Add global variable in Vue.js 3

How to add a global variable using Vue 3 and vue-cli (or Vite)

Note: You can drop the dollar sign from your $globalVariable and just use globalVariable, just like in the documentation.

Initially your main.js file looks something like this (adding router for common use case):

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

createApp(App).use(router).mount('#app')

To use add the global variable using Vue 3 and the vue-cli or Vite:

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

// 1. Assign app to a variable
let app = createApp(App)

// 2. Assign the global variable before mounting
app.config.globalProperties.globalVar = 'globalVar'

// 3. Use router and mount app
app.use(router).mount('#app')

Then to access the variables in components like this:

<script>
export default {
    data() {
        return {
            myVar: this.globalVar
        }
    }
}
</script>

like in the template like this:

<template>
  <h1>{{ globalVar }}</h1>
</template>

And that's it. Happy coding!

About Global Variables and Composition API

According to the very bottom of samayo's answer on this post, global variables are only available on the Options API.

Quoting the bottom of his answer:

Note: This is only for the Options API. Evan You (Vue creator) says: "config.globalProperties are meant as an escape hatch for replicating the behavior of Vue.prototype. In setup functions, simply import what you need or explicitly use provide/inject to expose properties to app.

I recommend to use provide/inject approach as follows :

in main.js :

import {createApp} from 'vue'

let app=createApp({
  provide:{
    globalVariable:123
  }

}).$mount('#app')

in some child or grand-child component do :

export default{
 name:'some-compo',
 inject:['globalVariable'],
 //then access this.globalVariable as property in you component
...
}

for composition api and script setup :

 import { inject } from 'vue'
 
 let globalVar=inject('globalVariable')

The most direct replacement is app.config.globalProperties. See:

https://vuejs.org/api/application.html#app-config-globalproperties

So:

Vue.prototype.$myGlobalVariable = globalVariable

becomes:

const app = createApp(RootComponent)
app.config.globalProperties.$myGlobalVariable = globalVariable

This is scoped to a particular application rather than being global as it was with Vue.prototype. This is by design, all 'global' configuration options are now scoped to an application.

The relevant RFC is here:

https://github.com/vuejs/rfcs/blob/master/active-rfcs/0009-global-api-change.md

Properties added to globalProperties will be available via the component instance for all components within the application. So if you're using the Options API you'll be able to access them using this.$myGlobalVariable, just like you could with Vue.prototype. They'll also be available in the template without the this., e.g. {{ $myGlobalVariable }}.

If you're using the Composition API then you'll still be able to use these properties within the template, but you won't have access to the component instance within setup, so these properties won't be accessible there.

While hacks involving getCurrentInstance() can be used to access globalProperties within setup, those hacks involve using undocumented APIs and are not the recommended approach.

Instead, application-level provide/inject (also discussed in that RFC) can be used as an alternative to Vue.prototype:

const app = createApp(RootComponent)
app.provide('myGlobalVariable', globalVariable)

In the descendant component this can then be accessed using inject. e.g. With <script setup>:

<script setup>
import { inject } from 'vue'
const myGlobalVariable = inject('myGlobalVariable')
</script>

Or with an explicit setup function:

import { inject } from 'vue'

export default {
  setup() {
    const myGlobalVariable = inject('myGlobalVariable')

    // Expose it to the template, if required
    return {
      myGlobalVariable
    }
  }
}

Or with the Options API:

export default {
  inject: ['myGlobalVariable']
}

Docs: https://vuejs.org/api/application.html#app-provide

The idea here is that the component can explicitly declare the property rather than inheriting it by magic. That avoids problems like name collisions, so there's no need to use a $ prefix. It can also help to make it clearer where exactly a property is coming from.

It is common for the inject function to be wrapped in a composable. For example, the useRoute composable exposed by Vue Router is just a wrapper around inject.

In addition to globalProperties and provide/inject, there are various other techniques that might be used to solve the same problems as Vue.prototype. For example, ES modules, stores, or even global mixins. These aren't necessarily direct answers to the specific question posted here, but I've gone into more detail describing the various approaches at:

https://skirtles-code.github.io/vue-examples/patterns/global-properties.html

Which approach you prefer will depend on your circumstances.

If possible you should use imports or provide/inject. Another way to define global variables/functions and use them would be using globalProperties (although this seems to be considered more of an anti-pattern). But if a library you use uses globalProperties then you can use it like this. This also works with global functions.

const app = Vue.createApp({})
app.config.globalProperties.$http = () => {} // global function
app.config.globalProperties.$globalVariable = 'Jimmy' // global variable

1. Using options API

mounted() {
  console.log(this.$globalVariable)
}

2. Using setup method

<script setup>
    import { getCurrentInstance } from 'vue'

    const app = getCurrentInstance()
    const progressBar = app.appContext.config.globalProperties.$globalVariable

    console.log(this.$globalVariable)
</script>