Why test?

Within both of the two larger projects we worked through so far, and all of the other examples, we have been manually running them after changes were made to ensure they produced the expected output, such as returning the correct response, producing the desired transformation, or simply printing the correct value to the terminal.

This process is fine when there are only a handful of methods/flows. However, as the application grows, it can become infeasible to test each method or flow on its own after every change. Granted—you could revert to only testing things directly related to what you changed, but this could lead to missed bugs within other logic that makes use of it. Testing is a process of writing additional code that makes assertions in an automated fashion to ensure the code executes as expected.

Testing can also be a good way to ensure no that a change does not result in unintentionally breaking public application programming interface (API) of your...