possible. If a value can be computed from existing state or props, derive it during render instead of storing it in state. Redundant state causes unnecessary re-renders and can drift out of sync.
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**Incorrect (redundant state):**
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```tsx
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function Cart({ items }: { items: Item[] }) {
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const [total, setTotal] = useState(0)
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const [itemCount, setItemCount] = useState(0)
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useEffect(() => {
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setTotal(items.reduce((sum, item) => sum + item.price, 0))
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setItemCount(items.length)
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}, [items])
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return (
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<View>
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<Text>{itemCount} items</Text>
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<Text>Total: ${total}</Text>
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</View>
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)
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}
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```
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**Correct (derived values):**
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```tsx
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function Cart({ items }: { items: Item[] }) {
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const total = items.reduce((sum, item) => sum + item.price, 0)
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const itemCount = items.length
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return (
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<View>
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<Text>{itemCount} items</Text>
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<Text>Total: ${total}</Text>
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</View>
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)
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}
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```
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**Another example:**
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```tsx
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// Incorrect: storing both firstName, lastName, AND fullName
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const [firstName, setFirstName] = useState('')
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const [lastName, setLastName] = useState('')
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const [fullName, setFullName] = useState('')
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// Correct: derive fullName
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const [firstName, setFirstName] = useState('')
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const [lastName, setLastName] = useState('')
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const fullName = `${firstName} ${lastName}`
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```
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State should be the minimal source of truth. Everything else is derived.
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Reference: [Choosing the State Structure](https://react.dev/learn/choosing-the-state-structure)
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