MSW 2.0のご紹介

MSW 2.0: Embracing the Fetch API and Unlocking New Possibilities

October 23, 2023

Artem Zakharchenko

Mock Service Worker (MSW) has come a long way since its inception five years ago. With the release of version 2.0, MSW has undergone significant changes, introducing a refined public API and first-class support for Fetch API primitives. In this blog post, we will explore the motivations behind these changes, the challenges faced during development, and the exciting possibilities that MSW 2.0 brings to the table.

The Evolution of MSW

Since its introduction in 2018, MSW has been widely used for mocking API responses in JavaScript applications. The library provided a composition API, allowing developers to declare mocked responses using the res() function. While this API was practical and scalable, it failed to educate developers about actual responses on the web.

Artem Zakharchenko, the maintainer of MSW, recognized the need for a change. He wanted developers to not only achieve their goals but also learn concepts and APIs that could be applied beyond MSW. With this in mind, version 2.0 of MSW was developed.

Embracing JavaScript Standards

One of the major changes in MSW 2.0 is the adoption of JavaScript standards, specifically the Fetch API. In previous versions, MSW had to support multiple versions of Node.js, which meant it couldn’t rely on the Fetch API for representing outgoing requests. To overcome this limitation, MSW coerced all requests to a single isomorphic request instance, resulting in contrived APIs and tedious implementation of new features.

Artem Zakharchenko decided to deprecate support for Node.js versions older than 16 in MSW 2.0, allowing the library to fully embrace the Fetch API and JavaScript standards. This change not only simplifies the codebase but also eliminates the need for polyfills and contrived support for platform features.

The Power of Fetch API

With the adoption of the Fetch API, MSW 2.0 introduces a new API for declaring request handlers and response resolvers. Now, both the request and response are instances of the Fetch API, providing a feature-rich and standardized way of handling requests and defining responses.

This change opens up a world of possibilities. Developers can now leverage the full power of the Fetch API, including features like reading the intercepted request’s body as FormData. MSW no longer relies on polyfills or custom representations of requests and responses, making the library more future-proof and compatible with different environments and tools.

Breaking Changes and Improvements

MSW 2.0 brings not only the adoption of the Fetch API but also numerous bug fixes and improvements. The library now has full compatibility with ESM, proper code splitting, improved internal architecture, and a refined interception algorithm in Node.js.

While these changes are exciting, they also come with breaking changes to the public API. However, the benefits of adopting the new API far outweigh the migration effort. Detailed migration guidelines are provided to help developers address each breaking change relevant to their setup.

Supporting MSW

Artem Zakharchenko’s dream is to work on MSW full-time and continue improving and evolving the library. To support this dream, he encourages developers to sponsor the project. Every contribution brings him a step closer to making this dream a reality.


MSW 2.0 marks a significant milestone in the evolution of the library. By embracing the Fetch API and JavaScript standards, MSW unlocks new possibilities and provides developers with a more powerful and standardized way of mocking API responses. The breaking changes in the API may require some effort to migrate, but the benefits are well worth it. MSW 2.0 sets a new threshold of reusability and empowers developers to write code that makes sense even outside of request handlers.

With the incredible contributions from the community and the dedication of Artem Zakharchenko, MSW continues to evolve and improve. The future of MSW looks bright, and developers can expect even more exciting features and enhancements in the coming years.

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Tags: MSW, Mock Service Worker, Fetch API, JavaScript, API mocking, Library, Software Development


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    Introducing MSW 2.0
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