HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>onselectionchange</title>
  <style>
    #out { border: 1px solid #ccc; padding: 10px; margin-bottom: 12px; }
  </style>
</head>
<body>
  <pre id="out">Select text below…</pre>
  <p>JavaScript Document selectionchange listens for highlight and caret updates.</p>
  <p>Try selecting part of this sentence, then click elsewhere to clear.</p>
  <script>
    var out = document.getElementById("out");
    document.onselectionchange = function () {
      var sel = document.getSelection();
      out.textContent = sel && sel.toString()
        ? "Selection active (" + sel.toString().length + " chars)"
        : "No text selected (caret or empty)";
    };
  </script>
</body>
</html>

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Document selectionchange — onselectionchange (Try It)