I've been using React in projects large and small for years and I can remember VERY FEW situations where I said to myself "gee I wish I could bundle a bunch of state or side effects together and share it between components like React mixins of lore". If you can't think of a situation where you need it, that's probably because you don't. Anything plain old JavaScript class components can't handle here were solved by using HOCs/render prop patterns or React context (often in the form of state management libraries such as Redux).