Vanilla JS - Trying to Shim Fake Data in place of JSON Response?

Question

I am trying to push fake data to test some code I have written that interacts with a third party, the reason it needs to be injected in this way is because the code doesn't exist in my test environment, so while I am receiving the correct data from the third party server, my test environment doesn't know what to do with it. So far I have created this;

const {fetch: origFetch} = window;

window.fetch = async (...args) => {
    console.log("fetch called with args:", args);
    const response = await origFetch(...args);

    response
        .clone()
        .json()
        .then(body => console.log("intercepted response:", body))
        .catch(err => console.error(err));

     /* the original response can be resolved unmodified: */
     //return response;

    /* or mock the response: */
    return {
        ok: true,
        status: 200,
        json: async () => ({
            'element': [
                'baseURI', 'https://#',
                'className', 'third-party-code',
                'dataSet', [
                    'displayMode', 'CAROUSEL',
                    'hideInMobile', 'true',
                    'navigationMethod', 'thirdParty',
                ],
            ],
            'productIds': [
                '1234567', 'https://#',
                'ABCDEFG', 'https://#',
                '8901112', 'https://#',
                'HIJKLMN', 'https://#',
            ],
        })
    };
};

Now while this kind of does what I want, it's not working as expected.

Firstly, it's intercepting the first AJAX JSON response it comes across, not the one I want, secondly, I'm still not sure it's going to work.

I really need someone to show me how I can create a SHIM that will a) use the JSON response if it is valid, and b) use the fake data when not on a production level environment, or the JSON response isn't valid.

Mainly I need help creating the SHIM so I can at least test this properly, but I'd also like to target a specific JSON response.

As always any help would be greatly appreciated.