stack/packages/mosaic/framework/skills/vue-best-practices/reference/rendering-excessive-rerenders-watch-vs-computed.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
Avoid Excessive Re-renders from Misused Watchers	HIGH	Using watch instead of computed, or deep watchers unnecessarily, triggers excessive component re-renders	gotcha	vue3 rendering performance watch computed reactivity re-renders

Avoid Excessive Re-renders from Misused Watchers

Impact: HIGH - Improper use of watchers is a common cause of performance issues. Deep watchers track every nested property change, and using watch when computed would suffice creates unnecessary reactive updates that trigger re-renders.

Task Checklist

• Use computed for derived values, not watch + manual state updates
• Avoid deep: true on large objects unless absolutely necessary
• When deep watching is needed, watch specific nested paths instead
• Never trigger state changes inside watch that cause the watch to re-fire

Incorrect:

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

const user = ref({ name: 'John', settings: { theme: 'dark', notifications: true } });
const displayName = ref('');

// BAD: Using watch to compute a derived value
// This triggers an extra reactive update cycle
watch(
  () => user.value.name,
  (name) => {
    displayName.value = `User: ${name}`;
  },
  { immediate: true },
);

// BAD: Deep watcher on a large object
// Fires on ANY nested change, even unrelated ones
const items = ref([
  /* 1000 items with nested properties */
]);
watch(
  items,
  (newItems) => {
    console.log('Items changed'); // Fires on every tiny change
  },
  { deep: true },
);
</script>

Correct:

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

const user = ref({ name: 'John', settings: { theme: 'dark', notifications: true } });

// GOOD: Use computed for derived values
// No extra reactive updates, automatically cached
const displayName = computed(() => `User: ${user.value.name}`);

// GOOD: Watch specific paths, not the entire object
const items = ref([
  /* 1000 items */
]);
watch(
  () => items.value.length, // Only watch the length
  (newLength) => {
    console.log(`Items count: ${newLength}`);
  },
);

// GOOD: Watch specific nested property
watch(
  () => user.value.settings.theme,
  (newTheme) => {
    applyTheme(newTheme); // Side effect - appropriate for watch
  },
);
</script>

When to Use Watch vs Computed

Use Case	Use This
Derive a value from state	computed
Format/transform data for display	computed
Perform side effects (API calls, DOM updates)	watch
React to route changes	watch
Sync with external systems	watch

Infinite Loop from Watch

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

const count = ref(0);

// DANGER: Infinite loop!
watch(count, (newVal) => {
  count.value = newVal + 1; // Modifies watched source -> triggers watch again
});

// CORRECT: Use computed or avoid self-modification
const doubleCount = computed(() => count.value * 2);
</script>

Efficient Deep Watching

When you must watch complex objects:

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

const formData = ref({
  personal: { name: '', email: '' },
  address: { street: '', city: '' },
  preferences: {
    /* many properties */
  },
});

// BAD: Watches everything, including preferences changes
watch(
  formData,
  () => {
    saveForm();
  },
  { deep: true },
);

// GOOD: Watch only the sections you care about
watch(
  () => formData.value.personal,
  () => savePersonalSection(),
  { deep: true }, // Deep only on this small subtree
);

// GOOD: Watch serialized version for change detection
watch(
  () => JSON.stringify(formData.value),
  () => {
    markFormDirty();
  },
);
</script>

Array Mutation Gotcha

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

const items = ref([1, 2, 3]);

// This watch won't trigger on sort/reverse without deep!
watch(items, () => {
  console.log('Items changed');
});

items.value.sort(); // Watch doesn't fire - array reference unchanged

// Solution 1: Use deep (performance cost)
watch(items, callback, { deep: true });

// Solution 2: Replace array instead of mutating
items.value = [...items.value].sort();
</script>

Reference

• Vue.js Watchers
• Vue.js Computed Properties
• Vue.js Performance - Reactivity