title: Why can't I "require" react-dom.js?
body: <p>I try to bring ReactDOM to my Jest tests. Like this.</p>
  
  <pre><code>const React = require('../src/js/vendor/react/build/react.js');
  const ReactDOM = require('../src/js/vendor/react/build/react-dom.js');
  </code></pre>
  
  <p>When I log ReactDOM in my test, I get undefined. Why?</p>
  
  <p>The path is correct. I can 'require' React.</p>