const errorElement = document.querySelector(selector) as HTMLElement; means code example

Example 1: js queryselector names

// This selects the first element with that name
document.querySelector('[name="your-selector-name-here"]');

Example 2: document.queryselector

<div id="foo\bar"></div>
<div id="foo:bar"></div>

<script>
  console.log('#foo\bar');               // "#fooar" (\b is the backspace control character)
  document.querySelector('#foo\bar');    // Does not match anything

  console.log('#foo\\bar');              // "#foo\bar"
  console.log('#foo\\\\bar');            // "#foo\\bar"
  document.querySelector('#foo\\\\bar'); // Match the first div

  document.querySelector('#foo:bar');    // Does not match anything
  document.querySelector('#foo\\:bar');  // Match the second div
</script>