What is this?
Mock Service Worker (MSW) is the industry-standard API mocking library for JavaScript. It intercepts the network requests your app makes and serves mock responses you define, so you never touch a real backend.
Mock Service Worker (MSW) is the industry-standard API mocking library for JavaScript. It intercepts the network requests your app makes and serves mock responses you define, so you never touch a real backend.Is it a testing framework or a library I use in my app?It's a library you add to a JavaScript project, not a test runner or framework: you write request handlers and register them with setupWorker in the browser or setupServer in Node.js.
It's a library you add to a JavaScript project, not a test runner or framework: you write request handlers and register them with setupWorker in the browser or setupServer in Node.js.How do I add it to my project?The README points to the official documentation and its Quick start guide as the place to begin. In short: install the msw package, write handlers, and register them via setupWorker (browser) or setupServer (Node).
The README points to the official documentation and its Quick start guide as the place to begin. In short: install the msw package, write handlers, and register them via setupWorker (browser) or setupServer (Node).Do I need anything special installed to run it?Node.js 18 or newer per the package engines field. No accounts or hosted services — it's an npm dependency, and graphql ships as a runtime dependency for mocking GraphQL APIs.
Node.js 18 or newer per the package engines field. No accounts or hosted services — it's an npm dependency, and graphql ships as a runtime dependency for mocking GraphQL APIs.Will I have to change my application code to use the mocks?No — requests are intercepted on the network level, after they leave your application, so app code and tests stay unaware of what is mocked. You can even inspect mocked responses in the browser's Network tab.
No — requests are intercepted on the network level, after they leave your application, so app code and tests stay unaware of what is mocked. You can even inspect mocked responses in the browser's Network tab.Who is this for — is it actually production-proven?Frontend and backend JavaScript developers who build or test against APIs. Hundreds of thousands of engineers use it, including teams at Google, Microsoft, Spotify, Amazon, and Netflix.
Frontend and backend JavaScript developers who build or test against APIs. Hundreds of thousands of engineers use it, including teams at Google, Microsoft, Spotify, Amazon, and Netflix.Does it work only in the browser, or in Node.js too?Both. In the browser it registers a real Service Worker; in Node.js, where Service Workers don't exist, it switches to a low-level interception algorithm — the same request handlers work in both environments.
Both. In the browser it registers a real Service Worker; in Node.js, where Service Workers don't exist, it switches to a low-level interception algorithm — the same request handlers work in both environments.How do I write a mock — is there a special DSL?No — handlers use an Express-like routing syntax. You match requests with parameters, wildcards, or regular expressions and respond with custom status codes, headers, cookies, delays, or fully custom resolvers.
No — handlers use an Express-like routing syntax. You match requests with parameters, wildcards, or regular expressions and respond with custom status codes, headers, cookies, delays, or fully custom resolvers.Can I reuse the same mocks across unit, integration, and E2E tests?Yes, that's a core design goal: one mock definition works for local development, debugging, and unit/integration/E2E testing alike, with no adapters — and your tests stay unaware anything is mocked.
Yes, that's a core design goal: one mock definition works for local development, debugging, and unit/integration/E2E testing alike, with no adapters — and your tests stay unaware anything is mocked.Does it cover GraphQL and WebSocket APIs, or only REST?More than REST: the README's course list covers mocking REST and GraphQL APIs as well as mocking and testing WebSocket APIs, and graphql is a runtime dependency of the package.
More than REST: the README's course list covers mocking REST and GraphQL APIs as well as mocking and testing WebSocket APIs, and graphql is a runtime dependency of the package.Is MSW free to use?Yes — it's MIT-licensed and free. The project runs on GitHub Sponsors with golden, silver, and bronze tiers, and royalties from paid Egghead courses about MSW also help sustain development.
Yes — it's MIT-licensed and free. The project runs on GitHub Sponsors with golden, silver, and bronze tiers, and royalties from paid Egghead courses about MSW also help sustain development.Which topics is it in?mocking-framework, service-worker, mocking, mock, testing-tools, rest, unit-testing, devtools, graphql, websocket, http, testing
mocking-framework, service-worker, mocking, mock, testing-tools, rest, unit-testing, devtools, graphql, websocket, http, testing