HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>movementX — DIY</title>
</head>
<body>
  <p>Move the mouse.</p>
  <pre id="out">Waiting…</pre>
  <script>
    var prev = null;
    document.addEventListener("mousemove", function (e) {
      var diyDx = prev == null ? 0 : e.screenX - prev;
      prev = e.screenX;
      document.getElementById("out").textContent = JSON.stringify({
        movementX: e.movementX,
        diyDx: diyDx
      }, null, 2);
    });
  </script>
</body>
</html>

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MouseEvent.movementX — DIY screenX delta (Try It)