JavaScript Calculate Distance Between Latitude Longitude Points

<html>
<head>
<title>Javascript Demo</title>
<script type="text/javascript">
function getDistanceFromLatLonInKm(latitude1,longitude1,latitude2,longitude2,units) {
  var earthRadius = 6371; // Radius of the earth in km
  var dLat = deg2rad(latitude2-latitude1);  
  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)); 
  var d = earthRadius * c; 
  var miles = d / 1.609344; 

if ( units == 'km' ) {  
document.getElementById("output").innerHTML = d+"Km"; 
 } else {
document.getElementById("output").innerHTML = miles+"Miles"; 
}
}
</script>
</head>
<body>
<p>Latitude 1 = 40.7128, Longitude 1 = 74.0059 (New York) </p>
<p>Latitude 2 = 51.5074, Longitude 2 = 0.1278 (London)</p>

<p>Distance Between London And New York is <span id="output"></span></p>
<a href="javascript:void(0)" onclick="getDistanceFromLatLonInKm(40.7128,74.0059,51.5074,0.1278,'km')">Get Distance</a>
</body>
</html>