window.open居中显示，模式窗口

var url="";//地址

var height = 500;
 var width = 400;
 var left = (screen.availWidth - width) / 2;
 var top = (screen.availHeight - height) / 2;
 if (top < 0) {
  top = 0;
 }
 if (left < 0) {
  left = 0;
 }
 var openWin = window.open(url, '', 'toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=no,copyhistory=yes,width=' + width
   + ',height=' + height + ',left=' + left + ',top=' + top);
 window.onfocus = function() {//模仿模式窗口
  openWin.focus();
 }