Unit Testing Skill

Unit testing validates individual functions, methods, and classes in isolation by mocking all external dependencies.

---

Workflows

For guided, step-by-step execution of unit testing tasks, use the appropriate workflow:

Workflow	Purpose	When to Use
Setup	Initialize test infrastructure	New project or missing test setup
Writing	Write new unit tests	Creating tests for components
Reviewing	Review existing tests	Code review, quality audit
Running	Execute tests	Running tests, analyzing results
Debugging	Fix failing tests	Tests failing, need diagnosis
Optimizing	Improve test performance	Slow tests, maintainability

Workflow Selection Guide

IMPORTANT: Before starting any testing task, identify the user's intent and load the appropriate workflow.

Detect User Intent → Select Workflow

User Says / Wants	Workflow to Load	File
"Set up tests", "configure Jest", "add testing to project", "install test dependencies"	Setup	workflows/setup/workflow.md
"Write tests", "add tests", "create tests", "test this service/controller"	Writing	workflows/writing/workflow.md
"Review tests", "check test quality", "audit tests", "are these tests good?"	Reviewing	workflows/reviewing/workflow.md
"Run tests", "execute tests", "check if tests pass", "show test results"	Running	workflows/running/workflow.md
"Fix tests", "debug tests", "tests are failing", "why is this test broken?"	Debugging	workflows/debugging/workflow.md
"Speed up tests", "optimize tests", "tests are slow", "fix open handles"	Optimizing	workflows/optimizing/workflow.md

Workflow Execution Protocol

1. ALWAYS load the workflow file first - Read the full workflow before taking action
2. Follow each step in order - Complete checkpoints before proceeding
3. Load knowledge files as directed - Each workflow specifies which references/ files to read
4. Verify compliance after completion - Re-read relevant reference files to ensure quality

---

Knowledge Base Structure

references/
├── common/              # Core testing fundamentals
│   ├── knowledge.md     # Testing philosophy and test pyramid
│   ├── rules.md         # Mandatory testing rules (AAA, naming, coverage)
│   ├── assertions.md    # Assertion patterns and matchers
│   ├── examples.md      # Comprehensive examples by category
│   ├── detect-open-handles.md   # Open handle detection and cleanup
│   └── performance-optimization.md  # Jest runtime optimization
│
├── nestjs/              # NestJS component testing
│   ├── services.md      # Service/usecase testing patterns
│   ├── controllers.md   # Controller testing patterns
│   ├── guards.md        # Guard testing patterns
│   ├── interceptors.md  # Interceptor testing patterns
│   └── pipes-filters.md # Pipe and filter testing
│
├── mocking/             # Mock patterns and strategies
│   ├── deep-mocked.md   # @golevelup/ts-jest patterns
│   ├── jest-native.md   # Jest.fn, spyOn, mock patterns
│   └── factories.md     # Test data factory patterns
│
├── repository/          # Repository testing
│   ├── mongodb.md       # mongodb-memory-server patterns
│   └── postgres.md      # pg-mem patterns
│
├── kafka/               # NestJS Kafka microservices testing
│   └── kafka.md         # ClientKafka, @MessagePattern, @EventPattern handlers
│
└── redis/               # Redis cache testing
    └── redis.md         # Cache operations, health checks, graceful degradation

Quick Reference by Task

Write Unit Tests

1. MANDATORY: Read references/common/rules.md - AAA pattern, naming, coverage
2. Read references/common/assertions.md - Assertion best practices
3. Read component-specific files:
   • Services: references/nestjs/services.md
   • Controllers: references/nestjs/controllers.md
   • Guards: references/nestjs/guards.md
   • Interceptors: references/nestjs/interceptors.md
   • Pipes/Filters: references/nestjs/pipes-filters.md

Setup Mocking

1. Read references/mocking/deep-mocked.md - DeepMocked patterns
2. Read references/mocking/jest-native.md - Native Jest patterns
3. Read references/mocking/factories.md - Test data factories

Test Repositories

1. MongoDB: references/repository/mongodb.md
2. PostgreSQL: references/repository/postgres.md

Test Kafka (NestJS Microservices)

• Read references/kafka/kafka.md - ClientKafka mocking, @MessagePattern/@EventPattern handlers, emit/send testing

