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Avoid testing anti-patterns

nestjs/nest
Based on 5 comments
TypeScript

Ensure your tests actually validate functionality by avoiding common testing anti-patterns: 1. **Don't mock what you're testing** - When testing a method, don't mock its implementation as this tests nothing meaningful:

Testing TypeScript

Reviewer Prompt

Ensure your tests actually validate functionality by avoiding common testing anti-patterns:

  1. Don’t mock what you’re testing - When testing a method, don’t mock its implementation as this tests nothing meaningful:
// WRONG - mocking the method under test
it('should find all cats', async () => {
  jest.spyOn(catsService, 'findAll').mockImplementation(() => result);
  expect(await catsService.findAll()).toEqual(result);
});

// RIGHT - setting up the state to test the actual implementation
it('should find all cats', async () => {
  // @ts-ignore - access internal state for test setup
  catsService.cats = result;
  expect(await catsService.findAll()).toEqual(result);
});
  1. Test public interfaces, not private functions - Private methods should be tested through public methods. If a private method is complex, consider extracting it to a separate class:
// WRONG - testing private methods directly
describe('getPaths', () => { // private method
  it('should return paths', () => {
    expect(middlewareModule['getPaths'](routes)).toEqual(['path1', 'path2']);
  });
});

// RIGHT - test via public interface or extract to a testable class
describe('register', () => { // public method
  it('should configure paths correctly', () => {
    middlewareModule.register(routes);
    // assert the expected behavior
  });
});
  1. Don’t make redundant test calls - Call the method once and make all assertions on that single call:
// WRONG - calling the method multiple times
it('should create a user', () => {
  usersController.create(createUserDto); // unnecessary call
  expect(usersController.create(createUserDto)).resolves.toEqual({
    id: 'a id',
    ...createUserDto,
  });
});

// RIGHT - single call with multiple assertions
it('should create a user', () => {
  expect(usersController.create(createUserDto)).resolves.toEqual({
    id: 'a id',
    ...createUserDto,
  });
  expect(usersService.create).toHaveBeenCalledWith(createUserDto);
});
  1. Test the actual behavior - Ensure your tests verify functionality, not just that methods were called:

```typescript // WRONG - only checking if a method was called it(‘should remove a user’, async () => { const removeUserSpy = jest.spyOn(service, ‘remove’); service.remove(‘anyid’); expect(removeUserSpy).toHaveBeenCalledWith(‘anyid’); });

// RIGHT - checking the actual behavior it(‘should remove a user’, async () => { const repositorySpy = jest.spyOn(repository, ‘delete’); await service.remove(‘anyid’); expect(repositorySpy).toHaveBeenCalledWith(‘anyid’); // and possibly check that the user is actually gone });

5
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TypeScript
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Testing
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