I shall respectfully disagree with you here. I mean, yes, obviously that is literally what mocks are for, but I have never found mocks to be a particularly effective or valuable tool for testing. They can take a disproportionate amount of time to write and maintain if whatever real external system they stand in for is complicated. They are inherently fragile if that system is subject to change. Most importantly, even if those tests pass, you don't actually know whether your real system is going to work, and IMHO the greatest benefits of automated testing are found where you can systematically and repeatably exercise exactly the behaviour and interactions you might see in production.
I shall respectfully disagree with you here. I mean, yes, obviously that is literally what mocks are for, but I have never found mocks to be a particularly effective or valuable tool for testing. They can take a disproportionate amount of time to write and maintain if whatever real external system they stand in for is complicated. They are inherently fragile if that system is subject to change. Most importantly, even if those tests pass, you don't actually know whether your real system is going to work, and IMHO the greatest benefits of automated testing are found where you can systematically and repeatably exercise exactly the behaviour and interactions you might see in production.