Using `watch` to observe a prop in a vue custom element, the other props accessed in the listener are the old values.

[hanshy1]

Vue version

3.5.11

Link to minimal reproduction

https://play.vuejs.org/#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

Steps to reproduce

1. Add custom components in the <template> of the parent component with different prop orders.

<template>
  <test-el :prop1="value1" :prop2="value2"></test-el>
  <test-el :prop2="value2" :prop1="value1"></test-el>
</template>

2. Use watch in the child component to monitor changes in a prop and record the value of another prop.

  watch: {
    prop1: {
      handler(val) {
        console.log('prop1 changed, prop2 is ', this.prop2)
        this.tempProp2 = this.prop2
      },
    },
    prop2: {
      handler(val) {
        console.log('prop2 changed, prop1 is', this.prop1)
        this.tempProp1 = this.prop1
      },
    },
  },

3. Update the prop value in the parent component.

const value1 = ref('old value1')
const value2 = ref('old value2')

setTimeout(() => {
  value1.value = 'new value1'
  value2.value = 'new value2'
}, 0)

4. Observe the results.

What is expected?

The watch listener in a custom element can access the latest prop values.

What is actually happening?

For a custom element, when using watch to observe a prop, accessing the values of other props defined later in the order within the listener will return their old values.

System Info

No response

Any additional comments?

No response

[catsmeatman]

bug exist from Vue 3.5.0. In Vue 3.4.38 all ok.

[edison1105]

@catsmeatman
It seems not, see Playground with v3.4.38. Does the issue you're experiencing seem different from this one?

The issue is caused by custom-element being rendered synchronously

core/packages/runtime-dom/src/apiCustomElement.ts

Lines 502 to 505 in 352bc88

	}
	if (shouldUpdate && this._instance) {
	this._update()
	}

When the watch callback for prop1 is triggered, the value of prop2 is still the old one. while normal components are rendered asynchronously, both prop1 and prop2 have the latest values.

a workaround: use watchEffect instead of watch, see Playground