From c21e7b24c66af270853d8ffefa9a424d99e21a75 Mon Sep 17 00:00:00 2001
From: Elliot Winkler <elliot.winkler@gmail.com>
Date: Sun, 31 Oct 2010 18:51:30 -0600
Subject: [PATCH] Fix reduce() so that if you don't pass in an initial value,
 the first item in the collection is used

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

diff --git a/test/collections.js b/test/collections.js
index 4e7c7ab6b..87d5ccfba 100644
--- a/test/collections.js
+++ b/test/collections.js
@@ -65,6 +65,9 @@ $(document).ready(function() {
 
     sum = _([1, 2, 3]).reduce(function(sum, num){ return sum + num; }, 0);
     equals(sum, 6, 'OO-style reduce');
+    
+    var sum = _.reduce([1, 2, 3], function(sum, num){ return sum + num; });
+    equals(sum, 6, 'default initial value');
   });
 
   test('collections: reduceRight', function() {
diff --git a/underscore.js b/underscore.js
index 6bd1ee3ca..5f57bc951 100644
--- a/underscore.js
+++ b/underscore.js
@@ -94,10 +94,16 @@
   _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
     if (nativeReduce && obj.reduce === nativeReduce) {
       if (context) iterator = _.bind(iterator, context);
-      return obj.reduce(iterator, memo);
+      var args = [iterator];
+      if (memo !== undefined) args.push(memo);
+      return obj.reduce.apply(obj, args);
     }
     each(obj, function(value, index, list) {
-      memo = iterator.call(context, memo, value, index, list);
+      if (memo === undefined && index == 0) {
+        memo = value;
+      } else {
+        memo = iterator.call(context, memo, value, index, list);
+      }
     });
     return memo;
   };