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)
})