MSW (Mock Service Worker) and Supertest are JavaScript testing tools that serve fundamentally different purposes, though they're often confused as alternatives. Supertest is an HTTP assertion library built on SuperAgent that tests Node.js server endpoints by making simulated requests directly to your Express, Koa, or other HTTP servers. MSW intercepts network requests at the protocol level using Service Workers in browsers and node-http interceptors in Node.js, allowing you to mock external API dependencies without modifying application code.
This comparison matters because choosing the wrong tool leads to brittle tests or unnecessary complexity. Supertest targets backend developers testing their own HTTP APIs and route handlers, while MSW targets frontend and full-stack developers who need to mock external services their application consumes. Understanding that these tools complement rather than compete with each other is crucial—most production applications benefit from using both simultaneously.
Choose Supertest when you're building and testing your own Node.js HTTP API. If you're validating that your Express routes return correct status codes, testing authentication middleware, or ensuring your API endpoints handle validation errors properly, Supertest provides the most direct and straightforward testing approach. It excels at unit and integration testing of backend services where you control the server implementation and need confidence in your route handlers, middleware stack, and response formatting.
Choose MSW when your application consumes external APIs or when you need consistent mocking across multiple environments. If you're testing React components that fetch data, building a frontend that calls microservices, or need to simulate error scenarios from third-party APIs during development, MSW's network-level interception is superior. The best architecture uses both: MSW mocks external dependencies while Supertest validates your own API endpoints, creating a comprehensive testing strategy that keeps tests fast, isolated, and independent from external services.