msw

Mock HTTP requests across browser and Node environments by using a unified service worker layer to intercept network calls, stabilize frontend testing, and simulate backend APIs deterministically. It enables consistent offline workflows during development and CI pipelines.

Override mocked API responses per test without cross-test leakage by applying server.use() to inject specific handlers. The setupServer lifecycle provides resetHandlers to clear custom handlers after each test, ensuring deterministic and isolated test execution.

Service worker HTTP mocking avoids server-side rendering issues by lazy loading the mock worker via msw/browser exclusively for browser environments. For server-side tests, it uses msw/node with setupServer to intercept requests without initializing a browser worker.

Simulate real network behavior while mocking HTTP requests by configuring flexible routing options including passthrough. This allows specific network calls to bypass the mock handlers and hit actual backend APIs when needed during development or integration validation.

Using a service worker for HTTP mocking provides a portable in-browser interception layer that standardizes API simulation across development and CI pipelines. Unlike traditional libraries, it eliminates flaky integration tests by intercepting requests at the network level deterministically.

Service workers cannot execute in Node tests, requiring msw/node and setupServer to intercept network requests server-side. Browser mock workers must be lazily loaded to avoid SSR issues, and handlers require manual resets using resetHandlers to prevent test state leakage.