What is the useContext hook?

useContext is a React hook that lets a component read and subscribe to a React Context. Context provides a way to share values (like themes, user data, or locale) across the component tree without passing props through every level ("prop drilling"). Setup: (1) Create context: const ThemeContext = createContext("light");. (2) Provide a value: <ThemeContext.Provider value="dark"><App /></ThemeContext.Provider>. (3) Consume anywhere in the tree: const theme = useContext(ThemeContext);. When the context value changes, all components using that context re-render. This is the main performance concern — if the context value is an object created inline (value={{ user, logout }}), it is a new object every render and causes all consumers to re-render. Fix by memoizing the value or splitting into multiple contexts. Use context for: authentication, themes, localization, and other "ambient" data. For frequently changing data (like a counter), state or external libraries (Zustand, Redux) are better.