From f729769d85390217c2be02697c195b2bb71e6967 Mon Sep 17 00:00:00 2001
From: Elliot Winkler <elliot.winkler@gmail.com>
Date: Sun, 31 Oct 2010 19:00:21 -0600
Subject: [PATCH] Fix reduceRight() so that if you don't pass in an initial
 value, the last item in the collection is used

---
 test/collections.js | 10 ++++++++--
 underscore.js       |  4 +++-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/test/collections.js b/test/collections.js
index 4e7c7ab6b..7896ef3ff 100644
--- a/test/collections.js
+++ b/test/collections.js
@@ -68,8 +68,14 @@ $(document).ready(function() {
   });
 
   test('collections: reduceRight', function() {
-    var list = _.foldr([1, 2, 3], function(memo, num){ return memo + num; }, '');
-    equals(list, '321', 'can perform right folds');
+    var list = _.reduceRight(["foo", "bar", "baz"], function(memo, str){ return memo + str; }, '');
+    equals(list, 'bazbarfoo', 'can perform right folds');
+    
+    var list = _.foldr(["foo", "bar", "baz"], function(memo, str){ return memo + str; }, '');
+    equals(list, 'bazbarfoo', 'aliased as "foldr"');
+    
+    var list = _.foldr(["foo", "bar", "baz"], function(memo, str){ return memo + str; });
+    equals(list, 'bazbarfoo', 'default initial value');
   });
 
   test('collections: detect', function() {
diff --git a/underscore.js b/underscore.js
index 6bd1ee3ca..476b85e2a 100644
--- a/underscore.js
+++ b/underscore.js
@@ -107,7 +107,9 @@
   _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
     if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
       if (context) iterator = _.bind(iterator, context);
-      return obj.reduceRight(iterator, memo);
+      var args = [iterator];
+      if (memo !== undefined) args.push(memo);
+      return obj.reduceRight.apply(obj, args);
     }
     var reversed = (_.isArray(obj) ? obj.slice() : _.toArray(obj)).reverse();
     return _.reduce(reversed, iterator, memo, context);