为什么2-7编程练习中prompt中的网址修改后，没有起作用
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>close()</title>
<script type="text/javascript">
function Wopen(){
var str = window.confirm("确定");
var mywin="http://www.imooc.com";
if(str){
var mywin;
window.prompt("输入你要打开的网址",mywin);
window.open(mywin);
}
}
</script>
</head>
<body>
<form>
<input name="button" type="button" value="新窗口打开" onclick="Wopen()"/>
</form>
</body>
</html>
