Adding Jest to vanilla JS (depends on JQuery and other vanilla js)

I’m trying to set up jest testing for a huge legacy vanilla js files that is consantly calling global variables as well as JQuery. All those dependencies are not imported, they are added to the HTML via <script> tags. Obviously, on a basic Jest set up, when I run the code that tests a specific file, all I see are error messages of "xxx is undefined".

I have tried the following but have come up short:

fileToTest.js

SomeGlobalVariable.hello();

AMillIoinOtherCallsToGlobalStuff.hey().whatsup();

$(document).ready(() => { 
    AnotherMillionCallsToGlobalStuff();
});

function theOneIWantToTest(){
    const a = Big("0"); // When running the test file, jest can't see Big.js library
    return false;
}

module.exports = {
    theOneIWantToTest
};

fileToTest.test.js

    const viewOrder = require('../js/fileToTest');
    
    describe("fileToTest.js", () => {
    
        describe("theOneIWantToTest", () => {
            it("return true", () => {
               
         
         
  expect(theOneIWantToTest()).toBeFalsy();
            })
        });
    });

What have I tried to achieve this? Well:

jestSetUpFiles.js

Defined all global variables here and mocked functions.

jest.config.js

Trie to add here all the scripts that are appended via

const config = {
    setupFiles: [
        "./node_modules/big.js/big.js",
        "./frontend/js/jquery-1.9.1",
        "./frontend/js/jquery-ui.js",
        "./frontend/js/jquery.dialogify.js",
        "./frontend/js/tappy.js",
        "./jestSetUpFiles"
    ],
    testEnvironment: "jsdom"
};

module.exports = config;

I tried to add Big.js which is a library that theOneIWantToTest uses directly in the code, but it didn’t work. When I try to run the test, it complains it can’t find Big. So I tried loading it direclty on jestSetUpFiles, such that:

/*
* Loading non mocked libraries because jestSetUpFiles did not work
* */
Big = require("./node_modules/big.js/big.js");

But obviously that doesn’t work, because then the code would have to access it such that Big.Big().

Not sure what else to try. Any ideas?