haversine.js

var haversine = (function () {
  var RADII = {
    km:    6371,
    mile:  3960,
    meter: 6371000,
    nmi:   3440
  }

  // convert to radians
  var toRad = function (num) {
    return num * Math.PI / 180
  }

  // convert coordinates to standard format based on the passed format option
  var convertCoordinates = function (format, coordinates) {
    switch (format) {
    case '[lat,lon]':
      return { latitude: coordinates[0], longitude: coordinates[1] }
    case '[lon,lat]':
      return { latitude: coordinates[1], longitude: coordinates[0] }
    case '{lon,lat}':
      return { latitude: coordinates.lat, longitude: coordinates.lon }
    case '{lat,lng}':
      return { latitude: coordinates.lat, longitude: coordinates.lng }
    case 'geojson':
      return { latitude: coordinates.geometry.coordinates[1], longitude: coordinates.geometry.coordinates[0] }
    default:
      return coordinates
    }
  }

  return function haversine (startCoordinates, endCoordinates, options) {
    options   = options || {}

    var R = options.unit in RADII
      ? RADII[options.unit]
      : RADII.km

    var start = convertCoordinates(options.format, startCoordinates)
    var end = convertCoordinates(options.format, endCoordinates)

    var dLat = toRad(end.latitude - start.latitude)
    var dLon = toRad(end.longitude - start.longitude)
    var lat1 = toRad(start.latitude)
    var lat2 = toRad(end.latitude)

    var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
            Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2)
    var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a))

    if (options.threshold) {
      return options.threshold > (R * c)
    }

    return R * c
  }

})()

if (typeof module !== 'undefined' && module.exports) {
  module.exports = haversine
}