Fix merging functions and avoid augmenting sources. [closes #1794, #1801]

lodash.js

@@ -2954,10 +2954,11 @@
      * @private
      * @param {Object} object The destination object.
      * @param {Object} source The source object.
+     * @param {number} srcIndex The index of `source`.
      * @param {Function} [customizer] The function to customize merged values.
      * @param {Object} [stack] Tracks traversed source values and their merged counterparts.
      */
-    function baseMerge(object, source, customizer, stack) {
+    function baseMerge(object, source, srcIndex, customizer, stack) {
       if (object === source) {
         return;
       }
@@ -2969,7 +2970,7 @@
         }
         if (isObject(srcValue)) {
           stack || (stack = new Stack);
-          baseMergeDeep(object, source, key, baseMerge, customizer, stack);
+          baseMergeDeep(object, source, key, srcIndex, baseMerge, customizer, stack);
         }
         else {
           var newValue = customizer ? customizer(object[key], srcValue, (key + ''), object, source, stack) : undefined;
@@ -2990,11 +2991,12 @@
      * @param {Object} object The destination object.
      * @param {Object} source The source object.
      * @param {string} key The key of the value to merge.
+     * @param {number} srcIndex The index of `source`.
      * @param {Function} mergeFunc The function to merge values.
      * @param {Function} [customizer] The function to customize assigned values.
      * @param {Object} [stack] Tracks traversed source values and their merged counterparts.
      */
-    function baseMergeDeep(object, source, key, mergeFunc, customizer, stack) {
+    function baseMergeDeep(object, source, key, srcIndex, mergeFunc, customizer, stack) {
       var objValue = object[key],
           srcValue = source[key],
           stacked = stack.get(srcValue) || stack.get(objValue);
@@ -3009,24 +3011,36 @@
       if (isCommon) {
         newValue = srcValue;
         if (isArray(srcValue) || isTypedArray(srcValue)) {
-          newValue = isArray(objValue)
-            ? objValue
-            : ((isArrayLikeObject(objValue)) ? copyArray(objValue) : baseClone(srcValue));
+          if (isArray(objValue)) {
+            newValue = srcIndex ? copyArray(objValue) : objValue;
+          }
+          else if (isArrayLikeObject(objValue)) {
+            newValue = copyArray(objValue);
+          }
+          else {
+            newValue = baseClone(srcValue);
+          }
         }
         else if (isPlainObject(srcValue) || isArguments(srcValue)) {
-          newValue = isArguments(objValue)
-            ? toPlainObject(objValue)
-            : (isObject(objValue) ? objValue : baseClone(srcValue));
+          if (isArguments(objValue)) {
+            newValue = toPlainObject(objValue);
+          }
+          else if (!isObject(objValue) || (srcIndex && isFunction(objValue))) {
+            newValue = baseClone(srcValue);
+          }
+          else {
+            newValue = srcIndex ? baseClone(objValue) : objValue;
+          }
         }
         else {
-          isCommon = isFunction(srcValue);
+          isCommon = false;
         }
       }
       stack.set(srcValue, newValue);
 
       if (isCommon) {
         // Recursively merge objects and arrays (susceptible to call stack limits).
-        mergeFunc(newValue, srcValue, customizer, stack);
+        mergeFunc(newValue, srcValue, srcIndex, customizer, stack);
       }
       assignMergeValue(object, key, newValue);
     }
@@ -3878,7 +3892,7 @@
         while (++index < length) {
           var source = sources[index];
           if (source) {
-            assigner(object, source, customizer);
+            assigner(object, source, index, customizer);
           }
         }
         return object;
@@ -5105,7 +5119,7 @@
     function mergeDefaults(objValue, srcValue, key, object, source, stack) {
       if (isObject(objValue) && isObject(srcValue)) {
         stack.set(srcValue, objValue);
-        baseMerge(objValue, srcValue, mergeDefaults, stack);
+        baseMerge(objValue, srcValue, undefined, mergeDefaults, stack);
       }
       return objValue === undefined
         ? baseClone(srcValue, undefined, undefined, key, object)
@@ -10438,7 +10452,7 @@
      * defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
      * // => { 'a': 1, 'b': 2 }
      */
-    var assignInWith = createAssigner(function(object, source, customizer) {
+    var assignInWith = createAssigner(function(object, source, srcIndex, customizer) {
       copyObjectWith(source, keysIn(source), object, customizer);
     });
 
@@ -10468,7 +10482,7 @@
      * defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
      * // => { 'a': 1, 'b': 2 }
      */
-    var assignWith = createAssigner(function(object, source, customizer) {
+    var assignWith = createAssigner(function(object, source, srcIndex, customizer) {
       copyObjectWith(source, keys(source), object, customizer);
     });
 
@@ -11148,8 +11162,8 @@
      * _.merge(users, ages);
      * // => { 'data': [{ 'user': 'barney', 'age': 36 }, { 'user': 'fred', 'age': 40 }] }
      */
-    var merge = createAssigner(function(object, source) {
-      baseMerge(object, source);
+    var merge = createAssigner(function(object, source, srcIndex) {
+      baseMerge(object, source, srcIndex);
     });
 
     /**
@@ -11187,8 +11201,8 @@
      * _.mergeWith(object, other, customizer);
      * // => { 'fruits': ['apple', 'banana'], 'vegetables': ['beet', 'carrot'] }
      */
-    var mergeWith = createAssigner(function(object, source, customizer) {
-      baseMerge(object, source, customizer);
+    var mergeWith = createAssigner(function(object, source, srcIndex, customizer) {
+      baseMerge(object, source, srcIndex, customizer);
     });
 
     /**