Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<p>Open a new window, and resize the width and height to 500px:</p>

<button onclick="openWin()">Create window</button>
<button onclick="resizeWin()">Resize window</button>

<script>
var myWindow;

function openWin() {
    myWindow = window.open("", "", "width=100, height=100");
}

function resizeWin() {
    myWindow.resizeTo(250, 250);
    myWindow.focus();
}
</script>

</body>
</html>