code: function (target, source) {
          if (source) {
              for (var propName in source) {
                  if (target[propName] === undefined) {
                      target[propName] = source[propName];
                  }
              }
          }
          return target;
      }
docs: Extend properties to target if not exist.
  @param  {Object} target
  @param  {Object} source
  @return {Object}
  @memberOf clay.core.util