code: public static function IFERROR($testValue = '', $errorpart = '')
      {
          $testValue = ($testValue === null) ? '' : Functions::flattenSingleValue($testValue);
          $errorpart = ($errorpart === null) ? '' : Functions::flattenSingleValue($errorpart);
  
          return self::statementIf(Functions::isError($testValue), $errorpart, $testValue);
      }
docs: IFERROR.
  
  Excel Function:
  =IFERROR(testValue,errorpart)
  
  @category Logical Functions
  
  @param mixed $testValue Value to check, is also the value returned when no error
  @param mixed $errorpart Value to return when testValue is an error condition
  
  @return mixed The value of errorpart or testValue determined by error condition