46 lines
833 B
Markdown
46 lines
833 B
Markdown
---
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title: Narrow Effect Dependencies
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impact: LOW
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impactDescription: minimizes effect re-runs
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tags: rerender, useEffect, dependencies, optimization
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---
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## Narrow Effect Dependencies
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Specify primitive dependencies instead of objects to minimize effect re-runs.
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**Incorrect (re-runs on any user field change):**
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```tsx
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useEffect(() => {
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console.log(user.id);
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}, [user]);
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```
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**Correct (re-runs only when id changes):**
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```tsx
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useEffect(() => {
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console.log(user.id);
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}, [user.id]);
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```
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**For derived state, compute outside effect:**
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```tsx
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// Incorrect: runs on width=767, 766, 765...
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useEffect(() => {
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if (width < 768) {
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enableMobileMode();
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}
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}, [width]);
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// Correct: runs only on boolean transition
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const isMobile = width < 768;
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useEffect(() => {
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if (isMobile) {
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enableMobileMode();
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}
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}, [isMobile]);
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```
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