Jasmine不承认Pusher.unsubscribe

Question

如果频道名称未知，如何查找pusher是否订阅频道？显然，Jasmine并不是这样知道的：

 subscribe: function(booking_request_id) {
      if(channel && pusher) pusher.unsubscribe(); // error :  not implemented thrown
      channel = pusher.subscribe('booking_requests_' + booking_request_id);
      channel.bind('async_notification', function (data) {
         $rootScope.$apply(function () {
         callback(data);
        });
      });
    }