Mock Service Worker
A JavaScript API mocking library that shares request handlers between browser development and automated tests.
About Mock Service Worker
Mock Service Worker is an open source JavaScript library for mocking API requests. It lets developers describe network responses once, using request handlers written in the Fetch API style, and reuse those handlers across frameworks, tools, and environments.
In the browser, it uses the standard Service Worker API to intercept requests. In Node.js, it intercepts requests at the socket level through @mswjs/interceptors. The library can mock REST, GraphQL, Server-Sent Events, and WebSocket APIs under one contract. Its examples show integration with Playwright, Vitest-style test setup, and Storybook, and it can be enabled only in development builds.
Key features
- Shared request handlers for browser development and Node.js tests
- Mocks REST, GraphQL, Server-Sent Events, and WebSocket APIs together
- Handlers written with the Fetch API request and response model
- Browser request interception through the standard Service Worker API
- Node.js request interception through @mswjs/interceptors
- Generates request handlers from OpenAPI documents and HAR files
Best for: JavaScript developers and teams who want the same API mocks in browser development and automated tests.
Good for
- Handlers use the Fetch API model, so the same mock code can be reused across tools
- One mock definition can cover browser development and Node.js test environments
- Free and open source
Mock Service Worker at a glance
- Website
- mswjs.io
- Category
- Development › APIs & Databases
- Type
- Online tool
- Pricing
- Free
- Open source
- Yes
- Last checked
- Oct 9, 2026 · Online when checked
- Listed
- Oct 9, 2026
Guides for APIs & Databases
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Mock Service Worker FAQ
Can the same mocks be used in browser development and in tests?
Yes. Handlers are described once and can be used with the browser setup for development and the Node.js setup for tests.
Can handlers be generated from existing API definitions?
The library lists the ability to generate request handlers from OpenAPI documents and HAR files instead of writing them by hand.
Can mocks return error responses?
Yes. Handlers can return a custom status code, such as a 429 rate limit response, to test how an interface handles errors.
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