js 用requestAnimationFrame api实现30s倒计时

可以使用以下代码实现30秒倒计时：

let countDown = 30;
let startTime = null;

function animate(timestamp) {
  if (!startTime) startTime = timestamp;
  const elapsedTime = timestamp - startTime;
  const seconds = Math.floor((countDown * 1000 - elapsedTime) / 1000);
  console.log(seconds);
  if (elapsedTime < countDown * 1000) {
    requestAnimationFrame(animate);
  }
}

requestAnimationFrame(animate);