mui.openWindow无法传递变量

        function jumpToChat(userId) {  
            console.log("I'm here");  
            var para = userId;  
/*            var webview_style = {  
                popGesture: "close"  
            };*/  
            mui.openWindow({  
                id: "id",  
                url: "im-chat.html",  
                //styles: webview_style,  
                show: {  
                    aniShow: "pop-in"  
                },  
                waiting: {  
                    autoShow: false  
                },  
                extras: {  
                    usrId: userId  
                }  
            });  
        }

如上我通过调用jumpToChat('ddddd')传递参数到im-chat.html，在im-chat.html中使用

                $.plusReady(function() {  

                    plus.webview.currentWebview().setStyle({  
                        softinputMode: "adjustResize"  
                    });  

                    var self = plus.webview.currentWebview();  
                                       alert(self.usrId);  

无法获取usrId的值