HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>resizeTo() — open</title>
</head>
<body>
  <button type="button" id="openBtn">Open and expand popup</button>
  <pre id="out"></pre>
  <script>
    document.getElementById("openBtn").onclick = function () {
      var popup = window.open("", "ExpandDemo", "width=400,height=300,left=150,top=100");
      if (popup) {
        popup.document.write("<p>Starting small…</p>");
        popup.resizeTo(700, 500);
        document.getElementById("out").textContent = "Popup opened at 400×300, then resizeTo(700, 500).";
      } else {
        document.getElementById("out").textContent = "Popup blocked.";
      }
    };
  </script>
</body>
</html>

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JavaScript resizeTo() — open then resize (Try It)