What is the useRef hook?

useRef returns a mutable ref object whose .current property persists across renders without causing re-renders when changed. Two primary uses: (1) Accessing DOM elements: const inputRef = useRef(null); <input ref={inputRef} /> — after mounting, inputRef.current is the DOM input element. Use this to imperatively focus, scroll, or measure elements: inputRef.current.focus(). (2) Storing mutable values that persist across renders but do not trigger re-renders — unlike state. Use cases: storing previous state values, interval/timeout IDs for cleanup, debounce timers, tracking if a component is mounted. Example: const countRef = useRef(0); countRef.current++; — this increments but does NOT cause a re-render. Key difference from state: changing ref.current does not trigger a re-render; changing state does. Do not read or write refs during rendering — refs are for imperative operations. useRef(initialValue) only uses the initial value on the first render.