HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>hasAttribute filter</title>
</head>
<body>
  <ul id="tasks">
    <li data-done>Buy milk</li>
    <li>Walk dog</li>
    <li data-done>Read docs</li>
  </ul>
  <pre id="out"></pre>
  <script>
    var items = document.getElementById("tasks").querySelectorAll("li");
    var doneCount = 0;
    for (var i = 0; i < items.length; i++) {
      if (items[i].hasAttribute("data-done")) doneCount++;
    }
    document.getElementById("out").textContent = doneCount;
  </script>
</body>
</html>

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Element.hasAttribute — filter data-done (Try It)