Show a JavaScript pop-up in the centre of a screen (Dual monitor)

in
/**
 * Pop up an external window center of the page (works with dual monitor setup's)
 *
 * @constructor
 */
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 width = window.innerWidth ? window.innerWidth : document.documentElement.clientWidth ? document.documentElement.clientWidth : screen.width;
    var height = window.innerHeight ? window.innerHeight : document.documentElement.clientHeight ? document.documentElement.clientHeight : screen.height;

    var left = ((width / 2) - (w / 2)) + dualScreenLeft;
    var top = ((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) {
        if(newWindow) {
            newWindow.focus();
        } else {
            $('<div class="has-error"><p class="help-block">Pop Up blocked. Please disable your pop up blocker to provide your bank details.</p></div>').insertBefore('.refNo');
        }
    }
    return newWindow;
}