Understanding a variable that is outside an export within an imported file

问题

Auth0 has some great documentation for its new javascript sdk to be used with Vue. See the Vue Quickstart page.

I'm confused by something they do with a variable that is outside the "export" of an imported module. Where does the instance variable live? Is it a global variable? Is it only available to the imported file itself?

If you import this module multiple times, I'd think it would run the let instance each time.

I must be misunderstanding how import works.

src/auth/index.js

import Vue from "vue";
import createAuth0Client from "@auth0/auth0-spa-js";

/** Define a default action to perform after authentication */
const DEFAULT_REDIRECT_CALLBACK = () =>
  window.history.replaceState({}, document.title, window.location.pathname);

let instance;

/** Returns the current instance of the SDK */
export const getInstance = () => instance;

/** Creates an instance of the Auth0 SDK. If one has already been created, it returns that instance */
export const useAuth0 = ({
  onRedirectCallback = DEFAULT_REDIRECT_CALLBACK,
  redirectUri = window.location.origin,
  ...options
}) => {
  if (instance) return instance;

  // The 'instance' is simply a Vue object
  instance = new Vue({
    data() {
        ...
    },
    methods: {
        ...
    } 
  });

  return instance;
};

// Create a simple Vue plugin to expose the wrapper object throughout the application
export const Auth0Plugin = {
  install(Vue, options) {
    Vue.prototype.$auth = useAuth0(options);
  }
};

来源：https://stackoverflow.com/questions/63125529/understanding-a-variable-that-is-outside-an-export-within-an-imported-file