code: function baseMergeDeep(object, source, key, srcIndex, mergeFunc, customizer, stack) {
      var objValue = object[key],
          srcValue = source[key],
          stacked = stack.get(srcValue);
  
      if (stacked) {
        assignMergeValue(object, key, stacked);
        return;
      }
      var newValue = customizer ? customizer(objValue, srcValue, key + '', object, source, stack) : undefined;
  
      var isCommon = newValue === undefined;
  
      if (isCommon) {
        newValue = srcValue;
        if (isArray(srcValue) || isTypedArray(srcValue)) {
          if (isArray(objValue)) {
            newValue = objValue;
          } else if (isArrayLikeObject(objValue)) {
            newValue = copyArray(objValue);
          } else {
            isCommon = false;
            newValue = baseClone(srcValue, true);
          }
        } else if (isPlainObject(srcValue) || isArguments(srcValue)) {
          if (isArguments(objValue)) {
            newValue = toPlainObject(objValue);
          } else if (!isObject(objValue) || srcIndex && isFunction(objValue)) {
            isCommon = false;
            newValue = baseClone(srcValue, true);
          } else {
            newValue = objValue;
          }
        } else {
          isCommon = false;
        }
      }
      if (isCommon) {
        // Recursively merge objects and arrays (susceptible to call stack limits).
        stack.set(srcValue, newValue);
        mergeFunc(newValue, srcValue, srcIndex, customizer, stack);
        stack['delete'](srcValue);
      }
      assignMergeValue(object, key, newValue);
    }
docs: 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 {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.