为什么新窗口中的网址自动添加了当前页面地址

<script type="text/javascript">  
    function openWindow(){
        var d = confirm("打开新窗口吗？");
            if(d==true){
                var url = prompt("输入网址：","http：//www.imooc.com/");
                    if(url!=null){
                        window.open(url,"_blank","width=400,height=500,menubar=on,toolbar=on");
            }
        }
    } 
  </script>