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fix(watch): fire callback when original value is undefined #688

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fix(watch): fire callback when original value is undefined #688

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9 changes: 9 additions & 0 deletions packages/runtime-core/__tests__/apiWatch.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -439,4 +439,13 @@ describe('api: watch', () => {
oldValue: 2
})
})

// #683
it('watching undefined', async () => {
let dummy = null
const r = ref(undefined)
watch(r, v => (dummy = v))
await nextTick()
expect(dummy).toBe(undefined)
})
})
6 changes: 5 additions & 1 deletion packages/runtime-core/src/apiWatch.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -153,14 +153,18 @@ function doWatch(
}
}

let oldValue = isArray(source) ? [] : undefined
const watchDefaultSymbol = (Symbol('watchDefault') as unknown) as undefined // Unique symbol for default, see #683
let oldValue = isArray(source) ? [] : watchDefaultSymbol
const applyCb = cb
? () => {
if (instance && instance.isUnmounted) {
return
}
const newValue = runner()
if (deep || hasChanged(newValue, oldValue)) {
if (oldValue === watchDefaultSymbol) {
oldValue = undefined
}
// cleanup before running cb again
if (cleanup) {
cleanup()
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