jest testing hapijs -> how to mock a decorated method call or override it during testing

[vivekVells]

Support plan

• is this issue currently blocking your project? (yes/no): yes.
• is this issue affecting a production system? (yes/no): no.

Context

• node version: node v14.19.1 (npm v6.14.17)
• module version: "@hapi/hapi": "20.1.5",
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): reactJS
• any other relevant information:

How can we help?

Objective: trying to figure out a way to mock return value of a decorated (server.decorate) function.

Example use-case:

simple plugin; uses server.decorate to define a function isUserSessionValid to return true or false value.

NEED HELP WITH: how to make this decorated isUserSessionValid (which would be like request.isUserSessionValid) in the jest test file to return true or false based on test case needs!

Sample code:

// user-session.ts

const isUserSessionValid = () => {
 // logic to return true if user session valid; else false
}

export const plugin = {
  name: 'session',
  register(server: Server) {
    // TODO: figure out a way to jest test mock this decorated method: isUserSessionValid return value to be true or false based on test cases
    server.decorate('request', 'isUserSessionValid', isUserSessionValid);

    // REST API endpoint to validate whether the received request contains valid user session or not; returns true if valid; else false. 
    server.route({
      method: 'GET',
      path: '/api/user-session-validate',
      async handler(request: Request, h: ResponseToolkit): Promise<boolean> {
        return await request.isUserSessionValid();
      },
    });
  },
};

in test file, trying to do

// user-session.spec.ts
...

import { plugin } from '../user-session';

describe('User session', () => {
  let server: Hapi.Server;

  beforeEach(async () => {
    server = Hapi.server();
    await server.initialize();

    server.register({
      plugin,
    });
  });

  afterEach(async () => {
    jest.clearAllMocks();
    await server.stop();
  });

  describe('GET /api/user-session-validate', () => {
    it('when session is valid, returns true', async () => {
      
      // doing this would result in expected error: "Request decoration already defined: isUserSessionValid"
      // HOW TO MOCK THIS TO RETURN "true" or "false" based on my needs?
      server.decorate('request', 'isUserSessionValid', () => Promise.resolve(true));

      await server
        .inject({
          method: 'GET',
          url: '/api/user-session-validate',
        })
        .then((res) => {
           // do some logic to validate.... 
        });
    });
  });
....

Can someone please help me on this? Been trying to tackle this for long time with no solid success. am also new to this framework & trying to learn..