It's all tradeoffs. Glue code should really only be tested in your integration or E2E tests, you have those, don't you? Now, if A would look like below, we mi

It's all tradeoffs. Glue code should really only be tested in your integration or E2E tests, you have those, don't you? Now, if A would look like below, we might accept not testing it at all, as the initial and upkeep costs to mock B might be higher then the potential cost of the one-time failure in the internal logic of A. function A(args) { return B(args) > 1 ? true : false } But, if A looks more like: function A(args) { const result = B(args); switch (result) { case 1: case 3: return result * .5; case 2: return result * result / 5; } } I would probably look to convert it to function A(args) { return A1(B(args)); } enabling me to unit test A1 without thinking about B (and without the cost associated in mocking it)