HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>activeElement MDN demo</title>
</head>
<body>
  <p>Select text from one of the text areas:</p>
  <form>
    <textarea id="ta-example-one" rows="4" cols="40">This is Text Area One. Lorem ipsum dolor sit amet.</textarea><br>
    <textarea id="ta-example-two" rows="4" cols="40">This is Text Area Two. Fusce ullamcorper nisl ac porttitor.</textarea>
  </form>
  <p>Active element ID: <em id="output-element"></em></p>
  <p>Selected text: <em id="output-text"></em></p>
  <script>
    function onMouseUp() {
      var activeTextarea = document.activeElement;
      var selection = activeTextarea.value.substring(
        activeTextarea.selectionStart,
        activeTextarea.selectionEnd
      );
      document.getElementById("output-element").textContent = activeTextarea.id;
      document.getElementById("output-text").textContent = selection;
    }
    document.getElementById("ta-example-one").addEventListener("mouseup", onMouseUp);
    document.getElementById("ta-example-two").addEventListener("mouseup", onMouseUp);
  </script>
</body>
</html>

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Document.activeElement — MDN textarea demo (Try It)