mswjs/msw

Health v2 · maintenance only

Adoption not part of the score

18168 stars · 619 forks observed · 2026-08-28

What it is AI-extracted, prompt v1, taxonomy v1, 2026-08-29, confidence not recorded

Mock Service Worker (MSW) is an API mocking library for JavaScript that intercepts HTTP, GraphQL, and WebSocket requests in browsers via the Service Worker API and in Node.js via class extension. It lets you define request handlers once and reuse the same mocks across development, unit/integration/e2e tests, Storybook, and demos.

Use cases

• mock rest api responses in jest or vitest tests
• intercept fetch and axios requests during development
• mock graphql queries for frontend testing
• reuse api mocks across storybook and e2e tests
• simulate network delays and error responses
• develop frontend before backend api exists
• mock websocket connections in tests

When to choose

• you need environment-agnostic mocks reusable across browser and Node.js
• you want mocks that don't patch your app code and stay visible in DevTools
• you're testing frontends that talk to REST, GraphQL, or WebSocket APIs
• you want a single source of truth for network behavior across tools

When to avoid

• you only need simple function/module mocking rather than network-level mocking
• you're mocking non-HTTP protocols or backend-to-backend gRPC services
• you need contract testing against real API specifications

Facets

library · maturity stable

mocking testing http-client graphql websocket developer-tools testing web-development frontend developer-tools apis browser cross-platform api-mocking service-worker request-interception mock-service-worker test-utils storybook nodejs web-server

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