Q1399 How can I open a popup window in the middle of the screen?

<script language="JavaScript"><!--
function openWindow(width,height) {
    x = (640 - width)/2, y = (480 - height)/2;

    if (screen) {
        y = (screen.availHeight - height)/2;
        x = (screen.availWidth - width)/2;
    }

    window.open('nextpage.htm','newWin','width='+width+',height='+height+',screenX='+x+',screenY='+y+',top='+y+',left='+x);
}
//--></script>

<form>
<input type="button" onClick="openWindow(100,200)" value="Click Me">
</form>