Using Promise.all() with mock-inspect to the same endpoint - response is always the same

[thomaschaplin]

Below is a question which I felt suitable to be added to the issues so we can track this and keep a log of the response in case anyone comes across the same question.

Question:

So, I've got some code that makes a number of api calls to the same endpoint with different request bodies concurrently in a Promise.all(). I'm writing tests for this code, mocking the responses with mock-inspect . What I found is that one mockRequest({...}) mocks all of the calls made within the Promise.all and the mocked response is the same for all calls. Is this expected behaviour? I would have assumed it to be possible to mock each of the api calls separately

[thomaschaplin]

Summary answer:

This is not supported by mock-inspect which is caused by our MSW dependency. But to our knowledge, no other mocking library would support this use case either since with Promise.all() there is no specific order. Therefore, no mocking library could know which request should use which response.

In-depth analysis:

This is indeed desired behaviour: By default, mock-inspect uses the same mock handler for all of the requests to that URL and method. This is happening because the persistent flag in the options defaults to “true”.
You could set this to “false” but it would mean that the output is only mocked for one request.

Since we are using MSW, it is not possible to set up different mocks for the same URL&method and them to be taken into account given on the order of how the calls are made. This would work with the library nock which we have been using previously. But even if you were using nock in this case, Promise.all() surely is designed to resolve these requests not in a specific order but just at random - so even nock couldn’t help you here.

So yeah, it would only work if they would for the purpose of tests somehow modify the URL so that MSW can understand what URL should respond with what. This is because MSW only uses the last mock handler which has been called last, it does not have any knowledge about order. nock would have such a knowledge but it wouldn’t work given that Promise.all() works without an order.

So in short, this is not a problem with mock-inspect but more or less with MSW which we depend on. But the problem would even occur with nock (a library we used to use).