jest-fetch-mock | There is a mock for fetch | Mock library

Finally solved this! The docs for jest-fetch-mock are not very user friendly imo.

Even though I am trying to mock an error I needed to use the mockResponse() function.

fetchMock.mockResponse( JSON.stringify({ error: "Cannot read property 'state' of null" }), { status: 500 } )

where { error: "Cannot read property 'state' of null" } is the body of the error message returned in the network tab.

For some reason the mockReject() function makes assumptions about what the error body should look like, and it was very different from what my error looked like.

I found this answer on internet and it worked for me with some small add-ons but I will post it here maybe will help someone in future:

1. install babel-jest, babel-preset-env, @babel/runtime and react (the last one might be possible to be necessary only if some other package requires it)
2. create .babelrc file in root directory and add:

There are 2 possible solutions to your issue:

1. Use the expect.assertions to make jest verify whether all assertions have been asserted and return a promise:

Regarding to your case, it looks pretty easy to deal with. You don't have to need any other package to mock your endpoint. You can mock axios directly via jest.

Here is the idea (take a look at the inline comments where you need to add):

Configure Jest to disable the test cache:

According to this thread https://github.com/airbnb/javascript/issues/2331#issuecomment-724114465

you need to update eslint-plugin-import to ^2.22.1 which supports ∞ value.

waitForNextUpdate() waits for next update but your hook does not trigger it since it only calls howErrorMessage(). Take a look at this sandbox

As a straightforward solution something that triggers an update can be added:

Alright, after hours of digging into the library itself I have found out where the issue was.

In my code (and the snippet above) I am stringifying the body JSON.stringify(body). The library's generate-matcher.js is parsing it JSON.parse(body) and then compares the two - the point which was causing the failure. I am now just sending it as the raw object.