Vue.js 3 Event Bus

As suggested in official docs you could use mitt library to dispatch events between components, let suppose that we have a sidebar and header which contains a button that close/open the sidebar and we need that button to toggle some property inside the sidebar component :

in main.js import that library and create an instance of that emitter and define as a global property:

Installation :

npm install --save mitt

Usage :

import { createApp } from 'vue'
import App from './App.vue'
import mitt from 'mitt';
const emitter = mitt();
const app = createApp(App);
app.config.globalProperties.emitter = emitter;
app.mount('#app');

in header emit the toggle-sidebar event with some payload :

<template>
<header>
<button @click="toggleSidebar"/>toggle</button>
</header>
</template>
<script >
export default {
data() {
return {
sidebarOpen: true
};
},
methods: {
toggleSidebar() {
this.sidebarOpen = !this.sidebarOpen;
this.emitter.emit("toggle-sidebar", this.sidebarOpen);
}
}
};
</script>

In sidebar receive the event with the payload:

<template>
<aside class="sidebar" :class="{'sidebar--toggled': !isOpen}">
....
</aside>
</template>
<script>
export default {
name: "sidebar",
data() {
return {
isOpen: true
};
},
mounted() {
this.emitter.on("toggle-sidebar", isOpen => {
this.isOpen = isOpen;
});
}
};
</script>

For those using composition api they could use emitter as follows :

Create a file src/composables/useEmitter.js

import { getCurrentInstance } from 'vue'


export default function useEmitter() {
const internalInstance = getCurrentInstance();
const emitter = internalInstance.appContext.config.globalProperties.emitter;


return emitter;
}

And from there on you can use useEmitter just like you would with useRouter:

import useEmitter from '@/composables/useEmitter'


export default {
setup() {
const emitter = useEmitter()
...
}
...
}

Using the composition API

You could also take benefit from the new composition API and define a composable event bus :

eventBus.js

import { ref } from "vue";
const bus = ref(new Map());


export default function useEventsBus(){


function emit(event, ...args) {
bus.value.set(event, args);
}


return {
emit,
bus
}
}

in component A do:

import useEventsBus from './eventBus';
...
//in script setup or inside the setup hook
const {emit}=useEventsBus()
...
emit('sidebarCollapsed',val)

in component B :

const { bus } = useEventsBus()


watch(()=>bus.value.get('sidebarCollapsed'), (val) => {
// destruct the parameters
const [sidebarCollapsedBus] = val ?? []
sidebarCollapsed.value = sidebarCollapsedBus
})

On version 3 of Vue.js, you can use either a third-party library, or use the functionality written in the publisher-subscriber(PubSub concept) programming pattern.

event.js

//events - a super-basic Javascript (publish subscribe) pattern


class Event{
constructor(){
this.events = {};
}


on(eventName, fn) {
this.events[eventName] = this.events[eventName] || [];
this.events[eventName].push(fn);
}


off(eventName, fn) {
if (this.events[eventName]) {
for (var i = 0; i < this.events[eventName].length; i++) {
if (this.events[eventName][i] === fn) {
this.events[eventName].splice(i, 1);
break;
}
};
}
}


trigger(eventName, data) {
if (this.events[eventName]) {
this.events[eventName].forEach(function(fn) {
fn(data);
});
}
}
}


export default new Event();

index.js

import Vue from 'vue';
import $bus from '.../event.js';


const app = Vue.createApp({})
app.config.globalProperties.$bus = $bus;

With Vue composition and defineEmit you can even make it easier :

<!-- Parent -->
<script setup>
import { defineEmit } from 'vue'
const emit = defineEmit(['selected'])
const onEmit = (data) => console.log(data)
</script>


<template>
<btnList
v-for="x in y"
:key="x"
:emit="emit"
@selected="onEmit"
/>
</template>

<!-- Children (BtnList.vue) -->
<script setup>
import { defineProps } from 'vue'
const props = defineProps({
emit: Function
})
</script>


<template>
<button v-for="x in 10" :key="x" @click="props.emit('selected', x)">Click \{\{ x }}</button>
</template>

I just showed it with one children, but you could pass though the emit function down to other children.

Content of EventBus class file:

class EventBusEvent extends Event {
public data: any


constructor({type, data} : {type: string, data: any}) {
super(type)
this.data = data
}
}


class EventBus extends EventTarget {
private static _instance: EventBus


public static getInstance() : EventBus {
if (!this._instance) this._instance = new EventBus()
return this._instance
}


public emit(type : string, data?: any) : void {
this.dispatchEvent(new EventBusEvent({type, data}))
}
}


export default EventBus.getInstance()

usage in project, emit event:

import EventBus from '...path to eventbus file with class'
//...bla bla bla... code...
EventBus.emit('event type', {..some data..}')

listen event:

import EventBus from '...path to eventbus file with class'
//...bla bla bla... code...
EventBus.addEventListener('event type', (event) => { console.log(event.data) })

I’ve adapted another answer to have an equivalent interface to a Vue instance so that the utility works as a drop-in replacement that doesn’t require changes in the consuming code.

