Using event bus in Vue.js to pass data between components

Editor’s note: This article was last updated on 14 June 2023 to compare the event bus in Vue 2 and Vue 3, such as the new need to install an external event emitter and listener package. Check out this article for more information about setting up event emitters in Vue.js, or this article for a deep dive into Vue event handling.

Vue has a way of communicating between two child components through a parent component using event emitters. When you set up an event in a child component and a listener in the parent component, the reaction is passed down through the parent to the nested components.

While this is a valuable solution, it can become clumsy as your project grows. Event bus is a Vue.js instance that can emit events in one component, and then listen and react to the emitted event in another component directly — without the help of a parent component. An event bus is more efficient than an emitter because it requires less code to run.

In this tutorial, we’re going to create an event bus in our Vue.js project to facilitate communication between two components via a private channel. This is commonly known as the publish-subscribe approach.

We’ll cover the following:

• Using the event bus pattern in Vue 2
• Getting started with the event bus in Vue 3
• Creating our child components
• Setting up events
• Listening and reacting to events
• The current state of the event bus in Vue.js

Prerequisites

This post is suited for developers of all stages, including beginners. Here are a few things you should already have before going through this article:

• Node.js version 14.18+ and above installed. You can verify that you have this version by running the command below in your terminal/command prompt: node -v
• npm version 6.x and above installed. Use the following command in your terminal to verify the version installed: npm -v
• Visual Studio Code editor or a similar code editor
• Run the following command to scaffold a Vite and Vue project:

  # npm 6.x
  npm create vite@latest event-bus-tutorial --template vue
  
  # npm 7+, extra double-dash is needed:
  npm create vite@latest event-bus-tutorial -- --template vue

• Navigate into the event-bus-tutorial directory and use npm to install the required dependencies:

  cd event-bus-tutorial
  npm install

Using the event bus pattern in Vue 2

In Vue 2.x, a Vue instance could be used to trigger handlers attached imperatively via the event emitter API ($on, $off, and $once).

In order to use the event bus pattern, all you had to do was create a new instance of the Vue constructor, assign the instance to a constant variable named eventBus, and then export it. This instance serves as the event bus, allowing different components in your Vue application to communicate with each other:

// eventBus.js
const eventBus = new Vue()
export default eventBus

Import the eventBus instance and add an event listener in the component that you want to receive events:

// ChildComponent.vue
import eventBus from './eventBus'

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

Import the eventBus instance and add an event emitter in the component that you want to send events:

// ParentComponent.vue
import eventBus from './eventBus'

export default {
  methods: {
    sendCustomEvent() {
      // sending the event
      eventBus.$emit('custom-event')
    }
  }
}

Getting started with the event bus in Vue 3

In Vue 3, the $on, $off, and $once methods were removed from the Vue instance completely. So, in order to use the event bus pattern, you have to install an external event emitter and listener package, such as mitt.

Use the following commands to install the mitt package and start serving the application:

npm install --save mitt
npm run dev

Open the main.js file located in the src directory, and modify the code inside it to look like the following:

import { createApp } from 'vue'
import mitt from 'mitt'
import App from './App.vue'

const emitter = mitt()
const app = createApp(App)

app.config.globalProperties.emitter = emitter
app.mount('#app')

This code integrates the mitt event emitter library with your Vue 3 application by creating an emitter instance, making it globally accessible through the Vue application instance, and mounting the application to the DOM. This allows components to emit and listen for events using the emitter without the need for direct imports.

Creating our child components

In our demo, we’re going to create two child components that need to communicate with each other without using the parent component as an intermediary. First, create a new file named Child1.vue inside the components directory and paste the following code block inside it:

<template>
  <div>
    <button>Increment</button>
  </div>
</template>

<script >
export default {
  name: "Child1",
  data: () => ({
    counter: 0,
  }),
  methods: {},
};
</script>

<style scoped>
button {
  margin: 4rem 0 0;
  padding: 1rem;
  background-color: #0d6efd;
  color: white;
  border-radius: 1rem;
  font-size: 2rem;
}
</style>

Overall, this code renders a <div> containing a styled <button>. The component also has an internal data property counter set to 0 and an empty methods property.

