A reactive swarm of animated circles demonstrates D3’s data join, updating 60 times per second via `setInterval`. Each circle’s x-position, y-position, and radius are recomputed from a sine wave, with the count oscillating between 5 and 15. The code uses `d3.select` to append an SVG, `d3.range` for data generation, and `selection.join('circle')` to create or update elements in place. The animation runs seamlessly in a React project, showing how D3’s selection methods handle dynamic data without explicit enter or exit handling.