There are cases where our code is not the only thing that will be present in the context of our tests. After all, the systems we design and build have to do something real, and that usually means connecting to external services (databases, storage services, external APIs, cloud services, and so on). Because they need to have those side-effects, they're inevitable. As much as we abstract our code, program towards interfaces, and isolate code from external factors in order to minimize side-effects, they will be present in our tests, and we need an effective way to handle that.
Mock objects are one of the best tactics to defend against undesired side-effects. Our code might need to perform an HTTP request or send a notification ...
