mock-service

MSW intercepts HTTP requests at the network level to mock APIs for browser and integration tests, while Nock intercepts requests for Node.js tests and API clients. Both produce deterministic test doubles by returning contract-accurate responses.

The best way to generate test data for mock services is using factory patterns. Test data factories create consistent, contract-accurate responses and integrate with reset patterns to keep mocks in sync across test runs.

Yes, MSW works with TypeScript for contract testing by providing properly typed mock services. TypeScript-safe mocks enforce contract-oriented practices, ensuring stateful mocks and error handling match real API dependencies.

You should mock APIs instead of using real services when you need deterministic test behavior, contract-accurate responses, or want to simulate error handling and stateful responses without external network dependencies or live data risks.

To reset mock state between test runs, apply test reset patterns alongside your data factories. These patterns clear network request interceptions and restore custom mock servers to their initial state for deterministic behavior.