title: 'React' refers to a UMD global, but the current file is a module
body: <p>I updated my project to create react app 4.0, and I'm slowing moving over my files to TypeScript. I know with this new version you don't have to repetitively import React from 'react'. However, within all of my TS files where I'm not importing React I receive this error:</p>
  <blockquote>
  <p>'React' refers to a UMD global, but the current file is a module.
  Consider adding an import instead.ts(2686)`</p>
  </blockquote>
  <p>I know I can fix it by importing React, but I thought this was no longer needed. Also, could someone explain the meaning of this error?</p>
  <p>My basic TSX file</p>
  <pre><code>const Users = () =&gt; {
      return &lt;&gt;Teachers aka Users&lt;/&gt;;
  };
  
  export default Users;
  </code></pre>