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.agents/skills/vue-debug-guides/reference/v-for-computed-reverse-sort.md

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+---
+title: Copy Arrays Before reverse() or sort() in Computed Properties
+impact: MEDIUM
+impactDescription: reverse() and sort() mutate the original array, causing unintended side effects in computed getters
+type: gotcha
+tags: [vue3, v-for, computed, array, mutation, list-rendering]
+---
+
+# Copy Arrays Before reverse() or sort() in Computed Properties
+
+**Impact: MEDIUM** - The `reverse()` and `sort()` methods mutate the original array in-place. When used directly in a computed property getter, this causes the source array to be modified, leading to infinite reactivity loops or incorrect data state.
+
+Computed properties should be pure - they calculate a value without side effects. Mutating the source data inside a computed getter violates this principle and causes unpredictable behavior.
+
+## Task Checklist
+
+- [ ] Always create a copy of the array before calling `reverse()` or `sort()` in computed properties
+- [ ] Use spread operator `[...array]` or `Array.from(array)` or `array.slice()` to copy
+- [ ] This applies to any in-place mutation method used in computed getters
+- [ ] Consider using `toSorted()` and `toReversed()` (ES2023) which return new arrays
+
+**Incorrect:**
+```javascript
+const numbers = ref([1, 2, 3, 4, 5])
+
+// WRONG: Mutates the original array
+const reversedNumbers = computed(() => {
+  return numbers.value.reverse()  // Modifies numbers.value!
+})
+
+// WRONG: Same issue with sort
+const sortedItems = computed(() => {
+  return items.value.sort((a, b) => a.name.localeCompare(b.name))
+})
+```
+
+**Correct:**
+```javascript
+const numbers = ref([1, 2, 3, 4, 5])
+
+// CORRECT: Create a copy first with spread operator
+const reversedNumbers = computed(() => {
+  return [...numbers.value].reverse()
+})
+
+// CORRECT: Create a copy before sorting
+const sortedItems = computed(() => {
+  return [...items.value].sort((a, b) => a.name.localeCompare(b.name))
+})
+
+// CORRECT: Using slice() to copy
+const reversedNumbers = computed(() => {
+  return numbers.value.slice().reverse()
+})
+
+// CORRECT: ES2023 non-mutating methods (if supported)
+const reversedNumbers = computed(() => {
+  return numbers.value.toReversed()
+})
+
+const sortedItems = computed(() => {
+  return items.value.toSorted((a, b) => a.price - b.price)
+})
+```
+
+## Also Applies to Methods Used in Templates
+
+When using methods to filter/sort in nested v-for loops, the same rule applies:
+
+```javascript
+// CORRECT: Method that doesn't mutate
+function getSortedChildren(parent) {
+  return [...parent.children].sort((a, b) => a.order - b.order)
+}
+```
+
+```html
+<ul v-for="parent in parents" :key="parent.id">
+  <li v-for="child in getSortedChildren(parent)" :key="child.id">
+    {{ child.name }}
+  </li>
+</ul>
+```
+
+## Reference
+- [Vue.js List Rendering - Displaying Filtered/Sorted Results](https://vuejs.org/guide/essentials/list.html#displaying-filtered-sorted-results)