This example demonstrates a React hook that tracks the browser viewport width in real time. It shows how components can adapt their layout based on the current window size, updating whenever the user resizes the browser. The code uses the `useState` and `useEffect` hooks from React, along with a `resize` event listener on the `window` object. The hook returns an object containing the current `width` value, which is initially undefined and then populated with `window.innerWidth` on mount and subsequent resize events. The implementation closely mirrors a `useWindowSize` hook, but only tracks width rather than both dimensions.