Testing Test Writing

Unit tests for DTOs and entities validate individual components in isolation using Jest's expect assertions. Create *.spec.ts files in your test directory, import the DTO or entity class, instantiate it with test data, and assert on expected behavior—for example, verifying that class-validator decorators reject invalid input like empty email fields.

Unit tests validate isolated components like DTOs or domain entities without external dependencies. Integration tests verify complete workflows using real database transactions, testing use cases and queries end-to-end. Both use Jest; integration tests exercise the full system including database persistence.

Jest provides built-in mocking capabilities to isolate components from external services. Use jest.mock() to replace dependencies, or manually create mock objects that return predictable values. This lets unit tests run fast and predictably without calling real APIs or databases.

Behavior-driven testing focuses on what the code should do—its observable outcome—rather than how it's coded internally. Test the expected result (e.g., article created, exception thrown for duplicates) not the specific function calls or internal state. This makes tests resilient to refactoring.

Jest works across TypeScript frontends and backends. This Skill applies Jest to NestJS backend projects for unit and integration testing, but Jest's test patterns—setup, mocking, assertions—are framework-agnostic and apply equally to frontend components.

Establish patterns: unit tests for DTOs and entities, integration tests with real database transactions for use cases, descriptive test naming, test fixtures in test/fixtures, and mocked external dependencies. Consistent patterns enforce reliability and make tests maintainable across your project.