title: Jest test fails with "window is not defined"
body: <p>I am trying to get started with state-of-the-art web development learning <a href="https://en.wikipedia.org/wiki/React_(JavaScript_library)" rel="nofollow noreferrer">React</a> and <a href="https://en.wikipedia.org/wiki/Redux_(JavaScript_library)" rel="nofollow noreferrer">Redux</a>.</p>
  <p>Right now I am stuck at getting tests running. For some reason, Jest fails with</p>
  <pre><code>Task :frontend:test
  yarn jest v1.0.2
  $ &quot;/Users/gunnar/git/app.oakstair.se/frontend/node_modules/.bin/jest&quot;
  FAIL src/containers/App/App.test.js
    ● Test suite failed to run
  
   ReferenceError: window is not defined
  
    at Object.&lt;anonymous&gt; (config/polyfills.js:18:1)
    at next (native)
    at process._tickCallback (internal/process/next_tick.js:109:7)
  </code></pre>
  <p>I have googled for a while without any success...</p>
  <p><a href="https://i.stack.imgur.com/8WKxV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8WKxV.png" alt="This is my starting test.js" /></a></p>
  <p><a href="https://i.stack.imgur.com/SbZK7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SbZK7.png" alt="And this my App test code" /></a></p>