Reverse and re-reverse object key-value maps

one-many-hash-interchange

var reverseMapFromMap = function(map, callback) {
  callback = callback || _.identity;
  return _.transform(map, function(reverseMap, one, manyone) {
    reverseMap[one] = reversedMap[one] || [];
    reverseMap[one].push(callback(manyone));
  }, {});
};

var mapFromReverseMap = function(reverseMap, callback) {
  callback = callback || _.identity;
  return _.transform(reverseMap, function(map, many, one) {
    _.forEach(many, function(manyone) {
      map[manyone] = callback(one);
    });
  }, {});
};

//Here's a performance overview of this method versus native and the version not using transform: http://jsperf.com/object-key-value-map-reversal-one-many
//As you can see from the above link, transform method and non-transform method are almost same in performance when it comes to large data, but I chose transform as it looks better to me in terms of readability. Native-only option does not perform so well when operating on large data.

Small typo on #L4?

reversedMap to reverseMap