运行完后有undefined并且没有打开新窗口？？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>window对象</title>
<script type="text/javascript">
function openNew()
{
document.write(alert("欢迎来到慕课网！！"));
window.open('https://www.imooc.com/','_blank','width=600','height=400');
}
</script>
</head>
<body>
<form>
<input type="button" value="点击我，打开新窗口" onclick="openNew()" />
</form>
</body>
</html>
