title: Add element to typescript Record<>
body: <p>I have a Record set up like this:
  <code>let myRecord = Record&lt;String, Set&lt;String&gt;&gt;</code></p>
  <p>How can I add an element to a set inside the record? I have tried the following without success:</p>
  <pre class="lang-js prettyprint-override"><code>let key = &quot;key&quot;;
  let stringToAdd = &quot;stringToAdd&quot;;
  myRecord[key].add(stringToAdd);
  </code></pre>