google.maps.geometry.spherical.computeDistanceBetween (latLngA, latLngB);

var haversine = (function() {

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

  return function haversine(start, end, options) {
    var km    = 6371
    var mile  = 3960
    options   = options || {}

    var R = options.unit === 'mile' ?
      mile :
      km

    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)
    } else {
      return R * c
    }
  }

})()

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

<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false&libraries=geometry"></script>