title: React 'Set is undefined', on Sharepoint
body: <p>I'm getting this error...</p>
  
  <pre><code>SCRIPT5009: 'Set' is undefined
  File: require-951f856e.js, Line: 1, Column: 1662
  </code></pre>
  
  <p>...on a Sharepoint site I developed for work. I have absolutely no idea what this means, other than somewhere in a React file there is a set that is not getting defined.</p>
  
  <p>This results in my site partly loading in Internet Explorer, but the contents of an iframe do not load. The site is working perfectly in firefox and chrome.</p>
  
  <p>I can't find any help elsewhere. Can someone help?</p>