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In this article, we’ll cover the simplest and quickest way of mocking any dependency-external or internal-with Jest just by calling jest.mock, without changing the implementation of the modules. There is plenty of helpful methods on returned Jest mock to control its input, output and implementation. If no implementation is provided, it will return the undefined value. It is the equivalent of manually calling mockReset on every mock you have (which can be tedious if you have a lot of them). to call jest.clearAllMocks to clear all mocks after each test. The simplest and most common way of creating a mock is jest.fn () method. The jest.resetAllMocks method resets the state of all mocks in use in your tests. to call to clear the mocked local.getData method after each test by calling it in the afterEach callback.

Due to Jest’s extensive list of features, the auto mock feature can be easily missed-especially because the documentation doesn’t explicitly focus on it (it’s mentioned in the The Jest Object, Mock Function and ES6 Class Mocks sections). To reset Jest mock functions calls count before every test with JavaScript, we can call mockClear on the mocked function or clearAllMocks to clear all mocks. Note: mockRestore works only with mocks created by jest.spyOn.

One that is very powerful and commonly used in unit tests is the auto mock feature, which is when Jest automatically mocks everything exported by a module that is imported as a dependency by any module we are testing. For one mock: fn.mockClear() // Clears mock usage date (fn.mock.calls, fn.mock.instances) fn.mockReset() // Clears and removes any mocked return values or implementations fn.mockRestore() // Resets and restores the initial implementation. The function checks if that the size is lower than 10,000 bytes, and returns boolean. Jest offers many features out of the box. I’m going to explain this problem with a function that validates an image file size.