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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollBy() — log</title>
</head>
<body>
  <button type="button" id="logBtn">Scroll and log</button>
  <pre id="out"></pre>
  <div style="height:150vh;padding:1rem">Tall page for scrolling</div>
  <script>
    document.getElementById("logBtn").onclick = function () {
      var before = window.scrollY;
      window.scrollBy(0, 100);
      var after = window.scrollY;
      document.getElementById("out").textContent = "Before: " + before + "px\nAfter:  " + after + "px";
    };
  </script>
</body>
</html>

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JavaScript scrollBy() — log scrollY (Try It)