msw
API mocking library for JavaScript that intercepts requests at the network level using Service Workers.
What is msw?
Mock Service Worker (MSW) is an API mocking library for JavaScript that intercepts requests at the network level, enabling seamless mocking for development, testing, and debugging. It works in both browser and Node.js environments, allowing developers to reuse the same mock definitions across different tools and stages of the development lifecycle. It uses a Service Worker in browsers and a low-level interception algorithm in Node.js to leave application code intact.
JavaScript developers building web applications who need to mock API calls for local development, unit testing, integration testing, and end-to-end testing without modifying their application's request logic. It is particularly useful for teams aiming to maintain a single source of truth for network behavior across environments.
Developers choose MSW because it intercepts requests externally at the network level, avoiding the need to stub internal libraries like fetch or axios, which keeps application code production-like and unaware of mocking. Its environment-agnostic design allows the same mock definitions to be reused across browsers, Node.js, and various testing frameworks, reducing configuration overhead and increasing confidence in tests.
Overview
Industry standard API mocking for JavaScript.
Use Cases
Best For
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