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.claude/skills/vercel-react-best-practices/rules/js-combine-iterations.md

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+---
+title: Combine Multiple Array Iterations
+impact: LOW-MEDIUM
+impactDescription: reduces iterations
+tags: javascript, arrays, loops, performance
+---
+
+## Combine Multiple Array Iterations
+
+Multiple `.filter()` or `.map()` calls iterate the array multiple times. Combine into one loop.
+
+**Incorrect (3 iterations):**
+
+```typescript
+const admins = users.filter((u) => u.isAdmin);
+const testers = users.filter((u) => u.isTester);
+const inactive = users.filter((u) => !u.isActive);
+```
+
+**Correct (1 iteration):**
+
+```typescript
+const admins: User[] = [];
+const testers: User[] = [];
+const inactive: User[] = [];
+
+for (const user of users) {
+  if (user.isAdmin) admins.push(user);
+  if (user.isTester) testers.push(user);
+  if (!user.isActive) inactive.push(user);
+}
+```