Center a new popup window even on dualscreen with javascript

function PopupCenter(url, title, w, h) {
    // Fixes dual-screen position                       Most browsers      Firefox
    var dualScreenLeft = window.screenLeft != undefined ? window.screenLeft : screen.left;
    var dualScreenTop = window.screenTop != undefined ? window.screenTop : screen.top;
    
    var left = ((screen.width / 2) - (w / 2)) + dualScreenLeft;
    var top = ((screen.height / 2) - (h / 2)) + dualScreenTop;
    var newWindow = window.open(url, title, 'scrollbars=yes, width=' + w + ', height=' + h + ', top=' + top + ', left=' + left);

    // Puts focus on the newWindow
    if (window.focus) {
        newWindow.focus();
    }
}

Usage example:

PopupCenter('http://google.com','Google','900','500');