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+---
+title: Avoid Expensive Operations in Updated Hook
+impact: MEDIUM
+impactDescription: Heavy computations in updated hook cause performance bottlenecks and potential infinite loops
+type: capability
+tags: [vue3, vue2, lifecycle, updated, performance, optimization, reactivity]
+---
+
+# Avoid Expensive Operations in Updated Hook
+
+**Impact: MEDIUM** - The `updated` hook runs after every reactive state change that causes a re-render. Placing expensive operations, API calls, or state mutations here can cause severe performance degradation, infinite loops, and dropped frames below the optimal 60fps threshold.
+
+Use `updated`/`onUpdated` sparingly for post-DOM-update operations that cannot be handled by watchers or computed properties. For most reactive data handling, prefer watchers (`watch`/`watchEffect`) which provide more control over what triggers the callback.
+
+## Task List
+
+- Never perform API calls in updated hook
+- Never mutate reactive state inside updated (causes infinite loops)
+- Use conditional checks to verify updates are relevant before acting
+- Prefer `watch` or `watchEffect` for reacting to specific data changes
+- Use throttling/debouncing if updated operations are expensive
+- Reserve updated for low-level DOM synchronization tasks
+
+**BAD:**
+```javascript
+// BAD: API call in updated - fires on every re-render
+export default {
+  data() {
+    return { items: [], lastUpdate: null }
+  },
+  updated() {
+    // This runs after every single state change!
+    fetch('/api/sync', {
+      method: 'POST',
+      body: JSON.stringify(this.items)
+    })
+  }
+}
+```
+
+```javascript
+// BAD: State mutation in updated - infinite loop
+export default {
+  data() {
+    return { renderCount: 0 }
+  },
+  updated() {
+    // This causes another update, which triggers updated again!
+    this.renderCount++ // Infinite loop
+  }
+}
+```
+
+```javascript
+// BAD: Heavy computation on every update
+export default {
+  updated() {
+    // Expensive operation runs on every keystroke, every state change
+    this.processedData = this.heavyComputation(this.rawData)
+    this.analytics = this.calculateMetrics(this.allData)
+  }
+}
+```
+
+**GOOD:**
+```javascript
+import debounce from 'lodash-es/debounce'
+
+// GOOD: Use watcher for specific data changes
+export default {
+  data() {
+    return { items: [] }
+  },
+  watch: {
+    // Only fires when items actually changes
+    items: {
+      handler(newItems) {
+        this.syncToServer(newItems)
+      },
+      deep: true
+    }
+  },
+  methods: {
+    syncToServer: debounce(function(items) {
+      fetch('/api/sync', {
+        method: 'POST',
+        body: JSON.stringify(items)
+      })
+    }, 500)
+  }
+}
+```
+
+```vue
+<!-- GOOD: Composition API with targeted watchers -->
+<script setup>
+import { ref, watch, onUpdated } from 'vue'
+import { useDebounceFn } from '@vueuse/core'
+
+const items = ref([])
+const scrollContainer = ref(null)
+
+// Watch specific data - not all updates
+watch(items, (newItems) => {
+  syncToServer(newItems)
+}, { deep: true })
+
+const syncToServer = useDebounceFn((items) => {
+  fetch('/api/sync', { method: 'POST', body: JSON.stringify(items) })
+}, 500)
+
+// Only use onUpdated for DOM synchronization
+onUpdated(() => {
+  // Scroll to bottom only if content changed height
+  if (scrollContainer.value) {
+    scrollContainer.value.scrollTop = scrollContainer.value.scrollHeight
+  }
+})
+</script>
+```
+
+```javascript
+// GOOD: Conditional check in updated hook
+export default {
+  data() {
+    return {
+      content: '',
+      lastSyncedContent: ''
+    }
+  },
+  updated() {
+    // Only act if specific condition is met
+    if (this.content !== this.lastSyncedContent) {
+      this.syncContent()
+      this.lastSyncedContent = this.content
+    }
+  },
+  methods: {
+    syncContent: debounce(function() {
+      // Sync logic
+    }, 300)
+  }
+}
+```
+
+## Valid Use Cases for Updated Hook
+
+```javascript
+// GOOD: Low-level DOM synchronization
+export default {
+  updated() {
+    // Sync third-party library with Vue's DOM
+    this.thirdPartyWidget.refresh()
+
+    // Update scroll position after content change
+    this.$nextTick(() => {
+      this.maintainScrollPosition()
+    })
+  }
+}
+```
+
+## Prefer Computed Properties for Derived Data
+
+```javascript
+// BAD: Calculating derived data in updated
+export default {
+  data() {
+    return { numbers: [1, 2, 3, 4, 5] }
+  },
+  updated() {
+    this.sum = this.numbers.reduce((a, b) => a + b, 0) // Causes another update!
+  }
+}
+
+// GOOD: Use computed property instead
+export default {
+  data() {
+    return { numbers: [1, 2, 3, 4, 5] }
+  },
+  computed: {
+    sum() {
+      return this.numbers.reduce((a, b) => a + b, 0)
+    }
+  }
+}
+```