Vue Composition API: Defining emits

When defining custom events Vue encourages us to define emitted events on the component via the emits option:

app.component('custom-form', {
  emits: ['inFocus', 'submit']
})

Using Vue 3's composition API, when a standalone composition function emits custom events, is it possible to define these in the composition function?

If you are using script setup you can use defineEmits which is a compiler macros and you don't have to import it:

<script setup>
const emit = defineEmits(['inFocus', 'submit'])

emit('inFocus')
</script>

You can also use an object syntax, which allows performing events validation:

<script setup>
const emit = defineEmits({
  // No validation
  inFocus: null,

  // Validate submit event
  submit: ({ email, password }) => {
    if (email && password) return true
    else return false
  }
})

function submitForm(email, password) {
  emit('submit', { email, password })
}
</script>

Note: the submit event will be emitted regardless of validation but if the validation doesn't pass, you will get a Vue warning:

[Vue warn]: Invalid event arguments: event validation failed for event "submit".

See it live

Typing with TS:

<script setup lang="ts">
const emit = defineEmits<{
  (e: 'change', id: number): void
  (e: 'update', value: string): void
}>()
</script>

The above code snippets are taken from Vue.js Emitting and Listening to Events