public class Handler
    {
        static AppFxClient BBClient;

        public Handler()

...

        // virtual so it can be overriden by test hander
        // ideally eventually will pull from a service class that can be overridden to return mock services with env vars
        public virtual IDataList<EmailInput, LookupOutput> getDataList(AppFxClient client, Guid callId) 
        {
            return new DataList<EmailInput, LookupOutput>(client, callId);            
        }

    public interface IDataList<InputsT, OutputsT>
        where OutputsT : new()
        where InputsT : class
    {
        Task<IEnumerable<OutputsT>> LoadAsync(InputsT inputs, DataListLoadOptions options);
    }

}

    public class HandlerTest : Handler, ILambdaTest
    {
        // xunit tests
        [Fact]
        public async void SingleUserSuccessTest()
...

        // overridden from Handler
        // ideally eventually will pull from a service class that can be overridden to return mock services with env vars
        public override IDataList<EmailInput, LookupOutput> getDataList(AppFxClient client, Guid callId) 
        {
           var mockDataList = new MockDataList<EmailInput, LookupOutput>(client, callId);
           mockDataList.HandlerTest = this;
           return mockDataList;
        }

const docClientMock = {
  put: function(params, callback) {
    if (params.Item.UserId === "test-user-already-added") callback(dupeUserErrorResponse);
    else callback(null, successResponse);
  }
};

describe('Tests index', function() {
  it('verifies successful response', function(done) {
    saveNewUser(newUser, docClientMock, (err, result) => {
      test.value(err).is(null);
      test.object({success:true}).is(result);
      done();
    });
  });
});
;