This example demonstrates a React component that displays the current value of a counter alongside the value from the previous render using the `useState`, `useRef`, and `useEffect` hooks. The code uses React’s `ReactDOM.render` to mount the component into the `#root` element defined in `index.html`. A custom `prevCountRef` stores the value from the previous state update, which is then displayed in the header. The counter starts at 20 and shows the before/after values on each render.