MSW (Mock Service Worker)
Industry standard API mocking for JavaScript
Mock Service Worker (MSW) is an API mocking library for browser and Node.js. It intercepts requests at the network level using Service Workers, enabling the same mock definitions for unit, integration, and E2E testing without modifying application code.
16.0k
GitHub Stars
free
Pricing
No
Self-Hostable
Features
- ✓ Service Worker-based interception
- ✓ REST API mocking
- ✓ GraphQL mocking
- ✓ WebSocket mocking
- ✓ Browser and Node.js support
- ✓ Framework agnostic
- ✓ Works with any HTTP client
- ✓ Request observation
- ✓ Type-safe with TypeScript
- ✓ Reusable across test types
Pros
- + Network-level interception
- + No code changes needed
- + Reusable mocks across tests
- + Trusted by Google, Microsoft, Spotify
- + Great developer experience
Cons
- − Service Worker complexity in browsers
- − Setup overhead for simple cases
- − Learning curve for new users
- − Browser Service Worker limitations