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.agents/skills/vercel-react-best-practices/rules/rerender-dependencies.md

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