sdcorejs-testing-unit-nestjs

To write NestJS unit tests without a real database, you mock repository dependencies and QueryRunners using Test.createTestingModule. This isolates business logic, validates transaction rollback behavior, and asserts thrown exception types without network or DB calls.

You test Zod validators in NestJS by using the it.each method for table-driven test cases. This approach verifies multiple inputs and confirms bilingual error messages without redundantly testing the Zod library's internal logic.

The best way to test transaction rollback in a mocked NestJS service is by mocking a QueryRunner object within your test module. This allows you to explicitly assert whether commit or rollback methods are called under specific error conditions.

Yes, you can test NestJS mappers by verifying field mappings and null-handling directly in pure mapper functions. This validates the transformation logic accurately without relying on brittle snapshot comparisons.

Yes, this approach provides light controller wiring tests that validate a controller delegates correctly to its service and returns expected results, while ensuring heavy business logic remains pushed into the service layer.

You should add a failing unit test first when addressing a bug in NestJS business logic. This test-first approach validates the defect, guides the fix, and prevents future regressions after the service method or utility lands.