Why isn't the Nextjs state updating when the route is changed?

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The Nextjs state is not updating when the route is changed because Nextjs uses server-side rendering and client-side rendering. When the route changes, the client-side rendering takes over but the state is not carried over from the server-side rendering. Therefore, if you want to update the state when the route changes, you need to use a React hook or a global state management library such as Redux or MobX.