title: Jest.js force window to be undefined
body: <p>I'm working with jest + enzyme setup for tests. I have function that conditionally renders something if window is defined.</p>
  <p>In my test suite I'm trying to reach second case, when window is not defined, but I can't force it.</p>
  <pre>
  
      it('makes something when window is not defined', () => {
         window = undefined;
         expect(myFunction()).toEqual(thisWhatIWantOnUndefinedWinow);
      });
  
  </pre>
  <p>But even if I force window to be undefined, it doesn't reach expected case, window is always window (jsdom?)</p>
  <p>Is it something with my jest setup or I should handle this another way?</p>