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

Written by Nwose Lotanna✏️

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 10.x and above installed. You can verify that you have this version by running the command below in your terminal/command prompt:

node -v

• Visual Studio Code editor or a similar code editor.
• Vue’s latest version installed globally on your machine
• Vue CLI 3.0 installed on your machine. To do this, uninstall the old CLI version first:

npm uninstall -g vue-cli

then install the new one:

npm install -g @vue/cli

• Download a Vue starter project here.
• Unzip the downloaded project
• Navigate into the unzipped file and run the command to keep all the dependencies up-to-date:

npm install

The emitter problem

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.

The solution: Event bus

Essentially, an 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.

By definition, using an event bus is more efficient than using event emitters because it requires less code to run.

We’re going to create an event bus instance as a separate file, import it into the two components that are going to share data, and then allow the components to communicate through this shared instance in a safe, private channel.

This is commonly known as the publish-subscribe approach.

Demo

Today, we’re going to walk through the process of creating and using the event bus to facilitate communication between two components.

Getting started with the event bus

First, we want to create the event bus. We’ll do this inside our main.js file. After definition, your main.js file should look like this:

import Vue from 'vue'
import App from './App.vue'
Vue.config.productionTip = false
export const bus = new Vue();
new Vue({
  render: h => h(App),
}).$mount('#app')

As you can see, we’ve created a new Vue instance — a secure abstraction where we can handle communication between components without involving the parent component in the correspondence.

Creating a new component

We need two child components to communicate. However, you’ll notice there’s only one test.vue component in your starter project.

Create a new file and call it test2.vue and paste the code block below inside it:

<template>
  <div>
  </div>
</template>
<script>
export default {
  name: 'Test2',
  props: {
    msg: String
  }
}
</script>
<!-- Add "scoped" attribute to limit CSS to this component only -->
<style scoped>
h3 {
  margin: 40px 0 0;
}
ul {
  list-style-type: none;
  padding: 0;
}
li {
  display: inline-block;
  margin: 0 10px;
}
a {
  color: #42b983;
}
</style>

Now, go to your App.vue file and import it like the Test.vue file. Register the file under components like this:

<template>
  <div id="app">
    <img alt="Vue logo" src="./assets/logo.png">
    <Test v-bind:header="header"/>
    <Test2 v-bind:header="header"/>
  </div>
</template>
<script>
import Test from './components/Test.vue';
import Test2 from './components/Test2.vue';
export default {
  name: 'app',
  components: {
    Test, Test2
  },
  data (){
    return {
      header:'initial header'
    }
  }
}
</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: 60px;
}
</style>

Setting up events

Now that your two components are ready, you can set up the event through emission in the Test component while you listen to the event in the Test2 component.

Open your Test.vue file and copy the code block below into it:

<template>
  <div>
      <h1 v-on:click="changeHeader">{{header}}</h1>
  </div>
</template>
<script>
import { bus } from '../main'
export default {
  name: 'Test',
  props: {
    header:{
        type: String
    } 
  },
  methods: {
      changeHeader (){
          this.header = "changed header";
          bus.$emit('changeIt', 'changed header');
      }
  }
}
</script>

Here, you’ll see that the event bus was imported from main.js, the template displays one header element through props, and there is a click event on it that points to the logic in the methods section.

The manual change of the Test.vue component occurs inside the method section and emits an event through the event bus.

The statement tells Vue to emit an event called changeIt and pass the string changed header as argument.

Listening to events and reacting

After setting up the event, we need to make the second component listen and react to the event. Open your Test2.vue file and copy in the code block below:

<template>
  <div> <h1>{{header}}</h1>
  </div>
</template>
<script>
import { bus } from '../main';
export default {
  name: 'Test2',
  props: {
    header:{
        type: String
    } 
  },
  created (){
    bus.$on('changeIt', (data) => {
      this.header = data;
    })
  }
}
</script>

When the event bus imports, all we see inside the template is the interpolation symbol. There isn’t a Vue directive or bindings.

We’ll use a lifecycle hook to initialize the listening process as the app is mounted on the DOM. The lifecycle hook is called created as the application is initialized.

The $on statement is now listening to a changeIt event, passing the data argument down, and setting it as the new header.

When you click the first header in the interface, both headers change.

Removing listeners

Vue automatically un-mounts and removes these listeners before the destruction of a Vue instance. However, if you want to manually destroy them, you can run this simple command:

bus.$off();

The complete code to this tutorial can be found here on GitHub.

Conclusion

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

The event bus is also cleaner and involves less code than other approaches, providing a great abstracted platform.

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Editor's note: Seeing something wrong with this post? You can find the correct version here.

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Comments

[Broderick Stadden]

Keep in mind this is not a replacement for parent-child events. You should preserve that data flow for most components because logic should be self contained when possible.

It’s very easy to make your app a mess if you are throwing everything into the event bus. This should only really be used if you need to pass data from one container component (a parent component responsible for its children) to another container component. There might be some other small use cases but just be careful. Also, if it’s state, consider using Vuex instead.