W3cubDocs

/Vue 3

Component Events

This page assumes you've already read the Components Basics. Read that first if you are new to components.

Emitting and Listening to Events

A component can emit custom events directly in template expressions (e.g. in a v-on handler) using the built-in $emit method:

<!-- MyComponent -->
<button @click="$emit('someEvent')">click me</button>

The $emit() method is also available on the component instance as this.$emit():

export default {
  methods: {
    submit() {
      this.$emit('someEvent')
    }
  }
}

The parent can then listen to it using v-on:

<MyComponent @some-event="callback" />

The .once modifier is also supported on component event listeners:

<MyComponent @some-event.once="callback" />

Like components and props, event names provide an automatic case transformation. Notice we emitted a camelCase event, but can listen for it using a kebab-cased listener in the parent. As with props casing, we recommend using kebab-cased event listeners in templates.

TIP

Unlike native DOM events, component emitted events do not bubble. You can only listen to the events emitted by a direct child component. If there is a need to communicate between sibling or deeply nested components, use an external event bus or a global state management solution.

Event Arguments

It's sometimes useful to emit a specific value with an event. For example, we may want the <BlogPost> component to be in charge of how much to enlarge the text by. In those cases, we can pass extra arguments to $emit to provide this value:

<button @click="$emit('increaseBy', 1)">
  Increase by 1
</button>

Then, when we listen to the event in the parent, we can use an inline arrow function as the listener, which allows us to access the event argument:

<MyButton @increase-by="(n) => count += n" />

Or, if the event handler is a method:

<MyButton @increase-by="increaseCount" />

Then the value will be passed as the first parameter of that method:

methods: {
  increaseCount(n) {
    this.count += n
  }
}

function increaseCount(n) {
  count.value += n
}

TIP

All extra arguments passed to $emit() after the event name will be forwarded to the listener. For example, with $emit('foo', 1, 2, 3) the listener function will receive three arguments.

Declaring Emitted Events

A component can explicitly declare the events it will emit using the defineEmits() macroemits option:

<script setup>
defineEmits(['inFocus', 'submit'])
</script>

The $emit method that we used in the <template> isn't accessible within the <script setup> section of a component, but defineEmits() returns an equivalent function that we can use instead:

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

function buttonClick() {
  emit('submit')
}
</script>

The defineEmits() macro cannot be used inside a function, it must be placed directly within <script setup>, as in the example above.

If you're using an explicit setup function instead of <script setup>, events should be declared using the emits option, and the emit function is exposed on the setup() context:

export default {
  emits: ['inFocus', 'submit'],
  setup(props, ctx) {
    ctx.emit('submit')
  }
}

As with other properties of the setup() context, emit can safely be destructured:

export default {
  emits: ['inFocus', 'submit'],
  setup(props, { emit }) {
    emit('submit')
  }
}

export default {
  emits: ['inFocus', 'submit']
}

The emits option and defineEmits() macro also support an object syntax. If using TypeScript you can type arguments, which allows us to perform runtime validation of the payload of the emitted events:

<script setup>
const emit = defineEmits({
  submit(payload: { email: string, password: string }) {
    // return `true` or `false` to indicate
    // validation pass / fail
  }
})
</script>

If you are using TypeScript with <script setup>, it's also possible to declare emitted events using pure type annotations:

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

More details: Typing Component Emits

export default {
  emits: {
    submit(payload: { email: string, password: string }) {
      // return `true` or `false` to indicate
      // validation pass / fail
    }
  }
}

Events Validation

Similar to prop type validation, an emitted event can be validated if it is defined with the object syntax instead of the array syntax.

To add validation, the event is assigned a function that receives the arguments passed to the this.$emitemit call and returns a boolean to indicate whether the event is valid or not.

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

  // Validate submit event
  submit: ({ email, password }) => {
    if (email && password) {
      return true
    } else {
      console.warn('Invalid submit event payload!')
      return false
    }
  }
})

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

export default {
  emits: {
    // No validation
    click: null,

    // Validate submit event
    submit: ({ email, password }) => {
      if (email && password) {
        return true
      } else {
        console.warn('Invalid submit event payload!')
        return false
      }
    }
  },
  methods: {
    submitForm(email, password) {
      this.$emit('submit', { email, password })
    }
  }
}