Create a new file named Child2.vue inside the components directory and paste the following code block inside it:

<template>
  <div>
    <h1>{{ counter }}</h1>
  </div>
</template>

<script>
export default {
  name: "Child2",
  data: () => ({
    counter: 0,
  }),
};
</script>

<style scoped>
h1 {
  margin: 5rem 0 0;
  font-size: 10.5rem;
}
</style>

This Vue component renders a <div> component that contains a styled <h1> heading. The counter value is displayed dynamically in the heading.

Now, go to your App.vue file and replace its entire code with the following:

<template>
  <div id="app">
    <Child2 />
    <Child1 />
  </div>
</template>
<script>
import Child1 from "./components/Child1.vue";
import Child2 from "./components/Child2.vue";
export default {
  name: "app",
  components: {
    Child1,
    Child2,
  },
};
</script>
<style>
#app {
  font-family: "Avenir", Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 5rem;
}
</style>

Inside the App.vue component, the code imports and renders the Child1 and Child2 components within a styled parent div.

Setting up events

Now that your two components are ready, you can set up the event through emission in the Child1 component while you listen to the event in the Child2 component. Open your Child1.vue file and copy the following code block into it:

<template>
  <div>
    <button v-on:click="sendEvent">Increment</button>
  </div>
</template>

<script >
export default {
  name: "Child1",
  data: () => ({
    counter: 0,
  }),
  methods: {
    sendEvent() {
      this.counter += 1;
      this.emitter.emit("increment", { msg: this.counter });
    },
  },
};
</script>

<style scoped>
button {
  margin: 4rem 0 0;
  padding: 1rem;
  background-color: #0d6efd;
  color: white;
  border-radius: 1rem;
  font-size: 2rem;
}
</style>

Here, an onclick event listener was added to the button element. When this event is triggered, a method named sendEvent() will be called.

The sendEvent() method increments the counter value by one and uses the global event emitter to emit an increment event with the updated counter value to any parent or sibling component.

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Listening and reacting to events

After setting up the event, we need to make the second component listen and react to the event. Open your Child2.vue file and copy in the following code block:

<template>
  <div>
    <h1>{{ counter }}</h1>
  </div>
</template>

<script>
export default {
  name: "Child2",
  data: () => ({
    counter: 0,
  }),
  mounted() {
    this.emitter.on("increment", (data) => {
      this.counter = data.msg;
    });
  },
};
</script>

<style scoped>
h1 {
  margin: 5rem 0 0;
  font-size: 10.5rem;
}
</style>

The code uses a mounted lifecycle hook to initialize the listening process as the app is mounted on the DOM.

The emitter.on statement is now listening to the increment event, passing the data argument down, and setting it as the new counter:

When you click the Increment button located on the Child1 component, the increment event is sent alongside the updated counter value to the Child2 component.

The current state of the event bus in Vue.js

Using a global event bus to facilitate communication between components is not recommended in most cases. While it may initially appear to be the easiest solution, it frequently leads to significant maintenance challenges over time.

Depending on the specific situation, the Vue developer team recommends several alternative approaches to consider instead of relying on an event bus:

• Prioritize using props and events as the primary means of communication between parent and child components. If necessary, siblings can communicate through their common parent
• The use of provide/inject allows for communication between components and their slot contents. This is especially useful for tightly-coupled components that are always used together
• Provide/inject can also facilitate communication between components that are far apart in the component hierarchy. It can help eliminate the need for excessive prop drilling, where props are passed down through multiple levels of components that do not require those props themselves
• To avoid prop drilling, consider refactoring the codebase to utilize slots. If an intermediate component doesn’t require certain props, it may indicate a concern with the separation of responsibilities. Introducing a slot in that component enables the parent to directly provide the contents, allowing for the passing of props without involving the intermediate component
• Global state management libraries like Pinia can be used to manage global state across components. These libraries offer a centralized approach to state management, allowing components to access and modify shared state without the need for explicit prop passing

Conclusion

This has been an introduction to the event bus in Vue.js. The event bus serves as an easy-to-achieve, independent communication between components without passing through a central or parent component.

While the event bus may initially seem like a convenient method for inter-component communication, it is advised to explore alternative options such as provide/inject or global state management. These alternatives offer more robust and maintainable solutions for facilitating communication between components.

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