HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollBy() — delta</title>
</head>
<body>
  <h1>scrollBy(0, 200)</h1>
  <p>
    <button type="button" id="down">Scroll down 200px</button>
    <button type="button" id="reset">Back to top</button>
  </p>
  <pre id="out">scrollY: 0</pre>
  <p>Top of page</p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p>More content below — keep scrolling.</p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p>Bottom area</p>
  <script>
    (function () {
      var out = document.getElementById("out");
      function show() {
        out.textContent = "scrollY: " + Math.round(window.scrollY || 0);
      }
      document.getElementById("down").onclick = function () {
        window.scrollBy(0, 200);
        show();
      };
      document.getElementById("reset").onclick = function () {
        window.scrollTo(0, 0);
        show();
      };
      window.addEventListener("scroll", show);
    })();
  </script>
</body>
</html>

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JavaScript Window scrollBy() — relative delta (Try It)