Vue Performance

Update Optimizations

Props Stability

• Keep props passed to child components as stable as possible.
• Instead of passing reactive IDs and comparing inside child:

<!-- Bad: Every ListItem updates when activeId changes -->
<ListItem
  v-for="item in list"
  :id="item.id"
  :active-id="activeId" />

• Compute the derived value in the parent and pass it directly:

<!-- Good: Only items whose active status changed will update -->
<ListItem
  v-for="item in list"
  :id="item.id"
  :active="item.id === activeId" />

Computed Stability

• Computed properties only trigger effects when their value changes.
• Avoid returning new objects from computed properties when the underlying data hasn't changed:

// Bad: Creates new object every time, always triggers updates
const computedObj = computed(() => {
  return {
    isEven: count.value % 2 === 0
  }
})

// Good: Returns old value if nothing changed
const computedObj = computed((oldValue) => {
  const newValue = {
    isEven: count.value % 2 === 0
  }
  if (oldValue && oldValue.isEven === newValue.isEven) {
    return oldValue
  }
  return newValue
})

• Always perform the full computation before comparing and returning the old value to ensure dependencies are collected.

Reduce Reactivity Overhead for Large Immutable Structures

• For large arrays of deeply nested objects, use shallowRef() and shallowReactive() to opt-out of deep reactivity.
• Shallow APIs create state reactive only at the root level, keeping nested property access fast.
• When using shallow reactivity, treat nested objects as immutable and trigger updates by replacing the root state:

const shallowArray = shallowRef([/* big list of deep objects */])

// Bad: Won't trigger updates
shallowArray.value.push(newObject)
shallowArray.value[0].foo = 1

// Good: Replace the root state
shallowArray.value = [...shallowArray.value, newObject]
shallowArray.value = [
  { ...shallowArray.value[0], foo: 1 },
  ...shallowArray.value.slice(1)
]

MaybeRefOrGetter with toValue

• Use MaybeRefOrGetter<T> type for flexible function parameters that can accept refs, getters, or plain values.
• Use toValue() to extract the actual value from MaybeRefOrGetter parameters:

7. Handle Async Operations with Error and Loading States

Always handle every possible state for data fetching or async logic, using separate components for each state: loading, success, error, and empty.

Example:

<script setup lang="ts">
import { ref } from "vue";
import type { User } from "@/types";

const user = ref<User | undefined>(undefined);
const loading = ref(true);
const error = ref<Error | undefined>(undefined);

async function fetchUserData(userId: string) {
  loading.value = true;
  error.value = undefined;

  try {
    const response = await fetch(`/api/users/${userId}`);
    if (!response.ok) throw new Error("Failed to fetch user data");
    user.value = await response.json();
  } catch (e) {
    error.value = e instanceof Error ? e : new Error("Unknown error");
    user.value = undefined;
  } finally {
    loading.value = false;
  }
}
</script>

<template>
  <div>
    <LoadingSpinner v-if="loading" />
    <ErrorMessage v-else-if="error !== undefined" :message="error.message" @retry="() => fetchUserData('replace-id')" />
    <UserProfile v-else-if="user !== undefined" :user="user" />
    <EmptyState v-else message="No user data available" />
  </div>
</template>

This ensures each state (loading, error, data, empty) is handled explicitly and the UI never displays an inconsistent result.

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