Why v-model on the сustom component throw a warning without emits?

[floydback]

According to the docs this code should be enough
MyCopmponent.vue

<template>
  <button @click="update">+1</button>
</template>

<script>
export default {
  name: 'MyComponent',

  // emits: ['update:modelValue'], <--- without this I got a warning

  props: {
    modelValue: {
      type: Number,
      required: true,
      default: 0
    }
  },

  methods: {
    update () {
      this.$emit('update:modelValue', this.modelValue + 1)
    }
  }
}
</script>

but every time I change modelValue I got warning
[Vue warn]: Component emitted event "update:modelValue" but it is neither declared in the emits option nor as an "onUpdate:modelValue" prop. runtime-core.esm-bundler.js:156

Adding emits: ['update:modelValue'] to the component solve this, but is this behavior correct? I didn't find in the docs adding this instruction is necessary.

I use @vue/cli 4.5.8 and vue 3.0.2

[skirtles-code]

I'll put in a PR for this. It looks like a few of the v-model examples need updating.

[skirtles-code]

There are some tweaks needed in the docs and I've got a first draft of those locally. However, there's something more going on here.

Most of the documentation examples do not show a warning when run in isolation, even though they don't include emits. The warning isn't supposed to be shown when emits isn't specified. It should only be shown when it is specified but an event is missing.

I did a bit of digging and came up with this test case:

https://jsfiddle.net/skirtle/yofbtL0h/

Notice this line:

app.mixin({})

With that line included it shows the warning. Without that line the warning goes away.

The global mixin triggers a merge operation, leading to an emits that's an empty object. That's enough to trigger the relevant check, leading to the warning.

I need to give this some more thought but I'll probably end up reporting it as a library bug.

@floydback Do you have any global mixins in your application, consistent with my theory?

[floydback]

Do you have any global mixins in your application, consistent with my theory?

Yes! I can confirm this. Right now I've remove a plugin (it add global mixin) and this warning go away.

I also tried to reproduce this warning but it didn't show up on a pure project. I could not understand what is the reason. I think your theory is right. Thank you very much.