Jest.js force window to be undefined

I'm working with jest + enzyme setup for tests. I have function that conditionally renders something if window is defined.

In my test suite I'm trying to reach second case, when window is now defined, but I can't force it.

    it('makes something when window is not defined', () => {
       window = undefined;
       expect(myFunction()).toEqual(thisWhatIWantOnUndefinedWinow);
    });

But even if I force window to be undefined, it doesn't reach expected case, window is always window (jsdom?)

Is it something with my jest setup or I should handle this another way?