# Events API removedbreaking

# Overview

$on, $off and $once instance methods are removed. Vue instances no longer implement the event emitter interface.

# 2.x Syntax

In 2.x, Vue instance could be used to trigger handlers attached imperatively via the event emitter API ($on, $off and $once). This was used to create event hubs to create global event listeners used across the whole application:

// eventHub.js

const eventHub = new Vue()

export default eventHub

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// ChildComponent.vue
import eventHub from './eventHub'

export default {
  mounted() {
    // adding eventHub listener
    eventHub.$on('custom-event', () => {
      console.log('Custom event triggered!')
    })
  },
  beforeDestroy() {
    // removing eventHub listener
    eventHub.$off('custom-event')
  }
}

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// ParentComponent.vue
import eventHub from './eventHub'

export default {
  methods: {
    callGlobalCustomEvent() {
      eventHub.$emit('custom-event') // if ChildComponent is mounted, we will have a message in the console
    }
  }
}

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# 3.x Update

We removed $on, $off and $once methods from Vue instance completely. $emit is still a part of the existing API as it's used to trigger event handlers declaratively attached by a parent component

# Migration Strategy

Existing event hubs can be replaced by using an external library implementing the event emitter interface, for example mitt.

These methods can also be supported in compatibility builds.

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