How do you unit test a custom pipe in NestJS?

Q: How do you unit test a custom pipe in NestJS?

// Stateless pipe — instantiate directly with new describe('ParsePositiveIntPipe', () => { let pipe: ParsePositiveIntPipe; beforeEach(() => { pipe = new ParsePositiveIntPipe(); }); it('transforms a valid positive integer string', () => { expect(pipe.transform('42')).toBe(42); }); it('throws BadRequestException for non-numeric input', () => { expect(() => pipe.transform('abc')).toThrow(BadRequestException); }); it('throws BadRequestException for zero', () => { expect(() => pipe.transform('0')).toThrow(BadRequestException); }); }); // Pipe with injected service — use TestingModule describe('UserExistsPipe', () => { let pipe: UserExistsPipe; const mockUsersService = { findOne: jest.fn() }; beforeEach(async () => { const module = await Test.createTestingModule({ providers: [ UserExistsPipe, { provide: UsersService, useValue: mockUsersService }, ], }).compile(); pipe = module.get(UserExistsPipe); }); it('returns the user when found', async () => { mockUsersService.findOne.mockResolvedValue({ id: 1 }); await expect(pipe.transform('1')).resolves.toEqual({ id: 1 }); }); it('throws NotFoundException when user does not exist', async () => { mockUsersService.findOne.mockResolvedValue(null); await expect(pipe.transform('99')).rejects.toThrow(NotFoundException); }); }); Call transform() directly — no need for a full HTTP request in unit tests. Test all branches: valid input, invalid input, boundary values, and missing input. For async pipes use await and resolves/rejects matchers from Jest. Mock injected services with useValue and jest.fn() to control return values per test. Test the exact exception type and message — BadRequestException vs NotFoundException matters.

Difficulty: 5/10

custom pipe implementation, jest unit testing, dependency injection

Call transform() directly — no HTTP stack needed. For stateless pipes, instantiate with new and test each branch. For pipes with injected services, use Test.createTestingModule() to provide mocks and retrieve the pipe via module.get().

Unit tests for stateless and service-dependent pipes

Pipe testing strategy:

1
Call transform() directly — no need for a full HTTP request in unit tests.

How do you unit test a custom pipe in NestJS?

Difficulty: 5/10

custom pipe implementation, jest unit testing, dependency injection

Call transform() directly — no HTTP stack needed. For stateless pipes, instantiate with new and test each branch. For pipes with injected services, use Test.createTestingModule() to provide mocks and retrieve the pipe via module.get().

Unit tests for stateless and service-dependent pipes

Pipe testing strategy:

1
Call transform() directly — no need for a full HTTP request in unit tests.

How do you unit test a custom pipe in NestJS?

Call transform() directly — no HTTP stack needed. For stateless pipes, instantiate with new and test each branch. For pipes with injected services, use Test.createTestingModule() to provide mocks and retrieve the pipe via module.get().

How do you unit test a custom pipe in NestJS?

Call transform() directly — no HTTP stack needed. For stateless pipes, instantiate with new and test each branch. For pipes with injected services, use Test.createTestingModule() to provide mocks and retrieve the pipe via module.get().

Scenario Questions

Follow-up Questions