HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>execCommand undo</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} textarea{width:100%;min-height:4rem;} button{margin-right:0.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <textarea id="note">Draft</textarea>
  <p>
    <button id="insert" type="button">Insert TEMP</button>
    <button id="undo" type="button">Undo</button>
  </p>
  <pre id="out">Try Insert, then Undo</pre>
  <script>
    var note = document.getElementById("note");
    var out = document.getElementById("out");
    document.getElementById("insert").addEventListener("click", function () {
      note.focus();
      document.execCommand("insertText", false, " TEMP");
      out.textContent = note.value;
    });
    document.getElementById("undo").addEventListener("click", function () {
      note.focus();
      document.execCommand("undo", false, null);
      out.textContent = note.value;
    });
  </script>
</body>
</html>

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Document.execCommand — undo after insertText (Try It)