HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getElementsByClassName live</title>
  <style>
    .color-box { background: #7c3aed; color: #fff; padding: 8px; margin: 6px; }
    .hue-frame { background: #0d9488; }
  </style>
</head>
<body>
  <div id="wrap">
    <div class="color-box">Box 1</div>
    <div class="color-box">Box 2</div>
    <div class="color-box">Box 3</div>
  </div>
  <button type="button" id="run">Transform boxes</button>
  <pre id="out"></pre>
  <script>
    document.getElementById("run").addEventListener("click", function () {
      var wrap = document.getElementById("wrap");
      var matches = wrap.getElementsByClassName("color-box");
      while (matches.length > 0) {
        matches.item(0).classList.add("hue-frame");
        matches[0].classList.remove("color-box");
      }
      document.getElementById("out").textContent = "Done: " + wrap.getElementsByClassName("hue-frame").length + " hue-frame";
    });
  </script>
</body>
</html>

Lines: 0 | Characters: 0

Live Preview Auto-refresh on Run

Element.getElementsByClassName — live loop (Try It)