Unhandled error during execution of watcher callback and scheduler flush

[kleinfreund]

Version

3.0.0

Reproduction link

https://codesandbox.io/s/vigorous-nash-8lxt8?file=/src/Child.vue

Steps to reproduce

• click the increment button

What is expected?

this.$el should be defined inside of the watcher like in Vue 2

What is actually happening?

it is null

[LinusBorg]

Does this only happen in the test, but works fine when using the code in an app? Then that mit be something related to the test-utils. I can transfer the issue if so.

[kleinfreund]

When using the component that the aforementioned test file is testing in a Vue app, it doesn’t render without issues. The most glaring issue is that in the mounted hook, the $refs object is still empty, even though I have two refs defined in the template section (both not being conditionally rendered). I’m getting two Missing ref owner context. ref cannot be used on hoisted vnodes. A vnode with ref must be created inside the render function. warnings (likely one for each ref instance in the template. Furthermore, five Invalid VNode type: Symbol(Comment) (symbol) warnings are printed after the created hook is executed, but before the mounted hook is executed. Finally, a Unhandled error during execution of mounted hook warning is shown because in the mounted hook, one of the refs is being accessed, but since it’s undefined, a TypeError is thrown.

Now I’m not certain that either of those issues is necessarily related to what I reported initially. It might be that I’ve done something wrong when migrating the consuming app to Vue 3 or something of my tooling dependencies isn’t quite ready to be used with Vue 3, yet. But also, both the component that is the subject of the report above and the consuming app are relatively small and basic. I’ll try to reduce the test setup for this to figure out more.

[amrnn90]

Here's a minimal reproduction:
https://codesandbox.io/s/vigorous-nash-8lxt8?file=/src/Child.vue

Click the increment button and open the console:
this.$el is null inside the Child component watcher.

[sixthgear]

I can confirm a similar issue while trying to migrate a v2 codebase to v3. Inside of a component watcher I expect this.$el to be set, but it is null.

[ajimix]

I'm having the same problem also when migrating from v2 to v3 this.$el ends up being null while in v2 it works properly

[LinusBorg]

https://github.com/vuejs/vue-next/blob/5d825f318f1c3467dd530e43b09040d9f8793cce/packages/runtime-core/src/renderer.ts#L1427-L1432

• updateComponentPreRender() sets the instance.vnode to the next vnode, and then calls all queued effects.
• at that moment, the new vnode doesn't have an element assigned
• only after that does the new vnode get the element assigned.

[kleinfreund]

In my naïve hopes to possibly fix this myself, I wrote a two test cases which — as far as I understand — should both pass:

• setting an instance’s watched data property
• rendering an instance’s watched prop

Both should have access to the instance’s $el property. I wrote them in packages/runtime-core/__tests__/rendererComponent.spec.ts.

should have access to instance’s “$el” property in watcher when setting instance data

// #2170
test('should have access to instance’s “$el” property in watcher when setting instance data', async () => {
  function returnThis(this: any) {
    return this
  }
  const dataWatchSpy = jest.fn(returnThis)
  let instance: any
  const Comp = {
    data() {
      return {
        testData: undefined
      }
    },

    watch: {
      testData() {
        // @ts-ignore
        dataWatchSpy(this.$el)
      }
    },

    created() {
      instance = this
    },

    render() {
      return h('div')
    },
  }


  const root = nodeOps.createElement('div')
  render(h(Comp), root)

  expect(dataWatchSpy).not.toHaveBeenCalled()
  instance.testData = 'data'

  await nextTick()
  expect(dataWatchSpy).toHaveBeenCalledWith(instance.$el)
})

This test passes.

should have access to instance’s “$el” property in watcher when rendereing with watched prop'

// #2170
test('should have access to instance’s “$el” property in watcher when rendereing with watched prop', async () => {
  function returnThis(this: any) {
    return this
  }
  const propWatchSpy = jest.fn(returnThis)
  let instance: any
  const Comp = {
    props: {
      testProp: String
    },

    watch: {
      testProp() {
        // @ts-ignore
        propWatchSpy(this.$el)
      }
    },

    created() {
      instance = this
    },

    render() {
      return h('div')
    },
  }


  const root = nodeOps.createElement('div')

  render(h(Comp), root)
  await nextTick()
  expect(propWatchSpy).not.toHaveBeenCalled()

  render(h(Comp, { testProp: 'prop ' }), root)
  await nextTick()
  expect(propWatchSpy).toHaveBeenCalledWith(instance.$el)
})

This test fails because propWatchSpy is called with null instead of what should be the instance’s $el.

[kleinfreund]

I now understand @LinusBorg’s investigation and think I can provide a fix for this. If that’s alright, I’d like to submit a pull request for this.