From 46341fea9633f93d5f12935e6042727127834842 Mon Sep 17 00:00:00 2001
From: John-David Dalton <john.david.dalton@gmail.com>
Date: Sat, 27 Dec 2014 21:51:13 -0600
Subject: [PATCH] Ensure `_.merge` handles source arrays with `undefined`
 values correctly with a `customizer`.

---
 lodash.js    | 21 +++++++++++----------
 test/test.js |  6 +++++-
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/lodash.js b/lodash.js
index af44453c9..3be6d1dcf 100644
--- a/lodash.js
+++ b/lodash.js
@@ -2490,11 +2490,13 @@
             value = object[key];
 
         if (!(isArr || isObj)) {
-          result = customizer ? customizer(value, srcValue, key, object, source) : undefined;
-          if (typeof result == 'undefined') {
+          var result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
+              isCommon = typeof result == 'undefined';
+
+          if (isCommon) {
             result = srcValue;
           }
-          if ((isSrcArr || typeof result != 'undefined') && !(customizer && result === value)) {
+          if ((isSrcArr || typeof result != 'undefined') && (isCommon || result !== value)) {
             object[key] = result;
           }
           return;
@@ -2510,10 +2512,10 @@
             return;
           }
         }
-        var result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
-            isDeep = typeof result == 'undefined';
+        result = customizer ? customizer(value, srcValue, key, object, source) : undefined;
+        isCommon = typeof result == 'undefined';
 
-        if (isDeep) {
+        if (isCommon) {
           result = isArr
             ? (isArray(value) ? value : [])
             : (isPlainObject(value) ? value : {});
@@ -2524,10 +2526,9 @@
         stackB.push(result);
 
         // Recursively merge objects and arrays (susceptible to call stack limits).
-        if (isDeep) {
-          baseMerge(result, srcValue, customizer, stackA, stackB);
-        }
-        if (!(customizer && result === value)) {
+        if (isCommon) {
+          object[key] = baseMerge(result, srcValue, customizer, stackA, stackB);
+        } else if (result !== value) {
           object[key] = result;
         }
       });
diff --git a/test/test.js b/test/test.js
index eb294592b..471113d53 100644
--- a/test/test.js
+++ b/test/test.js
@@ -8478,15 +8478,19 @@
       deepEqual(actual, { 'a': 1 });
     });
 
-    test('should handle merging if `customizer` returns `undefined`', 1, function() {
+    test('should handle merging if `customizer` returns `undefined`', 2, function() {
       var actual = _.merge({ 'a': { 'b': [1, 1] } }, { 'a': { 'b': [0] } }, _.noop);
       deepEqual(actual, { 'a': { 'b': [0, 1] } });
+
+      actual = _.merge([], [undefined], _.identity);
+      deepEqual(actual, [undefined]);
     });
 
     test('should defer to `customizer` when it returns a value other than `undefined`', 1, function() {
       var actual = _.merge({ 'a': { 'b': [0, 1] } }, { 'a': { 'b': [2] } }, function(a, b) {
         return _.isArray(a) ? a.concat(b) : undefined;
       });
+
       deepEqual(actual, { 'a': { 'b': [0, 1, 2] } });
     });
   }(1, 2, 3));