onfetch is a lightweight library for mocking the browser/Node `fetch()` API using the native `Request` and `Response` objects. It lets developers define intercept rules (by URL string, RegExp, `URLPattern`, or custom matcher) that can reply with static data, delays, redirects, or callbacks, and works in various contexts—including `globalThis`, Service Workers, and @mswjs interceptors—making it useful for testing and development of web applications that rely on `fetch`.