HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollWidth compare</title>
  <style>
    #strip {
      width: 180px;
      height: 80px;
      overflow-x: auto;
      border: 2px solid #00b4c5;
      border-radius: 6px;
    }
    #content {
      width: 420px;
      padding: 12px;
      background: #e8f7f8;
    }
  </style>
</head>
<body>
  <div id="strip">
    <div id="content">Compare scrollWidth and clientWidth</div>
  </div>
  <pre id="out"></pre>
  <script>
    var box = document.getElementById("strip");
    var lines = [];
    lines.push("scrollWidth: " + box.scrollWidth);
    lines.push("clientWidth: " + box.clientWidth);
    document.getElementById("out").textContent = lines.join("\n");
  </script>
</body>
</html>

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Element.scrollWidth — compare with clientWidth (Try It)