In which cases should you use local state and in which cases - global state?

Local state is used for data specific to a single component and not affecting other parts of the application. Examples include: the open/closed state of a modal window, the content of a form input field, the loading state for a specific item.

Global state is used for data that is shared among multiple components and requires centralized management. Examples include: user data after login, shopping cart, application theme settings, the loading state for the entire application.

The choice depends on how broad the scope of the data is and the number of components that need access and modification of this data. Global state is often managed using libraries like Redux, MobX, or Context API. Using local state where sufficient simplifies components and improves performance. Using global state for shared data helps avoid "prop drilling" and simplifies state management at the application level.