Jest Mock Generator

jest.mock() replaces an entire module with a mocked version — great for isolating tests from external dependencies. jest.spyOn() wraps an existing method on an object, letting you observe calls and optionally change behavior while preserving the original. Use jest.mock() for heavy isolation, jest.spyOn() for lighter observation with restore capability.

Use mockReturnValue when the mock simply needs to return a fixed value — it's concise and readable. Use mockImplementation when you need conditional logic, access to arguments, or multiple return paths. Use mockReturnValueOnce for chained sequential values: mock.mockReturnValueOnce('first').mockReturnValueOnce('second').

Use mockResolvedValue (equivalent to mockReturnValue(Promise.resolve(value))) or mockRejectedValue (equivalent to mockReturnValue(Promise.reject(error))). For async spies, use jest.spyOn(object, 'method').mockResolvedValue(data). In tests, use await to resolve the promise and assert the result.

Call jest.mock('./MyClass') to auto-mock the class. Then use MyClass as jest.MockedClass<typeof MyClass> to access mock methods. Override methods via MyClass.prototype.method = jest.fn() or use mockImplementation on the constructor to return a custom object. Always call mockClear() in beforeEach to reset call counts.

mockRestore() restores a jest.spyOn() spy back to its original implementation. It only works on spies created with jest.spyOn() — not manual jest.fn() mocks. Call it in afterEach to prevent test pollution. If you forget to restore, later tests may interact with stale spies. jest.restoreAllMocks() in afterEach restores all spies at once.

Use jest.clearAllMocks() to clear call history (calls, instances, results) without removing implementations. jest.resetAllMocks() also removes mock implementations, returning jest.fn(). jest.restoreAllMocks() restores spies created with jest.spyOn(). Configure these automatically in jest.config.js with clearMocks, resetMocks, or restoreMocks: true.