When I ask you about "What is ReactJS?", the first thing that comes to mind is "components". And the second is "reusability". Whenever you start a new project with React, you always do some clean-up work for your project. Deleting test files, deleting comments, deleting CSS codes, and even deleting the content of "App.js" files. That is not the only case for me. I am doing this clean-up work as well. However, I am also installing some libraries immediately. I don't even ask myself that "Am I gonna need this library in this project?". There are some libraries that are extremely useful and necessary to develop a new React project. In my opinion, those libraries are "must".