This example shows a smiley face rendered as an SVG that follows the mouse cursor across the full browser viewport. The React component tracks the pointer position through an `onMouseMove` event handler on the SVG element, updating the circle’s `cx` and `cy` coordinates via React state. The code uses React 16.9 with `useState` and `ReactDOM.render`, alongside core SVG primitives such as `<circle>` and `<g>`. The initial mouse position is centered at half the window’s width and height.