Comment by Lars Ejaas on "Mocking in Jest with TypeScript and React" | Hashnode

Lars Ejaas

Front end developer

Jan 23, 2022

Hi Rubert

Finally got to try this out in a project. I am however struggling with the following error:

TypeError: xxxxx.mockImplementation is not a function

Have you seen something like this before? Do you need to setup something specifically to handle mocks in jest.setup.ts or jest.config.js?

Rupert McKay

Staff Software Engineer at Lokalise

Jan 24, 2022

In general, any time you see "foo is not a function" that suggests that you have an undefined reference.

If you've followed along with the post above, the possible points of failure are either forgetting to jest.mock the relevant module path (e.g. jest.mock('my/path')) or forgetting to cast a mock type using the original type as the base (e.g. const mockFoo = foo as jest.MockedFunction<typeof foo>).

Rupert 'fildon' McKay Thanks Rupert. I will give it another go! I have another syntax to use, but yours looks so much more elegant, and I sometimes get edgecases where your pattern would be very helpful. Thanks again for a great article! There is way to little material out there on writing good tests. Best regards Lars

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