Unsurprisingly, the first thing that we saw after upgrading to Vue 3 was an error message: “TypeError: Vue is not a constructor”. In Vue 2, everything was a Vue instance, and you could `$mount()` a Vue instance to the DOM.
In Vue 3, the concepts of apps, components, and Vue instances are more cleanly separated. Instead of starting your app with `new Vue()`, you need to use `Vue.createApp()`.