Javascript window.open让窗口居中的代码

    function OpenEnvDefineWin()
    {
        var width = 400;
        var height = 300;
        var left = eval(screen.width - width) / 2;
        var top = eval(screen.height - height) / 2;
        var open_feature = "width=" + width + ", height=" + height + ", left=" + left + ", top=" + top + ", scrollbar=no";
        var hwnd = window.open("popup.html", "_blank", open_feature);
        if ((window != null) && (!hwnd.opener))
            hwnd.opener = window;
        hwnd.focus();
        return false;
    }