How to use Vue.prototype or global variable in Vue 3?

You could use provide/inject or define a global config property, which replaces Vue.prototype in Vue 3:

1. provide/inject (for Composition or Options API)

Provide

import axios from 'axios';

const app = Vue.createApp(App);
app.provide('$axios', axios);  // Providing to all components during app creation

Inject

Composition API:

const { inject } = Vue;
...
setup() {
  const $axios = inject('$axios');   // injecting in a component that wants it
  // $axios.get(...)
}

Options API:

inject: ['$axios'],   // injecting in a component that wants it

2. Global config (for Options API)

import axios from 'axios';

const app = Vue.createApp(App);
app.config.globalProperties.$axios = axios;

Options API:

this.$axios

^{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.“}