js计算某个经纬度不同角度和距离的坐标

/**
 * 度换成弧度
 * @param  {Float} d  度
 * @return {[Float}   弧度
 */
function rad (d){
    
    
    return d * Math.PI / 180.0;
};
/**
 * 弧度换成度
 * @param  {Float} x 弧度
 * @return {Float}   度
 */
function deg (x){
    
    
    return x * 180 / Math.PI;
};
/**
 *
 * @param {*} lng 经度 113.3960698
 * @param {*} lat 纬度 22.941386
 * @param {*} brng 方位角 45   ---- 正北方：000°或360°  正东方：090° 正南方：180°  正西方：270°
 * @param {*} dist 距离 9000
 *
 */
function getLngAndLat (lng,lat,brng,dist){
    
    
    //大地坐标系资料WGS-84 长半径a=6378137 短半径b=6356752.3142 扁率f=1/298.2572236
    var a=6378137;
    var b=6356752.3142;
    var f=1/298.257223563;

    var alpha1 = rad(brng);
    var sinAlpha1 = Math.sin(alpha1);
    var cosAlpha1 = Math.cos(alpha1);

    var tanU1 = (1 - f) * Math.tan(rad(lat));
    var cosU1 = 1 / Math.sqrt((1 + tanU1 * tanU1));
    var sinU1 = tanU1 * cosU1;
    var sigma1 = Math.atan2(tanU1, cosAlpha1);
    var sinAlpha = cosU1 * sinAlpha1;
    var cosSqAlpha = 1 - sinAlpha * sinAlpha;
    var uSq = cosSqAlpha * (a * a - b * b) / (b * b);
    var A = 1 + uSq / 16384 * (4096 + uSq * (-768 + uSq * (320 - 175 * uSq)));
    var B = uSq / 1024 * (256 + uSq * (-128 + uSq * (74 - 47 * uSq)));

    var cos2SigmaM=0;
    var sinSigma=0;
    var cosSigma=0;
    var sigma = dist / (b * A), sigmaP = 2 * Math.PI;
    while (Math.abs(sigma - sigmaP) > 1e-12) {
    
    
        cos2SigmaM = Math.cos(2 * sigma1 + sigma);
        sinSigma = Math.sin(sigma);
        cosSigma = Math.cos(sigma);
        var deltaSigma = B * sinSigma * (cos2SigmaM + B / 4 * (cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM)
            - B / 6 * cos2SigmaM * (-3 + 4 * sinSigma * sinSigma) * (-3 + 4 * cos2SigmaM * cos2SigmaM)));
        sigmaP = sigma;
        sigma = dist / (b * A) + deltaSigma;
    }

    var tmp = sinU1 * sinSigma - cosU1 * cosSigma * cosAlpha1;
    var lat2 = Math.atan2(sinU1 * cosSigma + cosU1 * sinSigma * cosAlpha1,
        (1 - f) * Math.sqrt(sinAlpha * sinAlpha + tmp * tmp));
    var lambda = Math.atan2(sinSigma * sinAlpha1, cosU1 * cosSigma - sinU1 * sinSigma * cosAlpha1);
    var C = f / 16 * cosSqAlpha * (4 + f * (4 - 3 * cosSqAlpha));
    var L = lambda - (1 - C) * f * sinAlpha
        * (sigma + C * sinSigma * (cos2SigmaM + C * cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM)));

    var revAz = Math.atan2(sinAlpha, -tmp); // final bearing
    var lngLatObj = [keepSixDecimal(lng+deg(L)),keepSixDecimal(deg(lat2))];
    return lngLatObj;
}