Here is a sample test written with mocha + chai + sinon

aggregates.js

describe('Error case(s)', () => {
        it('should return type required error', (done) => {
        repos.favorites.contentIdsCount.resolves(contentIdsCountResponse); 
          expect(services.aggregates.view({skip: 0, pageSize: 15, type: 'randomType'}))
            .to.eventually.be.rejected.and.be.an.instanceOf(errorHelper.BadRequestError)
            .and.eventually.deep.equal(new errorHelper.BadRequestError([messages.validation.aggregateType.enum]))
            .notify(done);
        });
});