title: Is there a way to mock a DOM before testing a React component?
body: <p>I'm trying to test my React class (using TestUtils, Enzyme and Jest), but because of a few workarounds we've had to put up with, it relies on elements in a server-rendered HTML page -- namely, it grabs certain elements from the DOM in componentDidMount before rendering. For example: </p>
  
  <pre><code>componentDidMount() {
    document.documentElement.classList.add('example');
    this.someOtherFunction(document.getElementsByClassName('my-great-className'))
  }
  </code></pre>
  
  <p>I'd like to test this component, but of course the functions in componentDidMount fail because the server-rendered HTML page does not exist in the tests. Is there a way to mock a specific DOM before rendering the React component?</p>