sinon

Sinon.js provides standalone test spies, stubs, and mocks for JavaScript that work with any test framework. Spies record information about function calls — arguments, return values, this context, and exceptions thrown — without changing the function's behavior. Stubs are spies with pre-programmed behavior that replace the target function entirely, useful for controlling function output and preventing side effects like network calls or file system access during tests. Mocks combine spies and stubs with built-in expectations that are verified at the end of the test. Sinon also includes a fake timer system for controlling setTimeout, setInterval, Date, and requestAnimationFrame, fake XMLHttpRequest and server implementations for intercepting HTTP requests, and sandboxes for automatically restoring all fakes after each test. Sinon works with any assertion library and test runner — it is most commonly paired with Mocha and Chai but integrates well with Jest and Vitest too. The library's approach to test doubles follows the patterns described by Gerard Meszaros in xUnit Test Patterns, providing a comprehensive toolkit for isolating code under test from its dependencies.