agent-skills/skills/vue-best-practices/reference/watch-reactive-property-getter.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
Use Getter Function When Watching Reactive Object Properties	HIGH	Watching reactive properties directly passes a primitive value, causing the watcher to never trigger	capability	vue3 watch watchers reactive getter common-mistake

Use Getter Function When Watching Reactive Object Properties

Impact: HIGH - Directly watching a property of a reactive object passes a primitive value to watch(), not a reactive reference. The watcher will never trigger because primitives are not reactive.

When you need to watch a specific property of a reactive object, always wrap it in a getter function () => obj.property.

Task Checklist

• Always use getter functions when watching properties of reactive objects
• Remember that watch(obj.count, ...) passes the current value, not a reactive reference
• For refs, you can watch directly: watch(myRef, ...)
• For entire reactive objects, you can watch directly (creates implicit deep watcher)

Incorrect:

import { reactive, watch } from 'vue'

const state = reactive({ count: 0, name: 'Vue' })

// WRONG: Passes the number 0 to watch(), not a reactive reference
// This watcher will NEVER fire!
watch(state.count, (newCount) => {
  console.log(`Count changed to: ${newCount}`)
})

// WRONG: Same problem with string property
watch(state.name, (newName) => {
  console.log(`Name changed to: ${newName}`)
})

Correct:

import { reactive, watch } from 'vue'

const state = reactive({ count: 0, name: 'Vue' })

// CORRECT: Use a getter function
watch(
  () => state.count,
  (newCount, oldCount) => {
    console.log(`Count changed from ${oldCount} to ${newCount}`)
  }
)

// CORRECT: Multiple properties with getter
watch(
  () => state.name,
  (newName) => {
    console.log(`Name changed to: ${newName}`)
  }
)

// CORRECT: Watching derived values
watch(
  () => state.count * 2,
  (doubledCount) => {
    console.log(`Doubled count: ${doubledCount}`)
  }
)

Watching Multiple Properties

import { reactive, watch } from 'vue'

const state = reactive({ firstName: 'John', lastName: 'Doe' })

// Watch multiple properties with array of getters
watch(
  [() => state.firstName, () => state.lastName],
  ([newFirst, newLast], [oldFirst, oldLast]) => {
    console.log(`Name changed from ${oldFirst} ${oldLast} to ${newFirst} ${newLast}`)
  }
)

When Direct Watching Works

import { ref, reactive, watch } from 'vue'

const count = ref(0)
const state = reactive({ nested: { value: 1 } })

// CORRECT: Refs can be watched directly
watch(count, (newVal) => {
  console.log(`Count: ${newVal}`)
})

// CORRECT: Entire reactive objects create implicit deep watcher
watch(state, (newState) => {
  // Fires on any nested change
  // Note: newState === oldState (same object reference)
})

Reference

• Vue.js Watchers - Watch Source Types