How can I set window to undefined in order to test the SSR rendering in an isomorphic application?

I have been trying to test an else block for when window is undefined. As I'm using Next.js the window will be undefined during server side rendering (SSR). At present I can not find a way to do this.

Any help is much appreciated.

I have tried setting window = undefined and global.window = undefined in the test and within the definition before function (both all and each options). Both approaches have not been successful.

When Googling the closest answer I can find is mocking or spying on a window method but this doesn't answer my problem. I have also found people noting you can run the test as a Node application (thus SSR) but not sure how this can be done for one test.

I have provided an example below of what I am trying to test and how.

Function to Test

export default () => {
   console.log(typeof window); // This is always an object and never undefined
   const language =
      typeof window !== 'undefined'
          ? window.navigator.userLanguage || window.navigator.language || 'en'
          : 'en';

   return language;
};

Jest Test

import getLanguage = './getLanguage';

describe('Get Language SSR', () => {
    const realWindow = window;

    beforeAll(() => {
        window = undefined;
    });

    afterAll(() => {
        window = realWindow;
    });

    it('should return "en"', () => {
         expect(getLanguage()).toEqual('en');
    })
});

The code coverage shows that the else block is never hit.