docs: 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) {
          var isArr = isArray(srcValue),
              isBuff = !isArr && isBuffer(srcValue),
              isTyped = !isArr && !isBuff && isTypedArray(srcValue);
  
          newValue = srcValue;
          if (isArr || isBuff || isTyped) {
            if (isArray(objValue)) {
              newValue = objValue;
            }
            else if (isArrayLikeObject(objValue)) {
              newValue = copyArray(objValue);
            }
            else if (isBuff) {
              isCommon = false;
              newValue = cloneBuffer(srcValue, true);
            }
            else if (isTyped) {
              isCommon = false;
              newValue = cloneTypedArray(srcValue, true);
            }
            else {
              newValue = [];
            }
          }
          else if (isPlainObject(srcValue) || isArguments(srcValue)) {
            newValue = objValue;
            if (isArguments(objValue)) {
              newValue = toPlainObject(objValue);
            }
            else if (!isObject(objValue) || (srcIndex && isFunction(objValue))) {
              newValue = initCloneObject(srcValue);
            }
          }
          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);
      }