Issue with calculating compass bearing between two GPS coordinates

Question

In my webapp, have a JSON data response from a database query that includes the lat/long coordinates of 1 to n locations. I want to calculate the bearing from the data[i]

Answer 1

This is an edit of the accepted answer with some modifications which made it work for me (mainly the use of toRad function on lat,lng values).

    var geo = {
        /**
         * Calculate the bearing between two positions as a value from 0-360
         *
         * @param lat1 - The latitude of the first position
         * @param lng1 - The longitude of the first position
         * @param lat2 - The latitude of the second position
         * @param lng2 - The longitude of the second position
         *
         * @return int - The bearing between 0 and 360
         */
        bearing : function (lat1,lng1,lat2,lng2) {
            var dLon = this._toRad(lng2-lng1);
            var y = Math.sin(dLon) * Math.cos(this._toRad(lat2));
            var x = Math.cos(this._toRad(lat1))*Math.sin(this._toRad(lat2)) - Math.sin(this._toRad(lat1))*Math.cos(this._toRad(lat2))*Math.cos(dLon);
            var brng = this._toDeg(Math.atan2(y, x));
            return ((brng + 360) % 360);
        },

       /**
         * Since not all browsers implement this we have our own utility that will
         * convert from degrees into radians
         *
         * @param deg - The degrees to be converted into radians
         * @return radians
         */
        _toRad : function(deg) {
             return deg * Math.PI / 180;
        },

        /**
         * Since not all browsers implement this we have our own utility that will
         * convert from radians into degrees
         *
         * @param rad - The radians to be converted into degrees
         * @return degrees
         */
        _toDeg : function(rad) {
            return rad * 180 / Math.PI;
        },
    };

    /** Usage **/
    var myInitialBearing = geo.bearing(0,0,45,45);

Find theory and online calculator at: http://www.movable-type.co.uk/scripts/latlong.html