bbb2f41591
- Created `openspec-ff-change` skill for fast-forward artifact creation. - Introduced `openspec-new-change` skill for structured change creation. - Developed `openspec-onboard` skill for guided onboarding through OpenSpec workflow. - Added `openspec-sync-specs` skill for syncing delta specs to main specs. - Implemented `openspec-verify-change` skill for verifying implementation against change artifacts. - Updated `.gitignore` to exclude OpenSpec generated files. - Added `skills-lock.json` to manage skill dependencies.
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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