计算地球表面两点间的距离大概有两种办法。

第一种是默认地球是一个光滑的球面，然后计算任意两点间的距离，这个距离叫做大圆距离(The Great Circle Distance)：

var EARTH_RADIUS = 6378137.0;    //单位m
    var PI = Math.PI;
    
    function getRad(d){
        return d*PI/180.0;
    }

    function getGreatCircleDistance(lat1,lng1,lat2,lng2){
        var radLat1 = getRad(lat1);
        var radLat2 = getRad(lat2);
        
        var a = radLat1 - radLat2;
        var b = getRad(lng1) - getRad(lng2);
        
        var s = 2*Math.asin(Math.sqrt(Math.pow(Math.sin(a/2),2) + Math.cos(radLat1)*Math.cos(radLat2)*Math.pow(Math.sin(b/2),2)));
        s = s*EARTH_RADIUS;
        s = Math.round(s*10000)/10000.0;
                
        return s;
    }

地球并不是一个真正的圆球体，而是椭球，有了下面的公式：

/** 计算两经纬度之间的距离，单位是m
* approx distance between two points on earth ellipsoid
*/
function getFlatternDistance (lat1, lng1, lat2, lng2) {
  const PI = Math.PI
  const EARTH_RADIUS = 6378137.0
  function getRad (d) {
    return d * PI / 180.0
  }
  let f = getRad((lat1 + lat2) / 2)
  let g = getRad((lat1 - lat2) / 2)
  let l = getRad((lng1 - lng2) / 2)
  let sg = Math.sin(g)
  let sl = Math.sin(l)
  let sf = Math.sin(f)

  let s, c, w, r, d, h1, h2
  let a = EARTH_RADIUS
  let fl = 1 / 298.257

  sg = sg * sg
  sl = sl * sl
  sf = sf * sf

  s = sg * (1 - sl) + (1 - sf) * sl
  c = (1 - sg) * (1 - sl) + sf * sl

  w = Math.atan(Math.sqrt(s / c))
  r = Math.sqrt(s * c) / w
  d = 2 * w * a
  h1 = (3 * r - 1) / 2 / c
  h2 = (3 * r + 1) / 2 / s

  return d * (1 + fl * (h1 * sf * (1 - sg) - h2 * (1 - sf) * sg))
}

精度比较

提供一个在线计算经纬度的网站：http://www.hhlink.com/%E7%BB%8F%E7%BA%AC%E5%BA%A6/

let EARTH_RADIUS = 6378137.0;    //单位m
let PI = Math.PI;

function getRad(d){
    return d*PI/180.0;
}

function getGreatCircleDistance(lat1,lng1,lat2,lng2){
    let radLat1 = getRad(lat1);
    let radLat2 = getRad(lat2);

    let a = radLat1 - radLat2;
    let b = getRad(lng1) - getRad(lng2);

    let s = 2*Math.asin(Math.sqrt(Math.pow(Math.sin(a/2),2) + Math.cos(radLat1)*Math.cos(radLat2)*Math.pow(Math.sin(b/2),2)));
    s = s*EARTH_RADIUS;
    s = Math.round(s*10000)/10000.0;

    return s;
}

/** 计算两经纬度之间的距离，单位是m
 * approx distance between two points on earth ellipsoid
 */
function getFlatternDistance (lat1, lng1, lat2, lng2) {
    const PI = Math.PI
    const EARTH_RADIUS = 6378137.0
    function getRad (d) {
        return d * PI / 180.0
    }
    let f = getRad((lat1 + lat2) / 2)
    let g = getRad((lat1 - lat2) / 2)
    let l = getRad((lng1 - lng2) / 2)
    let sg = Math.sin(g)
    let sl = Math.sin(l)
    let sf = Math.sin(f)

    let s, c, w, r, d, h1, h2
    let a = EARTH_RADIUS
    let fl = 1 / 298.257

    sg = sg * sg
    sl = sl * sl
    sf = sf * sf

    s = sg * (1 - sl) + (1 - sf) * sl
    c = (1 - sg) * (1 - sl) + sf * sl

    w = Math.atan(Math.sqrt(s / c))
    r = Math.sqrt(s * c) / w
    d = 2 * w * a
    h1 = (3 * r - 1) / 2 / c
    h2 = (3 * r + 1) / 2 / s

    return d * (1 + fl * (h1 * sf * (1 - sg) - h2 * (1 - sf) * sg))
}

let result1 = getGreatCircleDistance(30.1234, 140.1234, 30.3456, 140.3456)
let result2 = getFlatternDistance(30.1234, 140.1234, 30.3456, 140.3456)
console.log(result1) // 32688.3298
console.log(result2) // 32622.43244078783

可以看到两种方法计算出的距离还是比较接近的，做项目时我也跟后台开发确认过，他的计算结果与第二种计算方法吻合度好。