Sinon.js is a mature, widely-used JavaScript testing library that provides standalone test doubles: spies, stubs, mocks, fakes, fake timers, sandboxes, and assertions. It is designed to be test-framework agnostic and works in both Node.js and browser environments. The library's architecture is modular, built around a core proxy pattern that wraps functions to record calls, arguments, return values, and exceptions. Behaviors (return values, throws, callbacks, etc.) are defined separately and can be chained per invocation. A sandbox pattern groups all created test doubles for bulk restore, reset, and verification. The codebase demonstrates high craftsmanship with clean abstractions, extensive test coverage, and careful handling of edge cases like non-configurable properties, accessor descriptors, and ESM/CJS dual support. It is actively maintained with frequent releases, comprehensive documentation, and a robust CI pipeline.