di-test-example.js

// service file

export const createStockLevelChecker = (serviceLocator) => {
  return async (offerId) => {
    const offer = await serviceLocator.offerService.read(offerId);

    if (offer) {
      return "there is stock";
    }

    return "no stock";
  };
};

const createService = (serviceLocator) => {
  const service = new CrudService(serviceLocator)

  service.on("update", createStockLevelChecker(serviceLocator))

  return service
};

export default createService

// stock level checker test file

const { createStockLevelChecker } = require('../service')

describe("stockLevelChecker", () => {
    it('should return the right thing when there is stock', () => {
        let called = false
        const serviceLocator = {
            offerService: {
                read: (id) => {
                    return { id, title: 'blah' }
                }
            },
            
            mailer: () => {
                called = true
            }
        }
        const stockLevelChecker = createStockLevelChecker(serviceLocator)
        assert.equal(stockLevelChecker('1234'), 'there is stock')
        assert.equal(called, true)
    })

    it('should return the right thing when an offer is not found', () => {
        const serviceLocator = {
            offerService: {
                read: () => {
                    return null
                }
            }
        }
        const stockLevelChecker = createStockLevelChecker(serviceLocator)
        assert.equal(stockLevelChecker('1234'), 'there is no stock')
    })
})