Indeterminate Checkbox Demo

HTML

<h2>Indeterminate checkbox voorbeeld</h2>

<label>
  <input type="checkbox" id="parent">
  Hoofdcheckbox
</label>

<br><br>

<label>
  <input type="checkbox" class="child" checked>
  Item A
</label><br>

<label>
  <input type="checkbox" class="child">
  Item B
</label><br>

JavaScript

// De checkbox op indeterminate zetten
const parent = document.getElementById('parent');
parent.indeterminate = true;