Mock Service Worker is an API mocking library. It allows you to intercept requests and mock their responses. You've probably used tools like it before.

You've never used something quite like MSW though. Here's why.

1. You can use MSW anywhere. It works in the browser and in Node.js, which means any browser tooling and any Node.js tooling. Playwright, Cypress, Karma, Electron, Jest, Vitest, Express, Fastify, React Native. No configurations or adapters. MSW never belongs to a particular tooling.
2. You use web standards when mocking with MSW. You use the Fetch API to handle requests and responses in your app. You use the same Fetch API to intercept requests and declare mock responses. Consistent, feature-rich, and predictable. Learn the web, not the tools.
3. You allow requests to actually happen. MSW runs as much of the underlying network code as possible when mocking. It relies on the Service Worker API to provision request interception without touching your application's code. Control over the network without compromising the integrity of your app.

Learn more about MSW in the Documentation.

Why sponsor?

MSW has become a go-to choice for API mocking in JavaScript for hundreds of thousands of developers. It's used by Google, Microsoft, Amazon, Cloudflare, Adobe, Spotify, Vercel, and countless others. There are more than 100,000 repositories on GitHub alone that rely on MSW, and it's being downloaded about 3,000,000 times per week on NPM. People develop, test, debug, explore, and teach with MSW. It has become an essential tool in the JavaScript ecosystem.

By sponsoring the project, you support its present and invest in its future. You help it become less of an over-weekend hobby project and more of a stable, funded piece of software. By supporting MSW you support the future of API mocking, making it brighter for everyone. And we will make sure everyone knows about your investment in the future.

See the benefits you get as a sponsor in the list on the right →

How we use your contributions

MSW is a non-commercial open-source project. It's owned and actively maintained by a single person. Your contributions allow for it to exist and evolve. Let's be more specific.

By sponsoring the project you're creating a budget to:

• Investigate issues;
• Brainstorm, design, and implement features;
• Compensate active contributors for their work;
• Support the community on Discord, GitHub Discussions, StackOverflow, and Twitter.

Thank you for making these incredible things happen!