I need to test how my function would respond when only one of the methods under an object instantiated in the function script returns a certain value.

The function's script:

import TTLCache from '@isaacs/ttlcache';

const DATA_KEY = 'barks';

const dataCache = new TTLCache({ ttl: 100, checkAgeOnGet: true, max: 1 });

export const getCachedData = async (): Promise<string | undefined> => {    
    if (!dataCache.has(DATA_KEY)) {
        dataCache.set(DATA_KEY, 'new-item');
    }

    return dataCache.get(DATA_KEY);
};

My test:

import { getCachedData } from '../functionScript';

const mockGetRemainingTTL = jest.fn();

jest.mock('@isaacs/ttlcache', () => {
    return jest.fn().mockImplementation(() => ({
        ...jest.requireActual('@isaacs/ttlcache'),
        getRemainingTTL: () => mockGetRemainingTTL(),
    }));
})

describe('ttlCaching', () => {
    test('Return undefined when ttl has expired', async () => {
        mockGetRemainingTTL.mockReturnValue(0);
        expect(await getCachedData()).toEqual(undefined);
    });
});

getRemainingTTL is a method under the cache object that gets called internally.

For the above test, I get the error: "TypeError: dataCache.has is not a function"

I do not understand why the ...jest.requireActual('@isaacs/ttlcache') part did not fill in the original implementations.