React was the first interface library that got me thinking about component-based design. React promotes many good patterns and habits, but this one stands out for me. To understand why, we need to think about how React works under the hood. React is primarily a rendering engine. It was created (and is used) for generating user interfaces.

How interfaces used to work (and indeed still work apart from React) was that someone would come up with a design. That image file would then be split up into assets for each interactive part of the interface. A library such as jQuery would manage user interactions and connect different interface components, often with an assortment of plugins.

Individual interface components can be quite clean and complete individually. However, when they are combined, interactions between components and shared, mutable component state often make a messy codebase. One of the main reasons why React was created was to simplify the interactions between components, so they can remain clean and easy to understand.