HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getComputedStyle() — width</title>
</head>
<body>
  <div id="panel" style="width: 200px; padding: 10px; border: 1px solid #333;">Panel</div>
  <button type="button" id="growBtn">Grow width</button>
  <pre id="out"></pre>
  <script>
    document.getElementById("growBtn").onclick = function () {
      var panel = document.getElementById("panel");
      var currentWidth = parseFloat(window.getComputedStyle(panel).width);
      panel.style.width = (currentWidth + 20) + "px";
      document.getElementById("out").textContent = "New width: " + panel.style.width;
    };
  </script>
</body>
</html>

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JavaScript getComputedStyle() — measure width (Try It)