Sinon is a JavaScript test double library and mocking framework used to isolate code and verify behavior during unit testing. It provides a collection of spies, stubs, and mocks that replace real function implementations and object methods with controlled substitutes.
The project includes a specialized fake timer library for overriding the system clock, allowing the simulation of time progression to trigger timeouts and intervals without waiting for real-time delays. It also features a test sandbox mechanism to group multiple fakes together, ensuring the original state is restored between test cases to prevent environment leakage.
The toolkit covers a broad range of verification capabilities, including recording function call arguments and invocation counts, simulating dependency behaviors through predefined return values or errors, and verifying object interactions. It further supports intercepting network requests and stubbing object instances and accessors.
