Purpose of th:classappend and no-operation (_)

Referring back to my previous example of checking validity of input fields by user

Observe this html with no-operation (_)

<input type="text" th:classappend=
  "${errors?.containsKey('itemName')} ? 'field-error' : _"
 class="form-control">

Through classappend, if there is an error with a particular field, it is added to the “field-error” class info and makes the form red for emphasis.

No-operation, as marked by the underscore (_), does not do anything. So if there is no error, it does nothing.