Mock Service Worker

Mock Service Worker is an API mocking library for JavaScript applications. It uses Service Workers to intercept requests on the network level, once they were actually dispatched and left your application. Because of this, you don't have to alter your application for the sake of mocking.

By introducing a standardized way to write mocks — request handlers — MSW allows you to reuse the same mock definitions in multiple environments and for various purposes: testing, development, debugging. 

Rich support for REST and GraphQL API makes it silly-easy to cover your use cases and enable fast prototyping via a mock-first development.