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'

const 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'
 
 const globalVar=inject('globalVariable')

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>

Vue 3:

import { getCurrentInstance } from "vue";

const { proxy } = getCurrentInstance();

"proxy" will be contain the value from globalProperties. For example:

app.config.globalProperties.$toast = {...}

and then:

proxy.$toast.success()

For those of you who are confused about how to access globalProperties in the setup() method, you can use getCurrentInstance() as in the following documentation.

https://v3.vuejs.org/api/composition-api.html#getcurrentinstance

In my case I had to create a global var and get the data from a script.
Used provide and inject:

In main.js:

    import { createApp } from 'vue'
    import App from './App.vue'
    const app = createApp(App);
    app.provide('message',document.querySelector('script[name="nameSCRIPT"]').innerHTML.split('=').slice(1).join('=').slice(1,-1));
    app.mount('#app')

In index.html:

 <script name="nameSCRIPT">nameSCRIPT="HELLO"</script>

In child component:

    inject:['message'],
    mounted(){
    console.log(this.message)
  },