<input type='text'></input>
<div>First div</div>

<script>
  const divs = document.getElementsByTagName('div');
  // let divs = document.querySelectorAll('div');
  const result = document.getElementsByTagName('input')[0];
  result.value = divs.length
</script>

<div>Second div</div>

<script>
  result.value = divs.length
</script>