How to import prototype getter from custom library using Object.defineProperty()

Question

I want to create a reusable library for my projects. For this exercise, I've got one main library, and then an extension library for "identity" related functionality.

The idea is that I can include the identity module if / when needed, and extend the MainLibrary with a getter to access anything related to identity. Here's my code:

// Main library - index.js (Package name: @mainlib)
export class MainLibrary
{
   // ....
}

---

// Identity library - index.js (Package name: @mainlib/identity)
import { MainLibrary } from '@mainlib';

export function useIdentity()
{
  Object.defineProperty(MainLibrary.prototype, "identity", {
    get: function()
    {
       // Returns an identity object
       return {
          isAuth: function()
          {
             return false;
          }
       }
    },
    enumerable: true
 });
}

---

// App
import { MainLibrary } from '@mainlib';
import { useIdentity } from '@mainlib/identity';

useIdentity();

const myLib = new MainLibrary();
myLib.identity.isAuth(); // Throws error, identity is undefined

If I move the Object.defineProperty code into my App, it works just fine. How can I define this in the extension library and apply it to the MainLibrary class prototype, upon calling useIdentity() in my App?

Oh, I'm building with ViteJS, if that's relevant.