code: function (actual, message){
          YUITest.Assert._increment();
          if (!isNaN(actual)){
              throw new YUITest.ComparisonFailure(YUITest.Assert._formatMessage(message, "Value should be NaN."), NaN, actual);
          }
      }
docs: ------------------------------------------------------------------------- Special Value Assertion Methods ------------------------------------------------------------------------- 
  Asserts that a value is not a number.
  @param {Object} actual The value to test.
  @param {String} message (Optional) The message to display if the assertion fails.
  @method isNaN
  @static