Component emitted event "setTitle" but it is neither declared in the emits option nor as an "onSetTitle" prop

<child @setTitle=“setTitle” :title=“title” />

Vue 3 requires an ‘emits’ option in your component definition. For example if your custom component is called ‘child’ as your example above, and it emits a setTitle event then in the definition the ‘child’ component you should have something like this:

export default {
name: ‘child’,
components: {},
props: [‘title’],
emits: [‘setTitle’], …etc