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>
41 lines
1.2 KiB
Markdown
41 lines
1.2 KiB
Markdown
---
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title: Calculate Derived State During Rendering
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impact: MEDIUM
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impactDescription: avoids redundant renders and state drift
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tags: rerender, derived-state, useEffect, state
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---
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## Calculate Derived State During Rendering
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If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.
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**Incorrect (redundant state and effect):**
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```tsx
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function Form() {
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const [firstName, setFirstName] = useState('First')
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const [lastName, setLastName] = useState('Last')
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const [fullName, setFullName] = useState('')
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useEffect(() => {
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setFullName(firstName + ' ' + lastName)
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}, [firstName, lastName])
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return <p>{fullName}</p>
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}
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```
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**Correct (derive during render):**
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```tsx
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function Form() {
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const [firstName, setFirstName] = useState('First')
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const [lastName, setLastName] = useState('Last')
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const fullName = firstName + ' ' + lastName
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return <p>{fullName}</p>
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}
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```
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References: [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect)
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