The Rules of React
All libraries have subtle rules that you have to follow for them to work well. Often these are implied and undocumented rules that you have to learn as you go. This is an attempt to document the rules of React renders. Ideally a type system could enforce it.
What Functions Are "Pure"?
A number of methods in React are assumed to be "pure".
On classes that's the constructor, getDerivedStateFromProps, shouldComponentUpdate and render.