HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getSelection toString</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} p{line-height:1.6;} pre{border:1px solid #ccc;padding:0.75rem;white-space:pre-wrap;}</style>
</head>
<body>
  <p>Hello World — highlight any part of this sentence.</p>
  <button id="btn" type="button">Show selected text</button>
  <pre id="out">Highlight text first</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var selection = document.getSelection();
      var selectedText = selection ? selection.toString() : "";
      document.getElementById("out").textContent =
        selectedText ? selectedText : "(nothing selected)";
    });
  </script>
</body>
</html>

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Document.getSelection — toString selected text (Try It)