In that case, overriding the implementation allows us to create test cases that cover the relevant code paths. This is useful when the code under tests relies on the output of a mocked function.
#JEST RESET MOCKS HOW TO#
Now we’ll see how to set the implementation of a mock or spy using mockImplementation and mockImplementationOnce. We’ve looked at how to make sure call information is cleared between tests using jest.clearAllMocks(). Simple setting of mocked function output with mockImplementation In situation where one might use resetAllMocks/ mockReset, I opt for mockImplementationOnce/ mockReturnValueOnce/ mockResolvedValueOnce in order to set the behaviour of the stub for a specific test instead of resetting said mock. I’ve personally not found mockReset's use case to be too compelling. mockReset resets to mock to its initial implementation, on a spy makes the implementation be a noop (function that does nothing).mockClear clears only data pertaining to mock calls, which means we get a fresh dataset to assert over with toHaveBeenX methods.(Note that resetting a spy will result in a function with no return value). This is useful when you want to completely reset a mock back to its initial state. expect (method1 ()). This helps Jest correctly mock an ES6 module that uses a default export. We’ve just seen the clearAllMocks definition as per the Jest docs, here’s the mockReset() definition:ĭoes everything that mockFn.mockClear() does, and also removes any mocked return values or implementations. In order to successfully mock a module with a default export, we need to return an object that contains a property for esModule: true and then a property for the default export. Jest mockReset/resetAllMocks vs mockClear/clearAllMocks Reset/Clear with beforeEach/beforeAll and clearAllMocks/resetAllMocksĪssuming we have a global stub or spy that is potentially called mutliple times throughout our tests.īeforeEach (() => ) // testsĬlears the mock.calls and mock.instances properties of all mocks. This section goes through how to set, reset and clear mocks, stubs and spies in Jest using techniques such as the beforeEach hook and methods such as jest.clearAllMocks and jest.resetAllMocks. This is why we want to be able to set and modify the implementation and return value of functions in Jest. HTTP requests, database reads and writes are side-effects that are crucial to writing applications. Systems are inherently side-effectful (things that are not parameters or output values). In unit tests of complex systems, it’s not always possible to keep business logic in pure functions, where the only input are the parameters and the only output is the return value. This is a way to mitigate what little statefulness is in the system. Between test runs we need mocked/spied on imports and functions to be reset so that assertions don’t fail due to stale calls (from a previous test).
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