HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>moveTo() — center</title>
</head>
<body>
  <button type="button" id="openBtn">Open centered popup</button>
  <pre id="out"></pre>
  <script>
    function centerWindow(win) {
      var left = (window.screen.width - win.outerWidth) / 2;
      var top = (window.screen.height - win.outerHeight) / 2;
      win.moveTo(left, top);
    }
    document.getElementById("openBtn").onclick = function () {
      var popup = window.open("", "Centered", "width=400,height=300");
      if (popup) {
        centerWindow(popup);
        document.getElementById("out").textContent = "Popup centered.";
      } else {
        document.getElementById("out").textContent = "Popup blocked.";
      }
    };
  </script>
</body>
</html>

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JavaScript moveTo() — center popup (Try It)