React Hooks Principles and Best Practices

React Hooks are functions that let you hook into React state and lifecycle features from function components. Under the hood, React maintains a linked list of hooks for each component instance, which is why hooks must always be called in the same order — hence the rule that you cannot put hooks inside conditions or loops. useState returns a state variable and setter function; React batches state updates for performance. useEffect runs after render and handles side effects — it takes a dependency array that controls when the effect re-runs. An empty array means it runs once on mount; omitting the array means it runs every render. The cleanup function returned from useEffect runs before the next effect and on unmount. useRef creates a mutable ref object whose .current property persists across renders without triggering re-renders — useful for DOM references and storing mutable values. For performance, useMemo memoizes computed values and useCallback memoizes functions, both accepting dependency arrays. They prevent unnecessary recalculations and re-renders, but should not be used everywhere — only when there is a measurable performance benefit, as they add memory overhead. Custom hooks extract reusable logic: for example, useLocalStorage could encapsulate reading/writing to localStorage with state synchronization. The key insight is that custom hooks compose — you can call hooks inside custom hooks, building complex behavior from simple primitives. React 18 added useTransition for non-urgent state updates and useDeferredValue for deferring expensive re-renders.