/typescript-e2e-testing

E2E Testing Core Knowledge

What is E2E Testing?

End-to-End (E2E) testing validates the complete flow of an application from the user's perspective, testing the integration of all components including databases, message brokers, and external services.

E2E vs Unit vs Integration Testing

Test Pyramid Distribution

    /\      E2E Tests (10%)
   /  \     - Full Kafka flow, self-healing scenarios
  /----\    Integration Tests (30%)
 /      \   - RdKafka wrapper validation, Schema Registry
/--------\  Unit Tests (60%)
            - Core logic, utilities, error handling

When to Write E2E Tests

• MUST Test: Critical user flows, async message processing, multi-component interactions
• SHOULD Test: API endpoints, error handling flows, authentication/authorization
• NICE TO HAVE: Edge cases, performance scenarios, recovery mechanisms

Test File Organization

project/
├── src/
│   └── *.spec.ts          # Unit tests (co-located)
└── test/
    ├── e2e/               # E2E tests
    │   ├── *.e2e-spec.ts  # Test files
    │   ├── setup.ts       # Global setup
    │   └── helpers/       # Test utilities
    ├── fixtures/          # Reusable test data
    └── jest-e2e.config.ts # E2E Jest config

E2E Test Lifecycle

beforeAll (90s timeout)
├── Start Docker infrastructure
├── Wait for services healthy
├── Initialize NestJS app
├── Connect Kafka microservice
└── Pre-subscribe to topics (if Kafka)

beforeEach
├── Wait for in-flight operations (1-2s)
└── Clear data (DB collections, message buffers)

test
├── GIVEN: Setup test data
├── WHEN: Execute single action
└── THEN: Verify outcomes

afterEach
└── Clean up test data

afterAll (30s timeout)
├── Close app
├── Disconnect clients
└── Stop infrastructure (if applicable)

Key Principles

1. Test Isolation: Each test must be independent and not affect others
2. Real Infrastructure: Use real databases, message brokers, not mocks
3. Deterministic Results: Tests must produce same results on every run
4. Clear Intent: Test names and structure must clearly describe behavior
5. Fast Feedback: Optimize for quick failure detection while maintaining reliability