From ea44179d810ee8e551d783056d989c2b29de0ad2 Mon Sep 17 00:00:00 2001
From: Jeremy Ashkenas <jashkenas@gmail.com>
Date: Fri, 15 Apr 2011 17:46:16 -0400
Subject: [PATCH] Issue #123. _.extend shouldn't copy keys for undefined
 values.

---
 test/objects.js | 2 ++
 underscore.js   | 8 ++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/test/objects.js b/test/objects.js
index 67d05ecf2..93c441176 100644
--- a/test/objects.js
+++ b/test/objects.js
@@ -33,6 +33,8 @@ $(document).ready(function() {
     ok(_.isEqual(result, {x:'x', a:'a', b:'b'}), 'can extend from multiple source objects');
     result = _.extend({x:'x'}, {a:'a', x:2}, {a:'b'});
     ok(_.isEqual(result, {x:2, a:'b'}), 'extending from multiple source objects last property trumps');
+    result = _.extend({}, {a: void 0, b: null});
+    equals(_.keys(result).join(''), 'b', 'extend does not copy undefined values');
   });
 
   test("objects: defaults", function() {
diff --git a/underscore.js b/underscore.js
index 981a7ce8f..c24c34645 100644
--- a/underscore.js
+++ b/underscore.js
@@ -543,7 +543,9 @@
   // Extend a given object with all the properties in passed-in object(s).
   _.extend = function(obj) {
     each(slice.call(arguments, 1), function(source) {
-      for (var prop in source) obj[prop] = source[prop];
+      for (var prop in source) {
+        if (source[prop] !== void 0) obj[prop] = source[prop];
+      }
     });
     return obj;
   };
@@ -551,7 +553,9 @@
   // Fill in a given object with default properties.
   _.defaults = function(obj) {
     each(slice.call(arguments, 1), function(source) {
-      for (var prop in source) if (obj[prop] == null) obj[prop] = source[prop];
+      for (var prop in source) {
+        if (obj[prop] == null) obj[prop] = source[prop];
+      }
     });
     return obj;
   };