HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>mousewheel wheelDelta</title>
  <style>
    #stage {
      height: 120px; border: 2px solid #333; overflow: auto;
      padding: 12px; user-select: none;
    }
  </style>
</head>
<body>
  <p><strong>Note:</strong> these values are inconsistent across browsers/OS — prefer <code>deltaY</code>.</p>
  <div id="stage">Scroll here<br>Line 2<br>Line 3<br>Line 4</div>
  <pre id="out">(waiting)</pre>
  <script>
    var stage = document.getElementById("stage");
    var out = document.getElementById("out");
    stage.addEventListener("mousewheel", function (event) {
      out.textContent =
        "wheelDelta=" + event.wheelDelta +
        " X=" + event.wheelDeltaX +
        " Y=" + event.wheelDeltaY;
    });
  </script>
</body>
</html>

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Element mousewheel — wheelDelta (Try It)