hrsh7th/msw

Mock Service Worker (MSW) lets your front-end app pretend it's talking to a real back-end API, even when that API doesn't exist yet. Instead of waiting for the server team to finish building endpoints, you define fake responses locally, and your app behaves as though the server is already live. You can also simulate errors, delays, and edge cases that might be hard to trigger against a real server. In the browser, it uses a built-in browser feature called a Service Worker to intercept outgoing network requests at the network layer, before they reach the internet. Your app makes normal requests as usual, and MSW quietly steps in and returns the mock data you defined. In a Node.js testing environment, the same mock definitions work by intercepting requests at the module level instead. The key idea is that your application code doesn't change, it doesn't know or care whether the response is real or mocked. This is useful for front-end developers and teams practicing test-driven or front-end-first development. If you're building a login screen but the authentication endpoint isn't ready, you define what the response should look like and build against that. In testing, you can verify how your UI handles a successful login, a server error, or a slow response, all without hitting a real server. The same mock definitions serve both purposes, so you write them once and reuse them. What sets it apart is that it works at the network level rather than patching your HTTP libraries directly, so it plays nicely with any tool you use to make requests. You can even see the mocked requests appear in your browser's network tab, just like real ones.