Ok finally got it to not error out:

function u_INC_FilterAssignmentOnDisableTrigger(){

        var current_source = current.cmdb_ci;

        var answer = '';

        if (JSUtil.notNil(current.assignment_group))

  {

                  answer = current.assignment_group;

        }

  if(current.cmdb_ci.u_disable_auto_populate_assign == true)

  {

  answer='parentIN' + 'current.assignment_group' + ',' + 'current.cmdb_ci.support_group';

  }

  else {

  //if(current.cmdb_ci.u_disable_auto_populate_assign   != true)

                answer= 'assignment.cmdb_ci';

  }

        return answer;

}

Next STEPS:

Ok so I created an Advance Reference Qualifier javascript:u_INC_FilterAssignmentOnDisableTrigger

BUT The script is not doing what I need it to do….

If the u_disable_auto_populate_assign IS check then I DONOT want the Assignment group to auto populate the group, but rather populate the look up table with a tree view of the assignment group that is listed in the CI (this is a parent CI) and also show the children CI for that Parent. Instead it is still showing me the given assignment group for the CI and the pop up list is showing all available groups.

What am I missing