---

name: jest-testing-generator description: 'Generate Jest unit tests, integration tests, React Testing Library component tests, mock implementations, test fixtures, and edge case coverage for TypeScript codebases'

Purpose

Create comprehensive, maintainable Jest test suites for frontend and backend TypeScript code with proper mocking and edge case coverage.

When to Use

Use this skill when the task involves:

• Writing new unit or integration tests
• Adding tests for uncovered code paths
• Creating test fixtures and mock implementations
• Migrating from other test frameworks to Jest
• Improving test coverage metrics

Constraints

• Tests must be deterministic — no random values, fixed timestamps
• Mock external dependencies at adapter boundaries, not deep in business logic
• Use React Testing Library for component tests (query by role/label, not test IDs)
• Prefer integration tests for API boundaries, unit tests for pure business logic
• Follow AAA pattern: Arrange, Act, Assert
• Name tests descriptively: should [expected behavior] when [condition]

Execution Steps

1. Analyze target — Identify function/component inputs, outputs, and side effects
2. Define test cases — List happy paths, error cases, and edge cases
3. Create fixtures — Build reusable test data factories
4. Write mocks — Mock external dependencies (APIs, databases, time)
5. Implement tests — Write tests following AAA pattern
6. Verify coverage — Check uncovered branches and add missing tests

Examples

Input

Write tests for a UserService.register method that validates email and hashes password

Output

Copy// @/modules/users/__tests__/users.service.spec.ts
import { Test } from '@nestjs/testing';
import { UsersService } from '../users.service';
import { UsersRepository } from '../users.repository';
import { ConflictException } from '@nestjs/common';

describe('UsersService', () => {
  let service: UsersService;
  let repository: jest.Mocked<UsersRepository>;

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      providers: [
        UsersService,
        {
          provide: UsersRepository,
          useValue: {
            findByEmail: jest.fn(),
            create: jest.fn(),
          },
        },
      ],
    }).compile();

    service = module.get(UsersService);
    repository = module.get(UsersRepository);
  });

  describe('register', () => {
    const validDto = {
      email: 'test@example.com',
      password: 'securePass123',
      name: 'Test',
    };

    it('should create user when email is unique', async () => {
      repository.findByEmail.mockResolvedValue(null);
      repository.create.mockResolvedValue({ id: '1', ...validDto });

      const result = await service.register(validDto);

      expect(result.email).toBe(validDto.email);
      expect(repository.create).toHaveBeenCalledWith(
        expect.objectContaining({ email: validDto.email }),
      );
    });

    it('should throw ConflictException when email exists', async () => {
      repository.findByEmail.mockResolvedValue({ id: '1', ...validDto });

      await expect(service.register(validDto)).rejects.toThrow(
        ConflictException,
      );
    });

    it('should hash password before storing', async () => {
      repository.findByEmail.mockResolvedValue(null);
      repository.create.mockResolvedValue({ id: '1', ...validDto });

      await service.register(validDto);

      expect(repository.create).toHaveBeenCalledWith(
        expect.objectContaining({
          password: expect.not.stringMatching(validDto.password),
        }),
      );
    });
  });
});

Related Skills

• nest-backend-service-builder — Service implementation patterns
• nextjs-react-engineering — Component testing patterns