Add baseMergeDeep.

lodash.src.js

@@ -2479,13 +2479,16 @@
      * @returns {Object} Returns the destination object.
      */
     function baseMerge(object, source, customizer, stackA, stackB) {
-      var isSrcArr = isArray(source) || isTypedArray(source);
+      var isSrcArr = isLength(source.length) && (isArray(source) || isTypedArray(source));
 
       (isSrcArr ? arrayEach : baseForOwn)(source, function(srcValue, key, source) {
-        var value = object[key];
+        if (isObjectLike(srcValue)) {
-
+          stackA || (stackA = []);
-        if (!isObjectLike(srcValue)) {
+          stackB || (stackB = []);
-          var result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
+          return baseMergeDeep(object, source, baseMerge, key, customizer, stackA, stackB);
+        }
+        var value = object[key],
+            result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
             isCommon = typeof result == 'undefined';
 
         if (isCommon) {
@@ -2495,20 +2498,37 @@
             (isCommon || (result === result ? result !== value : value === value))) {
           object[key] = result;
         }
-          return;
+      });
+      return object;
     }
-        // Avoid merging previously merged cyclic sources.
-        stackA || (stackA = []);
-        stackB || (stackB = []);
 
-        var length = stackA.length;
+    /**
+     * A specialized version of `baseMerge` for arrays and objects which performs
+     * deep merges and tracks traversed objects enabling objects with circular
+     * references to be merged.
+     *
+     * @private
+     * @param {Object} object The destination object.
+     * @param {Object} source The source object.
+     * @param {Function} mergeFunc The function to merge values.
+     * @param {string} key The key of the property value to merge.
+     * @param {Function} [customizer] The function to customize merging properties.
+     * @param {Array} [stackA=[]] Tracks traversed source objects.
+     * @param {Array} [stackB=[]] Associates values with source counterparts.
+     * @returns {boolean} Returns `true` if the objects are equivalent, else `false`.
+     */
+    function baseMergeDeep(object, source, mergeFunc, key, customizer, stackA, stackB) {
+      var length = stackA.length,
+          srcValue = source[key];
+
       while (length--) {
         if (stackA[length] == srcValue) {
           object[key] = stackB[length];
           return;
         }
       }
-        result = customizer ? customizer(value, srcValue, key, object, source) : undefined;
+      var value = object[key],
+          result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
           isCommon = typeof result == 'undefined';
 
       if (isCommon) {
@@ -2524,14 +2544,12 @@
       stackA.push(srcValue);
       stackB.push(result);
 
-        // Recursively merge objects and arrays (susceptible to call stack limits).
       if (isCommon) {
-          object[key] = baseMerge(result, srcValue, customizer, stackA, stackB);
+        // Recursively merge objects and arrays (susceptible to call stack limits).
+        object[key] = mergeFunc(result, srcValue, customizer, stackA, stackB);
       } else if (result === result ? result !== value : value === value) {
         object[key] = result;
       }
-      });
-      return object;
     }
 
     /**
@@ -3034,7 +3052,10 @@
         }
         var index = 0;
         while (++index < length) {
-          assigner(object, arguments[index], customizer);
+          var source = arguments[index];
+          if (source) {
+            assigner(object, source, customizer);
+          }
         }
         return object;
       };