把半径换成以千米为单位的距离

如果要将半径换成以千米为单位的距离，只需要在计算距离时，将计算结果除以1000即可。下面是相应的修改后的代码：

function calculateDistance(lat1, lon1, lat2, lon2) {
  const R = 6371; // 地球半径，单位为千米
  const dLat = (lat2 - lat1) * Math.PI / 180;
  const dLon = (lon2 - lon1) * Math.PI / 180;
  const a =
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos(lat1 * Math.PI / 180) * Math.cos(lat2 * Math.PI / 180) *
    Math.sin(dLon / 2) * Math.sin(dLon / 2);
  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  const distance = R * c;
  return distance;
}

function checkPointInCircle(lat, lon, circleLat, circleLon, radius) { // 判断点是否在圆内
  return calculateDistance(lat, lon, circleLat, circleLon) <= radius;
}

function calculateNearestCircleDistance(lat, lon, circles) { // 计算距离最近的圆的距离
  let minDistance = Number.MAX_VALUE;
  for (let i = 0; i < circles.length; i++) {
    const circle = circles[i];
    const distance = (calculateDistance(lat, lon, circle.lat, circle.lon) - circle.radius) / 1000;
    if (distance < minDistance) {
      minDistance = distance;
    }
  }
  return minDistance;
}

// 示例数据
const pointLat = 31.2304;
const pointLon = 121.4737;
const circles = [
  { lat: 31.2165, lon: 121.4365, radius: 2000 }, // 单位为米
  { lat: 31.2460, lon: 121.4742, radius: 1000 },
  { lat: 31.2359, lon: 121.5090, radius: 3000 }
];

console.log(`距离最近的圆的距离为：${calculateNearestCircleDistance(pointLat, pointLon, circles)}千米`);

let isInCircle = false;
for (let i = 0; i < circles.length; i++) {
  const circle = circles[i];
  if (checkPointInCircle(pointLat, pointLon, circle.lat, circle.lon, circle.radius / 1000)) { // 单位为千米
    isInCircle = true;
    break;
  }
}
console.log(`该点${isInCircle ? "在" : "不在"}圆内`);

在这个实现中，半径的单位是米，使用时需要除以1000得到千米。另外需要注意的是，在 `checkPointInCircle` 函数中，需要将圆的半径从米转换为千米。