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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>movementX — accumulate</title>
</head>
<body>
  <p id="out">Horizontal travel ≈ 0</p>
  <script>
    var traveled = 0;
    var out = document.getElementById("out");
    document.addEventListener("mousemove", function (e) {
      traveled += Math.abs(e.movementX);
      out.textContent = "Horizontal travel ≈ " + Math.round(traveled);
    });
  </script>
</body>
</html>

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MouseEvent.movementX — accumulate travel (Try It)