HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>back to top</title>
  <style>
    #top {
      position: fixed; right: 16px; bottom: 16px;
      padding: 10px 14px; border: 1px solid #333; background: #fff;
    }
    .tall { height: 2000px; padding: 16px; }
  </style>
</head>
<body>
  <button type="button" id="top" hidden>Back to top</button>
  <div class="tall">Scroll down until the button appears…</div>
  <script>
    var btn = document.getElementById("top");
    document.addEventListener(
      "scroll",
      function () {
        btn.hidden = window.scrollY < 200;
      },
      { passive: true }
    );
    btn.addEventListener("click", function () {
      window.scrollTo({ top: 0, behavior: "smooth" });
    });
  </script>
</body>
</html>

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