HTML5 geolocation geofence location detection (without geofence API)

fence.js

/* PLEASE NOTE
   THE JS HAS BEEN ALTERED FOR RELEVANCE - simply copy and paste into your own JS and watch the console.
   THE HTML AND CSS ARE WHOLLY UNUSED FOR THIS PARTICULAR CASE
   please refer to the original gist here: https://gist.github.com/bmoren/22470de1cd47410e0902 for relevant jquery selectors and stuff.
 */


window.onload = function() {
        var startPos;
        var startPosLat;
        var startPosLong;
        var distance;
      
         // Check if browser has geolocation capability       
        if (navigator.geolocation) {

          startPosLat = 44.95716993150707; //target latitude of Type:Number
          startPosLong = -93.28439280496818; // target longitude of Type:Number
           
           
         // We dont need to watch position. just get the position on page load, ONCE!
         /*
            navigator.geolocation.getCurrentPosition(success, error, options);
         */            
           let success  = function(position) {
            // get the current position of the device               
            console.log(position.coords.latitude + " is my current latitude");
            console.log(position.coords.longitude+ " is my current longitude");
              
            // Calculate distance function to compare difference between your start position and where you are.      
            distance = calculateDistance(startPosLat, startPosLong,position.coords.latitude, position.coords.longitude)
           console.log(distance);
              
            // SUBSIDE, BRUHERIA!
              // Do what you want, play around
            if(distance < .05){
              console.log("Yes, were inside .05 KM!!! :) A+")
            } 
            if(distance > .05){
              console.log("No, not inside .05 KM :(")
              
            }
          }
           
         // Create an optional error handler
           let error = function(err){
              console.warn(`ERROR(${err.code}): ${err.message}`);
           }
          navigator.geolocation.getCurrentPosition(success, error);
        }
      };
      
      // Reused code - copyright Moveable Type Scripts - retrieved May 4, 2010.
      // http://www.movable-type.co.uk/scripts/latlong.html
      // Under Creative Commons License http://creativecommons.org/licenses/by/3.0/
      function calculateDistance(lat1, lon1, lat2, lon2) {
        var R = 6371; // km
        var dLat = (lat2-lat1).toRad();
        var dLon = (lon2-lon1).toRad();
        var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
                Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) *
                Math.sin(dLon/2) * Math.sin(dLon/2);
        var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
        var d = R * c;
        return d;
      }
      Number.prototype.toRad = function() {
        return this * Math.PI / 180;
      }