HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>resizeTo() — popup</title>
</head>
<body>
  <button type="button" id="openBtn">Open popup</button>
  <button type="button" id="resizePopupBtn">Set popup to 500×400</button>
  <pre id="out"></pre>
  <script>
    var popupWindow = null;
    document.getElementById("openBtn").onclick = function () {
      popupWindow = window.open("", "SizeDemo", "width=320,height=200,left=200,top=120");
      popupWindow.document.write("<p>Resize me from the opener</p>");
    };
    document.getElementById("resizePopupBtn").onclick = function () {
      if (popupWindow && !popupWindow.closed) {
        popupWindow.resizeTo(500, 400);
        document.getElementById("out").textContent = "Popup set to 500×400.";
      } else {
        document.getElementById("out").textContent = "Open the popup first.";
      }
    };
  </script>
</body>
</html>

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JavaScript resizeTo() — resize popup (Try It)