High performance JS function to return distance between two points using Haversine formula

haversine-distance.js

function distance(lat1, lon1, lat2, lon2) {
  var deg2rad = Math.PI / 180;
  lat1 *= deg2rad;
  lon1 *= deg2rad;
  lat2 *= deg2rad;
  lon2 *= deg2rad;
  var diam = 12742; // Diameter of the earth in km (2 * 6371)
  var dLat = lat2 - lat1;
  var dLon = lon2 - lon1;
  var a = (
    (1 - Math.cos(dLat)) +
    (1 - Math.cos(dLon)) * Math.cos(lat1) * Math.cos(lat2)
  ) / 2;

  return diam * Math.asin(Math.sqrt(a));
}

distance(48, -122, 49, -121);

Tested on Chrome 47.0.2526.106 on Mac OS X 10.11.2 to perform ~7.4million ops/sec as of 12-01-16