Note that derived state must set be inside a condition like `prevCount !== count`, and there must be a call like `setPrevCount(count)` inside of it. Otherwise, your component would re-render in a loop until it crashes.
When you can, try to avoid derived state. However, derived state is better than updating state in an effect. This is because calling the `set` function in render restarts a render immediately without waiting for child components.
If you can, try to avoid derived state. However, derived state is better than updating state in an effect. When you call the `set` function during render, React will render that component immediately after your component exits with a `return` statement. React will ignore the initial output so it won't need to render the children twice.
Dan Abramov
3 years ago