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.agents/skills/vercel-react-best-practices/rules/rerender-split-combined-hooks.md

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+---
+title: Split Combined Hook Computations
+impact: MEDIUM
+impactDescription: avoids recomputing independent steps
+tags: rerender, useMemo, useEffect, dependencies, optimization
+---
+
+## Split Combined Hook Computations
+
+When a hook contains multiple independent tasks with different dependencies, split them into separate hooks. A combined hook reruns all tasks when any dependency changes, even if some tasks don't use the changed value.
+
+**Incorrect (changing `sortOrder` recomputes filtering):**
+
+```tsx
+const sortedProducts = useMemo(() => {
+  const filtered = products.filter((p) => p.category === category);
+  const sorted = filtered.toSorted((a, b) =>
+    sortOrder === 'asc' ? a.price - b.price : b.price - a.price
+  );
+  return sorted;
+}, [products, category, sortOrder]);
+```
+
+**Correct (filtering only recomputes when products or category change):**
+
+```tsx
+const filteredProducts = useMemo(
+  () => products.filter((p) => p.category === category),
+  [products, category]
+);
+
+const sortedProducts = useMemo(
+  () =>
+    filteredProducts.toSorted((a, b) =>
+      sortOrder === 'asc' ? a.price - b.price : b.price - a.price
+    ),
+  [filteredProducts, sortOrder]
+);
+```
+
+This pattern also applies to `useEffect` when combining unrelated side effects:
+
+**Incorrect (both effects run when either dependency changes):**
+
+```tsx
+useEffect(() => {
+  analytics.trackPageView(pathname);
+  document.title = `${pageTitle} | My App`;
+}, [pathname, pageTitle]);
+```
+
+**Correct (effects run independently):**
+
+```tsx
+useEffect(() => {
+  analytics.trackPageView(pathname);
+}, [pathname]);
+
+useEffect(() => {
+  document.title = `${pageTitle} | My App`;
+}, [pageTitle]);
+```
+
+**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.