Add _.mapKeys. [closes #169]

lodash.src.js

@@ -3600,6 +3600,28 @@
       };
     }
 
+    /**
+     * Creates a function for `_.mapKeys` or `_.mapValues`.
+     *
+     * @private
+     * @param {boolean} [isMapKeys] Specify mapping keys instead of values.
+     * @returns {Function} Returns the new map function.
+     */
+    function createObjectMapper(isMapKeys) {
+      return function(object, iteratee, thisArg) {
+        var result = {};
+        iteratee = getCallback(iteratee, thisArg, 3);
+
+        baseForOwn(object, function(value, key, object) {
+          var mapped = iteratee(value, key, object);
+          key = isMapKeys ? mapped : key;
+          value = isMapKeys ? value : mapped;
+          result[key] = value;
+        });
+        return result;
+      };
+    }
+
     /**
      * Creates a function for `_.padLeft` or `_.padRight`.
      *
@@ -9782,6 +9804,28 @@
       return result;
     }
 
+    /**
+     * The opposite of `_.mapValues`; this method creates an object with the
+     * same values as `object` and keys generated by running each own enumerable
+     * property of `object` through `iteratee`.
+     *
+     * @static
+     * @memberOf _
+     * @category Object
+     * @param {Object} object The object to iterate over.
+     * @param {Function|Object|string} [iteratee=_.identity] The function invoked
+     *  per iteration.
+     * @param {*} [thisArg] The `this` binding of `iteratee`.
+     * @returns {Object} Returns the new mapped object.
+     * @example
+     *
+     * _.mapKeys({ 'a': 1, 'b': 2 }, function(value, key) {
+     *   return key + value;
+     * });
+     * // => { 'a1': 1, 'b2': 2 }
+     */
+    var mapKeys = createObjectMapper(true);
+
     /**
      * Creates an object with the same keys as `object` and values generated by
      * running each own enumerable property of `object` through `iteratee`. The
@@ -9823,15 +9867,7 @@
      * _.mapValues(users, 'age');
      * // => { 'fred': 40, 'pebbles': 1 } (iteration order is not guaranteed)
      */
-    function mapValues(object, iteratee, thisArg) {
-      var result = {};
-      iteratee = getCallback(iteratee, thisArg, 3);
-
-      baseForOwn(object, function(value, key, object) {
-        result[key] = iteratee(value, key, object);
-      });
-      return result;
-    }
+    var mapValues = createObjectMapper();
 
     /**
      * Recursively merges own enumerable properties of the source object(s), that
@@ -11931,6 +11967,7 @@
     lodash.keys = keys;
     lodash.keysIn = keysIn;
     lodash.map = map;
+    lodash.mapKeys = mapKeys;
     lodash.mapValues = mapValues;
     lodash.matches = matches;
     lodash.matchesProperty = matchesProperty;