code: function(key, target, source) {
  		var tval = target[key];
  		var sval = source[key];
  
  		if (helpers.isObject(tval) && helpers.isObject(sval)) {
  			helpers.mergeIf(tval, sval);
  		} else if (!target.hasOwnProperty(key)) {
  			target[key] = helpers.clone(sval);
  		}
  	}
docs: Merges source[key] in target[key] only if target[key] is undefined.
  @private