Ensure _.merge ignores undefined values of source object properties. [closes #573]

lodash.js

@@ -1962,7 +1962,8 @@
      * @returns {Object} Returns the destination object.
      */
     function baseMerge(object, source, callback, stackA, stackB) {
-      (isArrayLike(source) ? arrayEach : baseForOwn)(source, function(srcValue, key, source) {
+      var isSrcArr = isArrayLike(source);
+      (isSrcArr ? arrayEach : baseForOwn)(source, function(srcValue, key, source) {
         var isArr = srcValue && isArrayLike(srcValue),
             isObj = srcValue && isPlainObject(srcValue),
             value = object[key];
@@ -1972,10 +1973,9 @@
           if (typeof result == 'undefined') {
             result = srcValue;
           }
-          if (typeof result != 'undefined') {
-            value = result;
+          if (isSrcArr || typeof result != 'undefined') {
+            object[key] = result;
           }
-          object[key] = value;
           return;
         }
         // avoid merging previously merged cyclic sources
@@ -1992,22 +1992,21 @@
         var result = callback ? callback(value, srcValue, key, object, source) : undefined,
             isShallow = typeof result != 'undefined';
 
-        if (isShallow) {
-          value = result;
-        } else {
-          value = isArr
+        if (!isShallow) {
+          result = isArr
             ? (isArray(value) ? value : [])
             : (isPlainObject(value) ? value : {});
         }
-        // add `source` and associated `value` to the stack of traversed objects
+        // add the source value to the stack of traversed objects
+        // and associate it with its merged value
         stackA.push(srcValue);
-        stackB.push(value);
+        stackB.push(result);
 
         // recursively merge objects and arrays (susceptible to call stack limits)
         if (!isShallow) {
-          baseMerge(value, srcValue, callback, stackA, stackB);
+          baseMerge(result, srcValue, callback, stackA, stackB);
         }
-        object[key] = value;
+        object[key] = result;
       });
 
       return object;

test/test.js

@@ -2580,22 +2580,6 @@
 
       deepEqual(actual, { 'a':  1, 'b': 2, 'c': 3 });
     });
-
-    if (methodName == 'merge') {
-      test('`_.' + methodName + '` should treat sparse arrays as dense', 2, function() {
-        var array = Array(3);
-        array[0] = 1;
-        array[2] = 3;
-
-        var actual = func([], array),
-            expected = array.slice();
-
-        expected[1] = undefined;
-
-        ok('1' in actual);
-        deepEqual(actual, expected);
-      });
-    }
   });
 
   /*--------------------------------------------------------------------------*/
@@ -6365,6 +6349,20 @@
       ok(actual.bar.b === actual.foo.b && actual.foo.b.foo.c === actual.foo.b.foo.c.foo.b.foo.c);
     });
 
+    test('should treat sources that are sparse arrays as dense', 2, function() {
+      var array = Array(3);
+      array[0] = 1;
+      array[2] = 3;
+
+      var actual = _.merge([], array),
+          expected = array.slice();
+
+      expected[1] = undefined;
+
+      ok('1' in actual);
+      deepEqual(actual, expected);
+    });
+
     test('should not treat `arguments` objects as plain objects', 1, function() {
       var object = {
         'args': args
@@ -6389,8 +6387,8 @@
     });
 
     test('should not assign `undefined` values', 1, function() {
-      var actual = _.merge({ 'a': 1 }, { 'a': undefined });
-      strictEqual(actual.a, 1);
+      var actual = _.merge({ 'a': 1 }, { 'a': undefined, 'b': undefined });
+      deepEqual(actual, { 'a': 1 });
     });
 
     test('should handle merging if `callback` returns `undefined`', 1, function() {