title: Vue 3.5+ Deep Watch Numeric Depth
impact: MEDIUM
impactDescription: enables efficient array mutation watching with numeric deep option
type: capability
tags: watch, deep, vue-3.5, array, mutation, performance

Vue 3.5+ Deep Watch Numeric Depth

Impact: MEDIUM - enables efficient array mutation watching with numeric deep option

Vue 3.5 introduced deep: number for watch depth control. This allows watching array mutations without the performance cost of deep traversal.

Symptoms

• Array mutations not triggering watch callback
• Deep watch causing performance issues on large nested objects
• Unaware of new Vue 3.5 feature

Note: TypeScript error "Type 'number' is not assignable to type 'boolean'" no longer occurs with Vue 3.5+ and current TypeScript versions. The types now correctly support numeric deep values.

The Feature

// Vue 3.5+ only
watch(items, (newVal) => {
  // Triggered on array mutations (push, pop, splice, etc.)
}, { deep: 1 })

deep value	Behavior
true	Full recursive traversal (original behavior)
false	Only reference changes
1	One level deep - array mutations, not nested objects
2	Two levels deep
n	N levels deep

Fix

Step 1: Ensure Vue 3.5+

npm install vue@^3.5.0

Step 2: Update @vue/runtime-core types

npm install -D @vue/runtime-core@latest

Step 3: Use numeric depth

import { watch, ref } from 'vue'

const items = ref([{ id: 1, data: { nested: 'value' } }])

// Watch array mutations only (push, pop, etc.)
watch(items, (newItems) => {
  console.log('Array mutated')
}, { deep: 1 })

// Won't trigger on: items.value[0].data.nested = 'new'
// Will trigger on: items.value.push(newItem)

Performance Comparison

const largeNestedData = ref({ /* deeply nested structure */ })

// SLOW - traverses entire structure
watch(largeNestedData, handler, { deep: true })

// FAST - only watches top-level changes
watch(largeNestedData, handler, { deep: 1 })

// FASTEST - only reference changes
watch(largeNestedData, handler, { deep: false })

Alternative: watchEffect for Selective Tracking

// Only tracks properties actually accessed
watchEffect(() => {
  // Only re-runs when items.value.length or first item changes
  console.log(items.value.length, items.value[0]?.id)
})

TypeScript Note

If TypeScript complains about numeric deep, ensure:

1. Vue version is 3.5+
2. @vue/runtime-core types are updated
3. tsconfig targets correct node_modules types

Reference

• Vue Watchers Docs
• Vue 3.5 Release Notes