JavaScript: Distance between two coordinates

distance_between.js

 /**
  * Distance between two coordinates
  *
  * @author Salvador Dali
  *
  * @param { number } lat_1 - Latitude of first point
  * @param { number } lon_1 - Longitude of first point
  * @param { number } lat_2 - Latitude of second point
  * @param { number } lon_2 - Longitude of second point
  *
  * @see http://stackoverflow.com/a/21623256
  * @see https://en.wikipedia.org/wiki/Haversine_formula
  *
  * @return { number } Distance in km
 */
function distance_between( lat_1, lon_1, lat_2, lon_2 ) {
	var Earth = 6371; // Radius of the Earth in km
	var x = ( lat_2 - lat_1 ) * Math.PI / 180;
	var y = ( lon_2 - lon_1 ) * Math.PI / 180;
	var distance = 0.5 - Math.cos( x ) / 2 +
	                 Math.cos( lat_1 * Math.PI / 180 ) *
	                 Math.cos( lat_2 * Math.PI / 180 ) *
	           ( 1 - Math.cos( y ) ) / 2;

	return Earth * 2 * Math.asin( Math.sqrt( distance ) );
}

distance_between_v2.js

/**
 * Distance between two coordinates (lite)
 *
 * @author Salvador Dali
 * @author AnttiK / OiOi
 *
 * @param { number } lat_1 - Latitude of first point
 * @param { number } lon_1 - Longitude of first point
 * @param { number } lat_2 - Latitude of second point
 * @param { number } lon_2 - Longitude of second point
 *
 * @see http://stackoverflow.com/a/21623256
 * @see https://en.wikipedia.org/wiki/Haversine_formula
 *
 * @return { number } Distance in km
 */
	function distance_between( lat_1, lon_1, lat_2, lon_2 ) {
		var Earth = 6371; // Radius of the Earth in km
		var x = ( lon_2 - lon_1 ) * Math.PI / 180 * Math.cos( ( ( lat_1 + lat_2 ) / 2 ) * Math.PI / 180 );
		var y = ( lat_2 - lat_1 ) * Math.PI / 180;
		return Earth * Math.sqrt( x * x + y * y );
	}