mswjs/msw

Mock Service Worker is an API mocking library for JavaScript that uses service workers to intercept requests at the network level.

The tool solves the problem of testing applications against APIs that may not exist, are unstable, or need to be designed alongside the application. It works by leveraging the Service Worker API to intercept HTTP requests before they reach the network, allowing you to respond with mock data without modifying your application code. This approach means your app remains unaware of whether requests are being mocked, and you can test against the same request patterns you would use in production. The library provides Express-like routing syntax for defining handlers, supporting parameters, wildcards, regular expressions, and custom response configurations including status codes, headers, cookies, and delays.

Developers should choose this tool if they want mocks that live alongside their application code rather than in separate configuration, and if they need the same mocking layer to work across both browser-based tests and Node.js environments. The library suits projects where you want to avoid modifying application code to detect mocks, and where you need to test actual application behavior against realistic API interactions. The browser-based approach using Service Workers distinguishes it from other mocking solutions by allowing you to inspect mocked responses in browser DevTools just as you would real network requests.

The project maintains a substantial base of adopters, as evidenced by the fact that almost all open issues are raised by outside users rather than the core team. Maintainers typically respond to new issues and pull requests within a day. Work in the issue tracker is dominated by bug reports and triage activities, with a notable focus on Node.js scope improvements.