This version also supports the $off method with the first argument being an array of event names. It also avoids an issue in the $off method were de-registering multiple event listeners would actually delete a wrong one due to iterating over the array in forwards direction while also deleting items from it.

event-bus.js:

// @ts-check


/**
* Replacement for the Vue 2-based EventBus.
*
* @template EventName
*/
class Bus {
constructor() {
/**
* @type {Map<EventName, Array<{ callback: Function, once: boolean }>>}
*/
this.eventListeners = new Map()
}


/**
* @param {EventName} eventName
* @param {Function} callback
* @param {boolean} [once]
* @private
*/
registerEventListener(eventName, callback, once = false) {
if (!this.eventListeners.has(eventName)) {
this.eventListeners.set(eventName, [])
}


const eventListeners = this.eventListeners.get(eventName)
eventListeners.push({ callback, once })
}


/**
* See: https://v2.vuejs.org/v2/api/#vm-on
*
* @param {EventName} eventName
* @param {Function} callback
*/
$on(eventName, callback) {
this.registerEventListener(eventName, callback)
}


/**
* See: https://v2.vuejs.org/v2/api/#vm-once
*
* @param {EventName} eventName
* @param {Function} callback
*/
$once(eventName, callback) {
const once = true
this.registerEventListener(eventName, callback, once)
}


/**
* Removes all event listeners for the given event name or names.
*
* When provided with a callback function, removes only event listeners matching the provided function.
*
* See: https://v2.vuejs.org/v2/api/#vm-off
*
* @param {EventName | EventName[]} eventNameOrNames
* @param {Function} [callback]
*/
$off(eventNameOrNames, callback = undefined) {
const eventNames = Array.isArray(eventNameOrNames) ? eventNameOrNames : [eventNameOrNames]


for (const eventName of eventNames) {
const eventListeners = this.eventListeners.get(eventName)


if (eventListeners === undefined) {
continue
}


if (typeof callback === 'function') {
for (let i = eventListeners.length - 1; i >= 0; i--) {
if (eventListeners[i].callback === callback) {
eventListeners.splice(i, 1)
}
}
} else {
this.eventListeners.delete(eventName)
}
}
}


/**
* See: https://v2.vuejs.org/v2/api/#vm-emit
*
* @param {EventName} eventName
* @param {any} args
*/
$emit(eventName, ...args) {
if (!this.eventListeners.has(eventName)) {
return
}


const eventListeners = this.eventListeners.get(eventName)
const eventListenerIndexesToDelete = []
for (const [eventListenerIndex, eventListener] of eventListeners.entries()) {
eventListener.callback(...args)


if (eventListener.once) {
eventListenerIndexesToDelete.push(eventListenerIndex)
}
}


for (let i = eventListenerIndexesToDelete.length - 1; i >= 0; i--) {
eventListeners.splice(eventListenerIndexesToDelete[i], 1)
}
}
}


const EventBus = new Bus()


export default EventBus

old-event-bus.js:

import Vue from 'vue'


const EventBus = new Vue()


export default EventBus

example.js:

// import EventBus from './old-event-bus.js'
import EventBus from './event-bus.js'

I just want to mention here that you can also use useEventBus defined by VueUse.

Here is one example for TypeScript so using an injection key.

//myInjectionKey.ts
import type { EventBusKey } from '@vueuse/core'
export const myInjectionKey: EventBusKey<string> = Symbol('my-injection-key')

//emmitter
import { useEventBus } from '@vueuse/core'
import { mapInjectionKey } from "src/config/mapInjectionKey";


const bus = useEventBus(mapInjectionKey)
bus.emit("Hello")

//receiver
import { useEventBus } from '@vueuse/core'
import { mapInjectionKey } from "src/config/mapInjectionKey";
const bus = useEventBus(mapInjectionKey)
bus.on((e) => {
console.log(e) // "Hello"
})

Using https://www.npmjs.com/package/vue-eventer means minimal code changes when migrating from Vue 2.x to Vue 3.0 (just initialization)...

// Vue 2.x
Vue.prototype.$eventBus = new Vue();

->

// Vue 3.x
import VueEventer from 'vue-eventer';
YourVueApp.config.globalProperties.$eventBus = new VueEventer();