1---
2title: Use Getter Function When Watching Reactive Object Properties
3impact: HIGH
4impactDescription: Watching reactive properties directly passes a primitive value, causing the watcher to never trigger
5type: capability
6tags: [vue3, watch, watchers, reactive, getter, common-mistake]
7---
8 
9# Use Getter Function When Watching Reactive Object Properties
10 
11**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.
12 
13When you need to watch a specific property of a reactive object, always wrap it in a getter function `() => obj.property`.
14 
15## Task Checklist
16 
17- [ ] Always use getter functions when watching properties of reactive objects
18- [ ] Remember that `watch(obj.count, ...)` passes the current value, not a reactive reference
19- [ ] For refs, you can watch directly: `watch(myRef, ...)`
20- [ ] For entire reactive objects, you can watch directly (creates implicit deep watcher)
21 
22**Incorrect:**
23```javascript
24import { reactive, watch } from 'vue'
25 
26const state = reactive({ count: 0, name: 'Vue' })
27 
28// WRONG: Passes the number 0 to watch(), not a reactive reference
29// This watcher will NEVER fire!
30watch(state.count, (newCount) => {
31  console.log(`Count changed to: ${newCount}`)
32})
33 
34// WRONG: Same problem with string property
35watch(state.name, (newName) => {
36  console.log(`Name changed to: ${newName}`)
37})
38```
39 
40**Correct:**
41```javascript
42import { reactive, watch } from 'vue'
43 
44const state = reactive({ count: 0, name: 'Vue' })
45 
46// CORRECT: Use a getter function
47watch(
48  () => state.count,
49  (newCount, oldCount) => {
50    console.log(`Count changed from ${oldCount} to ${newCount}`)
51  }
52)
53 
54// CORRECT: Multiple properties with getter
55watch(
56  () => state.name,
57  (newName) => {
58    console.log(`Name changed to: ${newName}`)
59  }
60)
61 
62// CORRECT: Watching derived values
63watch(
64  () => state.count * 2,
65  (doubledCount) => {
66    console.log(`Doubled count: ${doubledCount}`)
67  }
68)
69```
70 
71## Watching Multiple Properties
72 
73```javascript
74import { reactive, watch } from 'vue'
75 
76const state = reactive({ firstName: 'John', lastName: 'Doe' })
77 
78// Watch multiple properties with array of getters
79watch(
80  [() => state.firstName, () => state.lastName],
81  ([newFirst, newLast], [oldFirst, oldLast]) => {
82    console.log(`Name changed from ${oldFirst} ${oldLast} to ${newFirst} ${newLast}`)
83  }
84)
85```
86 
87## When Direct Watching Works
88 
89```javascript
90import { ref, reactive, watch } from 'vue'
91 
92const count = ref(0)
93const state = reactive({ nested: { value: 1 } })
94 
95// CORRECT: Refs can be watched directly
96watch(count, (newVal) => {
97  console.log(`Count: ${newVal}`)
98})
99 
100// CORRECT: Entire reactive objects create implicit deep watcher
101watch(state, (newState) => {
102  // Fires on any nested change
103  // Note: newState === oldState (same object reference)
104})
105```
106 
107## Reference
108- [Vue.js Watchers - Watch Source Types](https://vuejs.org/guide/essentials/watchers.html#watch-source-types)
109