Test Redis

• Read references/redis/redis.md - Cache operations, health checks, graceful degradation

Examples

• Read references/common/examples.md for comprehensive patterns

Optimize Test Performance

1. Read references/common/performance-optimization.md - Worker config, caching, CI optimization
2. Read references/common/detect-open-handles.md - Fix open handles preventing clean exit

Debug Open Handles

• Read references/common/detect-open-handles.md - Detection commands, common handle types, cleanup patterns

---

Core Principles

0. Context Efficiency (Temp File Output)

ALWAYS redirect unit test output to temp files, NOT console. Test output can be verbose and bloats agent context.

IMPORTANT: Use unique session ID in filenames to prevent conflicts when multiple agents run.

# Initialize session (once at start of testing session)
export UT_SESSION=$(date +%s)-$$

# Standard pattern - redirect output to temp file (NO console output)
npm test > /tmp/ut-${UT_SESSION}-output.log 2>&1

# Read summary only (last 50 lines)
tail -50 /tmp/ut-${UT_SESSION}-output.log

# Get failure details
grep -B 2 -A 15 "FAIL\|✕" /tmp/ut-${UT_SESSION}-output.log

# Cleanup when done
rm -f /tmp/ut-${UT_SESSION}-*.log /tmp/ut-${UT_SESSION}-*.md

Temp Files (with ${UT_SESSION} unique per agent):

• /tmp/ut-${UT_SESSION}-output.log - Full test output
• /tmp/ut-${UT_SESSION}-failures.md - Tracking file for one-by-one fixing
• /tmp/ut-${UT_SESSION}-debug.log - Debug runs
• /tmp/ut-${UT_SESSION}-verify.log - Verification runs
• /tmp/ut-${UT_SESSION}-coverage.log - Coverage output

1. AAA Pattern (Mandatory)

ALL unit tests MUST follow Arrange-Act-Assert:

it('should return user when found', async () => {
  // Arrange
  const userId = 'user-123';
  mockRepository.findById.mockResolvedValue({
    id: userId,
    email: 'test@example.com',
    name: 'Test User',
  });

  // Act
  const result = await target.getUser(userId);

  // Assert
  expect(result).toEqual({
    id: userId,
    email: 'test@example.com',
    name: 'Test User',
  });
  expect(mockRepository.findById).toHaveBeenCalledWith(userId);
});

2. Use target for SUT

Always name the system under test as target:

let target: UserService;
let mockRepository: DeepMocked<UserRepository>;

3. DeepMocked Pattern

Use @golevelup/ts-jest for type-safe mocks:

import { createMock, DeepMocked } from '@golevelup/ts-jest';

let mockService: DeepMocked<UserService>;

beforeEach(() => {
  mockService = createMock<UserService>();
});

4. Specific Assertions

Assert exact values, not just existence:

// WRONG
expect(result).toBeDefined();
expect(result.id).toBeDefined();

// CORRECT
expect(result).toEqual({
  id: 'user-123',
  email: 'test@example.com',
  name: 'Test User',
});

5. Mock All Dependencies

Mock external services, never real databases for unit tests:

// Unit Test: Mock repository
{ provide: UserRepository, useValue: mockRepository }

// Repository Test: Use in-memory database
const mongoServer = await createMongoMemoryServer();

---

Standard Test Template

import { Test, TestingModule } from '@nestjs/testing';
import { createMock, DeepMocked } from '@golevelup/ts-jest';
import { MockLoggerService } from 'src/shared/logger/services/mock-logger.service';

describe('UserService', () => {
  let target: UserService;
  let mockRepository: DeepMocked<UserRepository>;

  beforeEach(async () => {
    // Arrange: Create mocks
    mockRepository = createMock<UserRepository>();

    const module: TestingModule = await Test.createTestingModule({
      providers: [
        UserService,
        { provide: UserRepository, useValue: mockRepository },
      ],
    })
      .setLogger(new MockLoggerService())
      .compile();

    target = module.get<UserService>(UserService);
  });

  afterEach(() => {
    jest.clearAllMocks();
  });

  describe('getUser', () => {
    it('should return user when found', async () => {
      // Arrange
      mockRepository.findById.mockResolvedValue({
        id: 'user-123',
        email: 'test@example.com',
      });

