HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>replaceChildren vs append</title>
  <style>
    #box { padding: 12px; border: 1px solid #ccc; margin: 8px 0; }
  </style>
</head>
<body>
  <div id="box">
    <span>a</span>
    <span>b</span>
  </div>
  <button type="button" id="appendBtn">append span</button>
  <button type="button" id="replaceBtn">replaceChildren span</button>
  <pre id="out">—</pre>
  <script>
    var box = document.getElementById("box");
    var out = document.getElementById("out");
    document.getElementById("appendBtn").addEventListener("click", function () {
      var extra = document.createElement("span");
      extra.textContent = "new";
      box.append(extra);
      out.textContent = "after append: " + box.childNodes.length;
    });
    document.getElementById("replaceBtn").addEventListener("click", function () {
      var only = document.createElement("span");
      only.textContent = "only";
      box.replaceChildren(only);
      out.textContent = "after replace: " + box.childNodes.length;
    });
  </script>
</body>
</html>

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replaceChildren vs append (Try It)