code: function (iterator) {
      var self = this;
  
      // Invoking bindEnvironment'd callbacks outside of a Fiber in Node doesn't
      // run them to completion (and exceptions thrown from onException are not
      // propagated), so we need to be in a Fiber.
      Meteor._nodeCodeMustBeInFiber();
  
      var ids = _.keys(self.callbacks);
      for (var i = 0;  i < ids.length;  ++i) {
        var id = ids[i];
        // check to see if the callback was removed during iteration
        if (_.has(self.callbacks, id)) {
          var callback = self.callbacks[id];
  
          if (! iterator(callback))
            break;
        }
      }
    }
docs: For each registered callback, call the passed iterator function with the callback.  The iterator function can choose whether or not to call the callback.  (For example, it might not call the callback if the observed object has been closed or terminated).  The iteration is stopped if the iterator function returns a falsy value or throws an exception.