A smiley face is rendered as a single SVG circle that follows the mouse cursor across a 960-by-500 viewport. The circle’s center coordinates update on every mouse movement via React state and the `onMouseMove` event handler. The example uses React 16.9 and D3 v7’s `arc` import (though unused), with SVG rendering managed by ReactDOM. The circle starts at the center and tracks the pointer’s client coordinates in real time.