Override behavior of a stubbed function using JEST not working as expected - unit-testing

Question

I have a test class that tests behavior of various HTTP methods in a Nest controller class. I am using Jest manual mocks to stub the behavior of various functions in the service class so that I do not have to rely on actual dependencies/services, eg. snowflake. I have a top level jest.mock() defined as follows which initializes the mocked version of the service class instead of the actual service class.The mocked service class is created inside mocks folder adjacent to the actual service class.
I am redefining the behavior of one of the mocked functions in the 'error scenario' describe block as shown in the code snippet below, for testing the error scenario . The test scenario : 'throws an error' is failing as it is still picking up the default mocked behavior. Any pointers or help is appreciated.
In short, I want to be able to define different mocked behavior for a single function of the same mocked class for various test scenarios.
Thanks
jest.mock('#modules/shipment-summary/shipment-summary.service');
describe('ShipmentSummaryController', () => {
let shipmentSummaryController: ShipmentSummaryController;
let shipmentSummaryService: ShipmentSummaryService;
beforeEach(async () => {
const moduleRef = await Test.createTestingModule({
imports: [],
controllers: [ShipmentSummaryController],
providers: [ShipmentSummaryService],
}).compile();
shipmentSummaryController = moduleRef.get<ShipmentSummaryController>(
ShipmentSummaryController,
);
shipmentSummaryService = moduleRef.get<ShipmentSummaryService>(
ShipmentSummaryService,
);
jest.clearAllMocks();
});
//All the tests inside this describe block work as expected
describe('valid shipment-mode scenario', () => {
describe('valid shipment modes for tenant', () => {
let modes: ShipmentMode[];
beforeEach(async () => {
modes = await shipmentSummaryController.getAllShipmentModes('256');
});
test('calls the service fn. with the correct arg', () => {
expect(shipmentSummaryService.getAvailableShipmentModes).toBeCalledWith(
'256',
);
});
test('all available shipment modes for 256 are returned', () => {
expect(modes).toEqual(validModeDropdown());
});
});
});
// redefining behavior of getAllshipmentModes() is not working
describe('error scenario', () => {
let modes: ShipmentMode[] = []
beforeEach(async () => {
modes = await shipmentSummaryController.getAllShipmentModes('256');
});
beforeAll(() => {
jest.clearAllMocks();
jest.mock('#modules/shipment-summary/shipment-summary.service.ts', () => {
return {
getAvailableShipmentModes: () => {
throw new Error('Test error');
},
}
});
});
test('throws an error', () => {
expect(() => shipmentSummaryController.getAllShipmentModes('256')).toThrow();
})
})
});
My mocked service class is as follows:
export const ShipmentSummaryService = jest.fn().mockReturnValue({
// Fn. to be mocked differently per test scenario.
getAvailableShipmentModes: jest.fn().mockResolvedValue(validModeDropdown()),
});

Answer 1

There are many ways of accomplishing this. The Nest docs outline a number of them. However, one of my preferred ways, useValue, is not as clear as it could be, so I'll added it here.
This example will also use jest in order to spy on a mock, changing its behavior depending on the test.
Imagine these two simple resources
Injectable();
export class SimpleService {
public sayHello(): string {
return "Hello, world!";
}
}
#Controller()
export class SimpleController {
constructor(
#Inject(SimpleService) private readonly simpleService: SimpleService
) {}
#Get()
public controllerSaysHello(): string {
return this.simpleService.sayHello();
}
}
Your tests could look something like this
describe("SimpleController", () => {
let controller: SimpleController;
const mockReturnValue = "Goodbye, world..",
mockSimpleService: SimpleService = {
sayHello: () => mockReturnValue,
};
beforeEach(() => {
jest.restoreAllMocks();
});
beforeEach(async () => {
const module = await Test.createTestingModule({
providers: [
SimpleController,
{ provide: SimpleService, useValue: mockSimpleService },
],
}).compile();
controller = module.get(SimpleController);
});
test("default mockSimpleService", () => {
const result = controller.controllerSaysHello();
expect(result).toBe(mockReturnValue);
});
test("spied on mockSimpleService", () => {
const differentReturnValue = "Hallo!";
jest
.spyOn(mockSimpleService, "sayHello")
.mockReturnValue(differentReturnValue);
const result = controller.controllerSaysHello();
expect(result).toBe(differentReturnValue);
});
});