This form is created with the TYPO3 form framework. The complete logic, including validation and conditional fields is added in the form definition in a YAML file. There is no additional validation or logic exposed within JavaScript.

Choose items from the select boxes. Once the message field is shown, submit the form to see the validation error, because the field is required. Type some text into the field and see the validation error magically disappear, without having to submit the form.