Repolode
Submit
mswjs

mswjs/msw

A JavaScript API mocking library that intercepts network requests in browsers and Node.js, so app code runs unchanged while tests and development use mock responses.

View on GitHubWebsite
Stars
18,307
Forks
642
Open issues
16
Last commit
8 days ago

What is it?

Mock Service Worker (MSW) mocks API requests at the network level. In the browser it uses the Service Worker API to respond to requests with your mock definitions, and in Node.js it uses a low-level interception layer. Because the application's own code, including fetch, axios and data-fetching libraries, is left untouched, the app behaves as it would in production.

Request handlers are written with an Express-like routing syntax that supports parameters, wildcards and regular expressions. Responses can set status codes, headers, cookies and delays, or be produced by fully custom resolvers. The same handlers can be reused across unit tests, integration tests, E2E tests and local development, and responses are visible in the browser's DevTools Network tab.

The project provides setupWorker for browser use and setupServer for Node.js, with a documentation site and example repositories for common setups.

Key features

  • 01Intercepts requests at the network level, so application code runs unmodified.
  • 02Works in browsers via Service Worker and in Node.js for tests and servers.
  • 03Express-like route matching with parameters, wildcards and regular expressions.
  • 04Custom status codes, headers, cookies, delays and resolver functions for responses.
  • 05Mocked responses are visible in the browser DevTools Network tab.

Use cases

  • Developing frontend features before the backend API is ready.
  • Writing unit, integration and E2E tests that mock HTTP and GraphQL calls.
  • Simulating slow or failing endpoints to test loading and error states.
  • Running a local Express server against stubbed third-party APIs.

Quick start

  1. 1

    Install the package as a dev dependency

    npm i msw --save-dev
  2. 2

    Read the official quick start guide to set up request handlers

Things to know

  • Browser mocking requires generating and shipping a Service Worker file with your app.

In collections

Testing frameworks & test automation tools

Topics

#api-mocking#testing#javascript#typescript#service-worker#mocking#http#api#devtools#mock

FAQ

What is msw?
A JavaScript API mocking library that intercepts network requests in browsers and Node.js, so app code runs unchanged while tests and development use mock responses. Mock Service Worker (MSW) mocks API requests at the network level. In the browser it uses the Service Worker API to respond to requests with your mock definitions, and in Node.js it uses a low-level interception layer. Because the application's own code, including fetch, axios and data-fetching libraries, is left untouched, the app behaves as it would in production.
Is msw free to use? What license does it have?
msw is open source under the MIT license. Check the license terms before using it in commercial products.
Is msw still maintained?
Actively maintained. The last commit to msw was on Oct 17, 2026.
What can I use msw for?
Developing frontend features before the backend API is ready. Writing unit, integration and E2E tests that mock HTTP and GraphQL calls. Simulating slow or failing endpoints to test loading and error states. Running a local Express server against stubbed third-party APIs.
How do I install msw?
Install the package as a dev dependency: npm i msw --save-dev Read the official quick start guide to set up request handlers
Where can I get msw?
The source code and installation instructions are on GitHub: https://github.com/mswjs/msw

Summary generated by AI from the project's README. GitHub stats refresh daily.

Related projects