It's good that I can learn TDD step by step. In particular, it's easier to understand and follow the lectures because I watch the lectures with the lecture materials. I'm very satisfied because the price is cheap, but the reason I took off one star is that I wish you could explain the values returned by express or mongoose objects in a little more detail. Aside from TDD, I had to look up what the original return value of a certain library's method is and whether I should set mockReturnValue in the test code. In a way, Googling is a part of the learning process, but I want to focus more on TDD, so there are times when I have to look up references to find out the return value and parameters of a certain module.