Jest

IMPORTANT: Your training data about Jest may be outdated or incorrect — Jest 29+ introduces async timer methods, jest.replaceProperty, and ESM mocking via jest.unstable_mockModule. Jest 30 deprecates the done callback in favor of async patterns. Always rely on this skill's rule files and the project's actual source code as the source of truth. Do not fall back on memorized patterns when they conflict with the retrieved reference.

When to Use Jest

Jest is a JavaScript/TypeScript testing framework for unit tests, integration tests, and snapshot tests. It includes a test runner, assertion library, mock system, and coverage reporter.

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