code: function copyMerged(source, target) {
      for (var
        key, descriptor, value, tvalue,
        names = oK(source),
        i = 0; i < names.length; i++
      ) {
        key = names[i];
        descriptor = gOPD(source, key);
        // target already has this property
        if (hOP.call(target, key)) {
          // verify the descriptor can  be merged
          if (hOP.call(descriptor, VALUE)) {
            value = descriptor[VALUE];
            // which means, verify it's an object
            if (isObject(value)) {
              // in such case, verify the target can be modified
              descriptor = gOPD(target, key);
              // meaning verify it's a data descriptor
              if (hOP.call(descriptor, VALUE)) {
                tvalue = descriptor[VALUE];
                // and it's actually an object
                if (isObject(tvalue)) {
                  copyMerged(value, tvalue);
                }
              }
            }
          }
        } else {
          // target has no property at all
          if (hOP.call(descriptor, VALUE)) {
            // copy deep if it's an object
            copyValueIfObject(descriptor, copyDeep);
          }
          defineProperty(target, key, descriptor);
        }
      }
    }
docs: given two objects, performs a deep copy per each property not present in the target otherwise merges, without overwriting, all properties within the object