JavaScript Calculate Distance Between Latitude Longitude Points

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<head>
<title>Javascript Demo</title>
<script type="text/javascript">
function getDistanceFromLatLonInKm(latitude1,longitude1,latitude2,longitude2,units) {
  var p = 0.017453292519943295;    //This is  Math.PI / 180
  var c = Math.cos;
  var a = 0.5 - c((latitude2 - latitude1) * p)/2 + 
          c(latitude1 * p) * c(latitude2 * p) * 
          (1 - c((longitude2 - longitude1) * p))/2;
  var R = 6371; //  Earth distance in km so it will return the distance in km
 var dist = 2 * R * Math.asin(Math.sqrt(a)); 
document.getElementById("output").innerHTML = dist+"Km"; 
}
</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>