How do you unit test an exception filter in NestJS?

Q: How do you unit test an exception filter in NestJS?

describe('HttpExceptionFilter', () => { let filter: HttpExceptionFilter; beforeEach(async () => { const module = await Test.createTestingModule({ providers: [ HttpExceptionFilter, { provide: LoggerService, useValue: { error: jest.fn(), warn: jest.fn() } }, ], }).compile(); filter = module.get(HttpExceptionFilter); }); it('sends 404 with correct body for NotFoundException', () => { const mockJson = jest.fn(); const mockStatus = jest.fn().mockReturnValue({ json: mockJson }); const host = { switchToHttp: () => ({ getResponse: () => ({ status: mockStatus }), getRequest: () => ({ url: '/users/99' }), }), getType: () => 'http', } as unknown as ArgumentsHost; filter.catch(new NotFoundException('User not found'), host); expect(mockStatus).toHaveBeenCalledWith(404); expect(mockJson).toHaveBeenCalledWith( expect.objectContaining({ statusCode: 404, message: 'User not found' }), ); }); }); Call catch() directly — no need to spin up an HTTP server for unit tests. Create the mock host as a plain object cast to ArgumentsHost using as unknown as ArgumentsHost. Mock switchToHttp() to return mock getResponse() and getRequest() functions. Use jest.fn().mockReturnValue() chaining to simulate res.status(n).json(body) fluent API. Test both HttpException branches and non-HttpException (unexpected error) branches separately.

Difficulty: 6/10

Exception Filters, Unit Testing, NestJS Testing

Create a mock ArgumentsHost as a plain object cast to the interface and call catch() directly — no HTTP server required. Use Test.createTestingModule() to resolve the filter with its injected dependencies. Assert that res.status() and res.json() were called with the expected arguments.

Unit test for an exception filter with mocked ArgumentsHost

Exception filter testing strategy:

1
Call catch() directly — no need to spin up an HTTP server for unit tests.
2

How do you unit test an exception filter in NestJS?

Difficulty: 6/10

Exception Filters, Unit Testing, NestJS Testing

Create a mock ArgumentsHost as a plain object cast to the interface and call catch() directly — no HTTP server required. Use Test.createTestingModule() to resolve the filter with its injected dependencies. Assert that res.status() and res.json() were called with the expected arguments.

Unit test for an exception filter with mocked ArgumentsHost

Exception filter testing strategy:

1
Call catch() directly — no need to spin up an HTTP server for unit tests.
2

How do you unit test an exception filter in NestJS?

Create a mock ArgumentsHost as a plain object cast to the interface and call catch() directly — no HTTP server required. Use Test.createTestingModule() to resolve the filter with its injected dependencies. Assert that res.status() and res.json() were called with the expected arguments.

How do you unit test an exception filter in NestJS?

Create a mock ArgumentsHost as a plain object cast to the interface and call catch() directly — no HTTP server required. Use Test.createTestingModule() to resolve the filter with its injected dependencies. Assert that res.status() and res.json() were called with the expected arguments.

Scenario Questions

Follow-up Questions