function createCountdown(elementId, seconds, onEnd) {
  const element = document.getElementById(elementId);
  let remaining = seconds;

  // 更新显示
  function updateDisplay() {
    element.textContent = remaining;
  }

  // 开始倒计时
  const timer = setInterval(() => {
    remaining--;
    updateDisplay();

    if (remaining <= 0) {
      clearInterval(timer);
      if (onEnd && typeof onEnd === 'function') {
        onEnd();
      }
    }
  }, 1000);

  // 初始化显示
  updateDisplay();

  // 返回控制方法（可选）
  return {
    pause: () => clearInterval(timer),
    reset: (newSeconds) => {
      remaining = newSeconds || seconds;
      updateDisplay();
    }
  };
}

window.onload = function() {
  createCountdown('countdown', 10, function() {
    document.getElementById('countdown').textContent = '时间到！';
    alert('倒计时结束');
  });
};