执行的时候说我没定义，哪里错了，为什么？求指导
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>浏览器对象</title>
</head>
<body>
<script type="text/javascript">
function demo(){
var i=window.open('http://www.imooc.com','_blank',width=600,height=600);
document.write(i);
}
demo();
</script>
</body>
</html>
