So Im unit testing some functions which contains Headers and I've managed to mock the global.Headers by running following code before executing the test.

beforeAll(() => {
  const append = jest.fn();
  // set test `Headers`
  global.Headers = () => ({
    append: append,
  });
});

I'm however trying to mock this within the setup.js, so I don't require to fire make similar beforeAll(()=>{} code for all my test files. I'm not to where I need to point the mock properly, below is what've done.

setup.js

jest.mock(global.Headers, () => {
  global.Headers = () => ({
    append: jest.fn(),
  });
});

Update (Found the solution)

Simply do the following in setup.js will work

global.Headers = () => ({
  append: jest.fn(),
});