Correct, but you as the component writer don't care. From your perspective, you are writing a purely functional component because you have no control over the injected state. So your function is "pure" with respect to the props that are passed in and the state that is injected by React. It's actually UI = F(props, state)
React can (and will) run your component function multiple times with different values before rendering, so it is extremely important that you have no actual side effects. Otherwise, these "intermediate renders" will affect the final render, which breaks the contract that your function return the same values for the same inputs.
> the local variable myState is const but the state is mutable
It is "mutable" in the sense that it changes between invocations of your function. But no, it is immutable within a single render of your function. The fact that you can treat all state as constant is one of the massive advantages of using React. You literally don't have to think about state mutating within a render cycle. You just write a function that spits out JSX given props and state.
> Third, I'm interesting in learning what kind of bugs mutable state would cause in React
You're going to run into issues where:
* the order that your function is called somehow affects the final render. Now you need to worry about how your function is called, and with what values. Not an issue with React.
* the underlying state is out of sync with the UI, because you changed it but didn't trigger a re-render. Not possible in React (except for refs).
* the internal state of the component ends up in an invalid state because of the ordering of set-state calls and renders. Again, not possible in React because all set-states are batched together and applied consistently to the next render cycle. There's no such thing as a "partial render".
