为什么confirm没有弹出
<!DOCTYPE html>
<html>
<head>
<title> new document </title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<script type="text/javascript">
function openWindow(){
var msg=confirm("是否打开新的网页");
if(msg==true)
{msg=prompt('打开',' http://www.imooc.com/');
window.open("http://www.imooc.com/","_blank","width=400,height=500px,
menubar=no,toolbar=no");
}
else{null}
}
</script>
</head>
<body>
<input type="button" value="新窗口打开网站" onclick="openWindow()" />
</body>
</html>