[vue-i18n] already installed.

[emileber]

vue-i18n is installing itself on the window even when imported as a module

vue & vue-i18n version

I'm using 7.4.1 but it's not necessarily new to this version.

Steps to reproduce

Load Vue via a script tag, then use another version of Vue in a bundled file (which exposes no globals).

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/1.0.28/vue.min.js"></script>
<script src="bundle-using-vue-2.js"></script>

Within the bundle, use vue-i18n.

import Vue from 'vue';
import VueI18n from 'vue-i18n';

Vue.use(VueI18n);

What is Expected?

vue-i18n to be installed only with my bundle's vue 2.

What is actually happening?

Two warnings and the installation fails with my bundle's Vue.

[vue-i18n] vue-i18n (undefined) need to use Vue 2.0 or later (Vue: 1.0.28).
[vue-i18n] already installed.

---

After examining the vue-i18n.esm.js source, it looks like it's installing itself by default on the global Vue instance.

if (typeof window !== 'undefined' && window.Vue) {
  window.Vue.use(VueI18n);
}

To avoid this, Vuex installs itself with the global Vue instance only if it was not installed yet with another Vue instance. (source)

var Vue; // bind on install

var Store = function Store (options) {
  var this$1 = this;
  if ( options === void 0 ) options = {};

  // Auto install if it is not done yet and `window` has `Vue`.
  // To allow users to avoid auto-installation in some cases,
  // this code should be placed here. See #731
  if (!Vue && typeof window !== 'undefined' && window.Vue) {
    install(window.Vue);
  }

And the install function uses the Vue variable.

function install (_Vue) {
  if (Vue && _Vue === Vue) {
    if (process.env.NODE_ENV !== 'production') {
      console.error(
        '[vuex] already installed. Vue.use(Vuex) should be called only once.'
      );
    }
    return
  }
  Vue = _Vue;
  applyMixin(Vue);
}