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120 lines
3.6 KiB
Markdown
120 lines
3.6 KiB
Markdown
---
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title: Use Computed Properties for Cached Reactive Derivations
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impact: MEDIUM
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impactDescription: Methods recalculate on every render while computed properties cache results
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type: efficiency
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tags: [vue3, computed, methods, performance, caching]
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---
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# Use Computed Properties for Cached Reactive Derivations
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**Impact: MEDIUM** - Computed properties are cached based on their reactive dependencies and only re-evaluate when dependencies change. Methods run on every component re-render, causing performance issues for expensive operations.
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When you need to derive a value from reactive state, prefer computed properties over methods for automatic caching and optimized re-renders.
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## Task Checklist
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- [ ] Use computed properties for values derived from reactive state
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- [ ] Use methods only when you need to pass parameters or don't want caching
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- [ ] Never use computed for non-reactive values like `Date.now()`
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- [ ] Consider performance impact of expensive operations in methods vs computed
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**Incorrect:**
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```vue
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<template>
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<!-- BAD: Method runs on every re-render -->
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<p>{{ getFilteredItems() }}</p>
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<p>{{ calculateTotal() }}</p>
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<p>{{ getCurrentTime() }}</p>
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</template>
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<script setup>
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import { ref } from 'vue'
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const items = ref([/* large array */])
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const prices = ref([100, 200, 300])
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// BAD: Expensive operation runs every render
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function getFilteredItems() {
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return items.value
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.filter(item => item.active)
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.sort((a, b) => a.name.localeCompare(b.name))
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}
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// BAD: Calculation runs every render even if prices unchanged
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function calculateTotal() {
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return prices.value.reduce((sum, price) => sum + price, 0)
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}
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// This looks like a computed use case, but Date.now() is non-reactive
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function getCurrentTime() {
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return Date.now() // Will appear to work but won't update reactively
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}
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</script>
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```
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**Correct:**
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```vue
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<template>
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<!-- GOOD: Computed only recalculates when items change -->
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<p>{{ filteredItems }}</p>
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<p>{{ total }}</p>
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<!-- GOOD: Method for non-reactive current time -->
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<p>{{ getCurrentTime() }}</p>
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</template>
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<script setup>
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import { ref, computed } from 'vue'
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const items = ref([/* large array */])
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const prices = ref([100, 200, 300])
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// GOOD: Cached - only recalculates when items.value changes
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const filteredItems = computed(() => {
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return items.value
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.filter(item => item.active)
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.sort((a, b) => a.name.localeCompare(b.name))
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})
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// GOOD: Cached - only recalculates when prices change
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const total = computed(() => {
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return prices.value.reduce((sum, price) => sum + price, 0)
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})
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// GOOD: Use method for non-reactive values
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// (or use setInterval to update a ref)
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function getCurrentTime() {
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return Date.now()
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}
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</script>
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```
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## When to Use Each
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| Scenario | Use Computed | Use Method |
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|----------|--------------|------------|
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| Derived from reactive state | Yes | No |
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| Expensive calculation | Yes | No |
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| Need to pass parameters | No | Yes |
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| Non-reactive value (Date.now()) | No | Yes |
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| Don't want caching | No | Yes |
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| Triggered by user action | No | Yes |
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## Non-Reactive Values Warning
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Computed properties only track reactive dependencies. Non-reactive values like `Date.now()` will cause the computed to be evaluated once and never update:
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```javascript
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// BAD: Date.now() is not reactive - computed will never update
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const now = computed(() => Date.now())
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// GOOD: Use a ref with setInterval for live time
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const now = ref(Date.now())
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setInterval(() => {
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now.value = Date.now()
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}, 1000)
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```
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## Reference
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- [Vue.js Computed Properties - Computed Caching vs Methods](https://vuejs.org/guide/essentials/computed.html#computed-caching-vs-methods)
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