How to mock a typescript interface to test some use cases in a lib

[zedryas]

I did ask this question on StackOverflow but did not get any answers so i try to post it here as i did in typescript repo

I would like to have an interface populated only for test environment.

Let say i have this interface in the lib code base

// file MyInterface.ts

export interface MyInterface {}

And later in end user code base so he can populates MyInterface like so

// in the library user code base

declare module 'my-lib/MyInterface'
{
    interface MyInterface
    {
        user-property: unknown // some type here
    }
}

I need to test types in the library that uses Myinterface, and to do so i might need to test some keys and values associated to that key that are not relevant toward the lib but need some tests

I know I can define an interface _TestMyInterface which extends MyInterface and is not exported by the library but I will also have to redefine all the tests using MyInterface to use the _TestMyInterface interface instead and this will require me to do so duplicate the entire codebase. For example :

import {MyInterface} from './MyInterface'
import {_TestMyInterface} from './_TestMyInterface'

export type TypeThatUseMyInterface<K extends keyof MyInterface> = K // do something with K

type _TestTypeThatUseMyInterface<K extends keyof _TestMyInterface> = K // do something with K to allow testing for specific values that are not relevant to the library but might be relevant to the end user.

Is they're a better way to achieve this and mock MyInterface for testing purposes only?

I am using vitest for my testing environment so i do post it here as well