SVGCircleElement

<svg
  xmlns="http://www.w3.org/2000/svg"
  viewBox="0 0 250 250"
  width="250"
  height="250">
  <circle cx="100" cy="100" r="50" fill="gold" id="circle" />
</svg>

const circle = document.getElementById("circle");

function clickCircle() {
  // Randomly determine if the circle radius will increase or decrease
  const change = Math.random() > 0.5 ? 10 : -10;
  // Clamp the circle radius to a minimum of 10 and a maximum of 250,
  // so it won't disappear or get bigger than the viewport
  const newValue = Math.min(Math.max(circle.r.baseVal.value + change, 10), 250);
  circle.setAttribute("r", newValue);
}

circle.addEventListener("click", clickCircle);