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The above test is clear and easy to understand, but the reality is that, while this can be applied to a lot of cases, things get very complicated when you have to start to mock things like dependencies, API calls, etc... And one very tricky case is when you have to test a function that invoke some models from an ODM like Mongoose, like doing some CRUD operations against a database. In some cases a solution could be to use an actual test database, so you don't mock anything but you use real data. The only problem with that is that is assuming that you must have a database at your disposal to run unit tests, and that's not always possible, plus you have to clean the database, and a pletora of other problems. Another solution could be to use a database that lives only in memory and only for the duration of your tests, like the excellent mongodb-memory-server package. But while this will work most of the times, if you deploy your code on any CI/CD you might encounter problems (and I did!). Also a key factor of unit testing, is that you shouldn't rely on external services run them, unlike E2E tests for example. What you should do is to mock most of the dependencies you need, as your goal is to just test the function and no deeper than that.