How do I make API methods throw an exception

[Denis Tuzhik]

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I am trying to write a test, which assumes that sometimes getItem or setItem throw, this can happen f.e. if you are browser LS is running out of space.

What I've tried:

• localStorage.setItem.mockImplementation(() => { throw new Error('error') })
• jest.spyOn(localStorage, 'setItem').mockImplementation(() => { throw new Error('error') })

Both examples work, however I cannot restore mocks to original behaviour defined in the library, I've tried:

• jest.resetAllMocks or localStorage.setItem.mockReset for 1st example
• jest.restoreAllMocks or localStorage.setItem.mockRestore for 2nd example

Am I missing something obvious or is it methods defined through defineProperty prevents me from doing this?

[Bryan Clark]

I would have thought the first approach with mockReset would have worked. 🤔

[Denis Tuzhik]

In a way it does, it just doesn't work properly after reset. I submitted failed test.

[Daniel Stockman]

localStorage.setItem.mockImplementationOnce(() => { throw new Error('error'); }); should work, I think? mockImplementationOnce doesn't replace the default jest.fn(impl) (which is effectively an alias for jest.fn().mockImplementation(impl)), and automatically clears itself from the mock when triggered, thus preserving the default implementation for other tests.

[Bryan Clark]

☝️ seems to work for me, closing this out.