haversineDistance.js

function haversineDistance(latitude1, longitude1, latitude2, longitude2, unit) {
   if(typeof latitude1 === "undefined" || typeof longitude1 === "undefined" || typeof latitude2 === "undefined" || typeof longitude2 === "undefined") return -1;
   unit = (typeof unit === "undefined") ? "km" : unit;
   // convert to Number
   latitude1 = Number(latitude1);
   longitude1 = Number(longitude1);
   latitude2 = Number(latitude2);
   longitude2 = Number(longitude2);
   var R = 6371;
   // Radius of the earth in km
   var dLat = deg2rad(latitude2 - latitude1);
   // deg2rad below
   var dLon = deg2rad(longitude2 - longitude1);
   var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.cos(deg2rad(latitude1)) * Math.cos(deg2rad(latitude2)) * Math.sin(dLon / 2) * Math.sin(dLon / 2);
   var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
   // Distance in km
   var d = R * c;

   switch (unit) {
      case "km":
         return d;
      case "m":
         return d * 1000;
      case "nm":
         return d / 1.852;
      case "miles":
         return d / 1.609344;
      case "feet":
         return d / 0.0003048;
   }
}

function deg2rad(deg) {
   return deg * (Math.PI / 180);
}