A computed property is a reactive value derived from other reactive state. It automatically recalculates when its dependencies change, and — crucially — caches its result so it only re-runs when something it depends on actually changes.
vue
<script setup>
import { ref, computed } from "vue";
const firstName = ref("Ann");
const lastName = ref("Lee");
const fullName = computed(() => `${firstName.value} ${lastName.value}`);
// fullName updates automatically whenever firstName OR lastName changes
</script>
<template>
<p>{{ fullName }}</p> <!-- auto-unwrapped, like a ref -->
</template>