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fargo 1a822493ba format: apply repo prettier (3.8.1) to the folded skills tree

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2026-08-19 14:37:17 -05:00

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title, impact, impactDescription, type, tags

title

impact

impactDescription

type

Use Computed Properties for Cached Reactive Derivations

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.

When you need to derive a value from reactive state, prefer computed properties over methods for automatic caching and optimized re-renders.

Task Checklist

Use computed properties for values derived from reactive state
Use methods only when you need to pass parameters or don't want caching
Never use computed for non-reactive values like Date.now()
Consider performance impact of expensive operations in methods vs computed

Incorrect:

<template>
  <!-- BAD: Method runs on every re-render -->
  <p>{{ getFilteredItems() }}</p>
  <p>{{ calculateTotal() }}</p>
  <p>{{ getCurrentTime() }}</p>
</template>

<script setup>
import { ref } from 'vue';

const items = ref([
  /* large array */
]);
const prices = ref([100, 200, 300]);

// BAD: Expensive operation runs every render
function getFilteredItems() {
  return items.value.filter((item) => item.active).sort((a, b) => a.name.localeCompare(b.name));
}

// BAD: Calculation runs every render even if prices unchanged
function calculateTotal() {
  return prices.value.reduce((sum, price) => sum + price, 0);
}

// This looks like a computed use case, but Date.now() is non-reactive
function getCurrentTime() {
  return Date.now(); // Will appear to work but won't update reactively
}
</script>

Correct:

<template>
  <!-- GOOD: Computed only recalculates when items change -->
  <p>{{ filteredItems }}</p>
  <p>{{ total }}</p>
  <!-- GOOD: Method for non-reactive current time -->
  <p>{{ getCurrentTime() }}</p>
</template>

<script setup>
import { ref, computed } from 'vue';

const items = ref([
  /* large array */
]);
const prices = ref([100, 200, 300]);

// GOOD: Cached - only recalculates when items.value changes
const filteredItems = computed(() => {
  return items.value.filter((item) => item.active).sort((a, b) => a.name.localeCompare(b.name));
});

// GOOD: Cached - only recalculates when prices change
const total = computed(() => {
  return prices.value.reduce((sum, price) => sum + price, 0);
});

// GOOD: Use method for non-reactive values
// (or use setInterval to update a ref)
function getCurrentTime() {
  return Date.now();
}
</script>

When to Use Each

Scenario	Use Computed	Use Method
Derived from reactive state	Yes	No
Expensive calculation	Yes	No
Need to pass parameters	No	Yes
Non-reactive value (Date.now())	No	Yes
Don't want caching	No	Yes
Triggered by user action	No	Yes

Non-Reactive Values Warning

Computed properties only track reactive dependencies. Non-reactive values like Date.now() will cause the computed to be evaluated once and never update:

// BAD: Date.now() is not reactive - computed will never update
const now = computed(() => Date.now());

// GOOD: Use a ref with setInterval for live time
const now = ref(Date.now());
setInterval(() => {
  now.value = Date.now();
}, 1000);

Reference

Vue.js Computed Properties - Computed Caching vs Methods

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