Vue Composition API: Defining emits

Question

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?

Answer 1

No, because composition functions are used inside the setup hook which doesn't have access to the other options like methods and emits:

export default defineComponent({
    name: "layout",
    emits: ['showsidebar'],
    setup(props, { emit }) {
        const showSidebar = ref(true);
        const { breakpoints } = useBreakpoint();
        watch(breakpoints, (val) => {
            showSidebar.value = !(val.is === "xs" || val.is === "sm");
            emit('showsidebar',showSidebar.value);
        });
        return {
            showSidebar,
        };
    },
    data() {
        // ...
    },
});

In the example, useBreakpoint offers only some logic that the component could use. If there was a way to define the emits option in the composition function, then the function would always emit the event, even if the function is only used inside the component that defines the handler of the emitted event.

with the new script setup syntax you could do it as follows:

<script setup>
 import { defineEmits,watch,ref } from 'vue'
    
    const emit = defineEmits(['showsidebar'])
    const showSidebar = ref(true);
    const { breakpoints } = useBreakpoint();
    watch(breakpoints, (val) => {
            showSidebar.value = !(val.is === "xs" || val.is === "sm");
            emit('showsidebar',showSidebar.value);
        });
</script>

Answer 2

I did it like this with the script setup syntax:

<script setup>
    import { defineEmits } from 'vue'

    const emit = defineEmits(['close'])

    const handleClose = () => {
        emit('close')
    }
</script>

Answer 3

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>