17. useRef and mutable values

`useRef` holds a mutable `.current` value that survives re-renders without causing one when changed — used for timers, previous values, or references to native components.

import { useRef, useEffect } from "react";

function usePrevious<T>(value: T): T | undefined {
  const ref = useRef<T>();
  useEffect(() => { ref.current = value; }, [value]);
  return ref.current;
}

Refs are also how you imperatively call methods on native components, e.g. `inputRef.current?.focus()` on a `TextInput` after a modal opens.

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