      // Act
      const result = await target.getUser('user-123');

      // Assert
      expect(result).toEqual({ id: 'user-123', email: 'test@example.com' });
    });

    it('should throw NotFoundException when user not found', async () => {
      // Arrange
      mockRepository.findById.mockResolvedValue(null);

      // Act & Assert
      await expect(target.getUser('invalid')).rejects.toThrow(NotFoundException);
    });
  });
});

---

Test Coverage Requirements

Category	Priority	Description
Happy path	MANDATORY	Valid inputs producing expected outputs
Edge cases	MANDATORY	Empty arrays, null values, boundaries
Error cases	MANDATORY	Not found, validation failures
Exception behavior	MANDATORY	Correct type, error code, message
Business rules	MANDATORY	Domain logic, calculations
Input validation	MANDATORY	Invalid inputs, type mismatches

Coverage Target: 80%+ for new code

---

Failure Resolution Protocol

CRITICAL: Fix ONE test at a time. NEVER run full suite repeatedly while fixing.

When unit tests fail:

1. Initialize session (once at start):

   export UT_SESSION=$(date +%s)-$$

2. Create tracking file: /tmp/ut-${UT_SESSION}-failures.md with all failing tests
3. Select ONE failing test - work on only this test
4. Run ONLY that test (never full suite):

   npm test -- -t "test name" > /tmp/ut-${UT_SESSION}-debug.log 2>&1
   tail -50 /tmp/ut-${UT_SESSION}-debug.log

5. Fix the issue - analyze error, make targeted fix
6. Verify fix - run same test 3-5 times:

   for i in {1..5}; do npm test -- -t "test name" > /tmp/ut-${UT_SESSION}-run$i.log 2>&1 && echo "Run $i: PASS" || echo "Run $i: FAIL"; done

7. Mark as FIXED in tracking file
8. Move to next failing test - repeat steps 3-7
9. Run full suite ONLY ONCE after ALL individual tests pass
10. Cleanup: rm -f /tmp/ut-${UT_SESSION}-*.log /tmp/ut-${UT_SESSION}-*.md

WHY: Running full suite wastes time and context. Each failing test pollutes output, making debugging harder.

---

Naming Conventions

Test Files

• Pattern: *.spec.ts
• Location: Co-located with source file

Test Structure

describe('ClassName', () => {
  describe('methodName', () => {
    it('should [expected behavior] when [condition]', () => {});
  });
});

Variable Names

Variable	Convention
SUT	target
Mocks	mock prefix (mockRepository, mockService)
Mock Type	DeepMocked<T>

---

What NOT to Unit Test

Do NOT create unit tests for:

• Interfaces - Type definitions only, no runtime behavior
• Enums - Static value mappings, no logic to test
• Constants - Static values, no behavior
• Type aliases - Type definitions only
• Plain DTOs - Data structures without logic

Only test files containing executable logic (classes with methods, functions with behavior).

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Anti-Patterns to Avoid

Don't	Why	Do Instead
Assert only existence	Doesn't catch wrong values	Assert specific values
Conditional assertions	Non-deterministic	Separate test cases
Test private methods	Couples to implementation	Test via public interface
Share state between tests	Causes flaky tests	Fresh setup in beforeEach
Mock repositories in services	Tests implementation	Mock interfaces
Skip mock verification	Doesn't validate behavior	Verify mock calls
Test interfaces/enums/constants	No behavior to test	Skip these files

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Checklist

Setup:

• [ ] Use target for system under test
• [ ] Use mock prefix for all mocks
• [ ] Use DeepMocked<T> type
• [ ] Include .setLogger(new MockLoggerService())
• [ ] Follow AAA pattern with comments
• [ ] Reset mocks in afterEach

Coverage:

• [ ] Happy path tests for all public methods
• [ ] Edge case tests (empty, null, boundaries)
• [ ] Error case tests (not found, validation failures)
• [ ] Exception type and error code verification
• [ ] Mock call verification (parameters + count)

Quality:

• [ ] 80%+ coverage on new code
• [ ] No assertions on log calls
• [ ] No test interdependence
• [ ] Tests fail when any field differs