Introduction

Mock Service Worker (MSW) is an API mocking library for browser and Node.js. With MSW, you can intercept outgoing requests, observe them, and respond to them using mocked responses.

What sets MSW apart is that it heavily advocates for a standalone API mocking layer, creating a single source of truth for your network behavior and integrating it across whichever tools you’re using. This leads to a more resilient setup and, paired with other library features, creates a truly seamless API mocking experience.

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Features

Agnostic

MSW is designed to be fully environment-, framework- and tool-agnostic. You can use it in any browser or Node.js process without additional configurations, adapters, or plugins. It works with all request clients, be it a native window.fetch() or third-party libraries like Axios, React Query, or Apollo.

Seamless

MSW uses the Service Worker API to intercept actual production requests on the network level. Instead of patching fetch and meddling with your application’s integrity, MSW bets on the platform, utilizing the standard browser API to implement a revolutionary request interception logic.

Even in Node.js, where there are no standard means to intercept requests, MSW uses class extension instead of module patching to ensure your tests run in the environment as close to production as possible.

Reusable

By treating API mocking as a standalone layer, MSW can integrate throughout your entire stack, allowing you to reuse and customize network behavior on demand. Imagine using the same API mocks during development, integration and end-to-end testing, and then in your Storybook or during a live demo. Well, with MSW, you can.

Start here

If you have never tried MSW before or unsure where to start, look no further than the Getting started tutorial: