Add partitionInitializer helper.

lodash.js

@@ -463,6 +463,16 @@
     return typeof value.toString != 'function' && typeof (value + '') == 'string';
   }
 
+  /**
+   * Used by `_.partition` to create partitioned arrays.
+   *
+   * @private
+   * @returns {Array} Returns the new array.
+   */
+  function partitionInitializer() {
+    return [[], []];
+  }
+
   /**
    * A fallback implementation of `String#trim` to remove leading and trailing
    * whitespace or specified characters from `string`.
@@ -1979,22 +1989,22 @@
     }
 
     /**
-     * Creates a function that aggregates a collection, creating an object or
-     * array composed from the results of running each element in the collection
+     * Creates a function that aggregates a collection, creating an accumulator
+     * object composed from the results of running each element in the collection
      * through a callback. The given setter function sets the keys and values of
-     * the composed object or array.
+     * the accumulator object. If `initializer` is provided will be used to
+     * initialize the accumulator object.
      *
      * @private
-     * @param {Function} setter The setter function.
-     * @param {boolean} [retArray=false] A flag to indicate that the aggregator
-     *  function should return an array.
+     * @param {Function} setter The function to set keys and values of the accumulator object.
+     * @param {Function} [initializer] The function to initialize the accumulator object.
      * @returns {Function} Returns the new aggregator function.
      */
-    function createAggregator(setter, retArray) {
+    function createAggregator(setter, initializer) {
       return function(collection, callback, thisArg) {
-        var result = retArray ? [[], []] : {};
-
+        var result = initializer ? initializer() : {};
         callback = lodash.createCallback(callback, thisArg, 3);
+
         if (isArray(collection)) {
           var index = -1,
               length = collection.length;
@@ -4364,7 +4374,7 @@
      */
     var partition = createAggregator(function(result, value, key) {
       result[key ? 0 : 1].push(value);
-    }, true);
+    }, partitionInitializer);
 
     /**
      * Retrieves the value of a specified property from all elements in the collection.